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PALEOLIMNOLOGICAL RECONSTRUCTIONS OF TWO COASTAL DUNE LAKES IN NORTHWEST FLORIDA USING SEDIMENT ORGANIC MATTER PROXIES
By
BRANDY ELIZABETH SARA FOLEY
A THESIS PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF
MASTER OF SCIENCE
UNIVERSITY OF FLORIDA
2018
© 2018 Brandy Elizabeth Sara Foley
To my love, Matthew
4
ACKNOWLEDGMENTS
Foremost, I would like to express my sincere gratitude to Dr. Stephens for her
continuous support, enthusiasm and generous knowledge. My sincere appreciation of
the funding and support from the Mattie Kelly Environmental Institute and
Choctawhatchee Basin Alliance of Northwest Florida State College, and Walton County.
A special thank you to Dr. Brenner and Dr. Curtis for your exceptional assistance and
dedication that helped to develop this research project.
I would also like to express my immense gratitude to my advisor, Dr. Wright, and
committee members Dr. Stephens, Dr. Reynolds and Dr. Ogram for their patience,
support and commitment to my academic success.
Finally, I must express my very profound gratitude to my parents, family and
friends for providing me with unfailing support and continuous encouragement
throughout my life and during this time researching and writing my thesis. This
accomplishment would not have been possible without them. Thank you.
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TABLE OF CONTENTS page
ACKNOWLEDGMENTS .................................................................................................. 4
LIST OF TABLES ............................................................................................................ 7
LIST OF FIGURES .......................................................................................................... 8
ABSTRACT ..................................................................................................................... 9
CHAPTER
1 REVIEW OF LITERATURE .................................................................................... 11
Coastal Dune Lakes of Northwest Florida ............................................................... 11 Global Distribution ............................................................................................ 16
Similarities to Estuaries .................................................................................... 19 Similarities to Lagoons ..................................................................................... 24 Geomorphology ................................................................................................ 26
Physicochemical Variables ............................................................................... 27 Paleolimnology ....................................................................................................... 29
Objectives ............................................................................................................... 39
2 MATERIALS AND METHODS ................................................................................ 43
Site Description ....................................................................................................... 43 Eastern Lake .............................................................................................. 43 Big Redfish Lake ........................................................................................ 44
Surface Water Chemistry Collection ....................................................................... 45 Statistical Analyses ................................................................................................. 46
Collection and Preparation of Sediment Samples ................................................... 46 Geochronology ....................................................................................................... 47 Total Phosphorus .................................................................................................... 49
Geochemistry .......................................................................................................... 49
3 RESULTS ............................................................................................................... 53
Surface Water Chemistry ........................................................................................ 53
Nutrients ........................................................................................................... 53
Eastern Lake .............................................................................................. 53 Big Redfish Lake ........................................................................................ 54
Physicochemical Variables ............................................................................... 55 Eastern Lake .............................................................................................. 55 Big Redfish Lake ........................................................................................ 55
Geochronology and Sedimentation Rates .............................................................. 56 Eastern Lake .................................................................................................... 56
6
Big Redfish Lake .............................................................................................. 57
Total Phosphorus .................................................................................................... 58 Eastern Lake .................................................................................................... 58
Big Redfish Lake .............................................................................................. 59 Geochemistry .......................................................................................................... 59
Eastern Lake .................................................................................................... 59 Big Redfish Lake .............................................................................................. 60
4 DISCUSSION ......................................................................................................... 78
Surface Water Chemistry ........................................................................................ 79 Water Column Nitrogen to Phosphorus Ratios ....................................................... 89 Geochronology ....................................................................................................... 92 Sediment Total Phosphorus .................................................................................... 94
Organic Matter ........................................................................................................ 96 Paleoproductivity .................................................................................................. 101
Paleosalinity .......................................................................................................... 102
5 CONCLUSION ...................................................................................................... 111
LIST OF REFERENCES ............................................................................................. 115
BIOGRAPHICAL SKETCH .......................................................................................... 125
7
LIST OF TABLES
Table page 1-1 The 15 Recognized Coastal Dune Lakes of Walton County morphometric
summary statistics (Hoyer and Canfield 2008). .................................................. 42
3-1 Eastern Lake long term summary statistics (CBA 2017) .................................... 62
3-2 Big Redfish Lake long term summary statistics (CBA 2017). ............................. 63
4-1 Trophic state classification (Forsberg and Ryding 1980). ................................. 105
4-2 Eastern and Big Redfish Lake mean long term trophic variables (CBA 2017). . 106
4-3 Surface and bottom physicochemical variables between Eastern and Big Redfish Lake..................................................................................................... 107
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LIST OF FIGURES
Figure page 1-1 Coastal Dune Lakes of Walton County (Walton Outdoors 2018). ....................... 41
2-1 Eastern Lake and catchment area ...................................................................... 51
2-2 Big Redfish Lake and catchment area ................................................................ 52
3-2 Big Redfish Lake long term trend analysis (CBA 2017) ...................................... 65
3-3 Eastern Lake Lead-210 geochronology .............................................................. 66
3-4 Eastern Lake sedimentation rate ........................................................................ 67
3-5 Big Redfish Lake geochronology ........................................................................ 68
3-6 Big Redfish Lake sedimentation rate .................................................................. 69
3-7 Eastern Lake sediment total phosphorus ........................................................... 70
3-8 Big Redfish Lake sediment total phosphorus ..................................................... 71
3-9 Eastern Lake C/N ratio ....................................................................................... 72
3-10 Eastern Lake 13C ................................................................................................ 73
3-11 Eastern Lake δ15N .............................................................................................. 74
3-12 Big Redfish Lake C/N ratio ................................................................................. 75
3-13 Big Redfish Lake δ13C ........................................................................................ 76
3-14 Big Redfish Lake δ15N ........................................................................................ 77
4-1 Organic matter origins relationships between δ13C and C/N ratio (Lamb et al., 2006). ............................................................................................................... 108
4-2 Eastern Lake sediment δ13C and C/N ratio relationship. .................................. 109
4-3 Big Redfish Lake δ13C and C/N ratio relationship. ............................................ 110
9
Abstract of Thesis Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Master of Science
PALEOLIMNOLOGICAL RECONSTRUCTIONS OF TWO COASTAL DUNE LAKES IN
NORTHWEST FLORIDA USING SEDIMENT ORGANIC MATTER PROXIES
By
Brandy Elizabeth Sara Foley
May 2018
Chair: Alan Wright Major: Soil and Water Sciences
Coastal dune lakes in Northwest Florida frequently exchange seawater with the
Gulf of Mexico, but remain undefined in their geomorphology and hydrologic functions
due to limited published literature. A 20-year record of surface water quality data
indicated that two coastal dune lakes, Eastern and Big Redfish Lake, exhibited
significant positive trends in nutrients and chlorophyll indicating shifts toward more
biologically productive systems. To gain an understanding of historical changes within
Eastern and Big Redfish Lake, paleolimnological analyses of sediment organic matter
were conducted. Geochemical signatures of 210Lead, total phosphorus (TP), total
carbon (TC) and total nitrogen (TN) ratios, and the stable isotopes δ13C and δ15N were
analyzed to depict historical variability of lake hydrologic conditions. Results of these
analyses reconstructed historical nutrient status, organic matter sources,
paleoproductivity and periods of marine or freshwater dominance. TP and TN exhibited
significant increases (p<0.05) towards surface sediments, indicating a change in the
lake’s nutrient balance. The TC/TN ratios demonstrated significant decreases (p<0.05)
towards surface sediments, indicating a shift to autochthonous organic matter sources.
10
A combination of terrestrial C3 plants, freshwater and marine organic matter inputs were
revealed through stable isotopes δ13C and δ15N values. Paleo-productivity inferred from
stable isotopes did not correlate with increasing TP and TN, indicating that primary
productivity may not be the dominant source of organic matter in these lakes. Periods of
marine or freshwater dominance were revealed through stable isotope trends indicating
historical variability between both lakes. This research has improved our understanding
of two coastal dune lakes’ hydrologic functioning through the last century.
11
CHAPTER 1 REVIEW OF LITERATURE
The purpose of this literature review is to examine the global distribution of
similar coastal waterbody subclasses to the coastal dune lakes of Northwest Florida,
examine classification designations, and compare the morphologic and hydrologic
characteristics among these systems. This information will help to bolster our
understanding of the dynamic interactions between morphologic and hydrologic
interactions of the coastal dune lakes ecosystems and may lead to innovative
management techniques.
Coastal Dune Lakes of Northwest Florida
Coastal dune lakes of Northwest Florida are referenced as unique and rare
coastal waterbodies found in select locations around the world (FNAI 2010). They are
considered distinctive due to their intermittent connections to seawater of the Gulf of
Mexico and the subsequent role this interaction has on physicochemical conditions
within the lakes. This cluster of coastal dune lakes in Northwest Florida are the only
formations that exist in the state of Florida and are considered internationally rare
formations due to intermittent connections to seawater, geological characteristics of this
region in Florida, Gulf of Mexico geomorphology and regionally distinct climate of
Northwest Florida.
Walton County recognizes 15 coastal dune lakes between Walton and Bay
County in Northwest Florida (Figure 1-1). Within this region, barrier islands have not
formed, therefore, the lakes interact directly with the Gulf of Mexico waters (Gross
2015). The lakes are located between the maritime forest and the Gulf of Mexico coast,
occurring behind the barrier sand dune system. The hydrologic functions and
12
characteristics of the coastal dune lakes are unique when compared to other fresh,
estuary and marine waterbodies in the region. They are characterized by shallow,
irregularly shaped waterbodies located within three kilometers of the coastline (FNAI
2010; Hoyer and Canfield 2008). On average, coastal dune lakes have small
catchments areas of less than two square kilometers (km2) and morphometric features,
with an average surface area of 0.26 km2 and an average depth of two meters (Table 1-
1). Twelve of the fifteen recognized coastal dune lakes have an intermittent connection
to the Gulf of Mexico.
Physicochemical properties range among Northwest Florida’s coastal dune lakes
and are presumed to be strongly associated with local geologic conditions, lake
watershed and morphometry, climate, and frequency and duration of outlet connection
to seawater (Hoyer and Canfield 2008). Northwest Florida waterbodies exhibit soft
water and a high mineral content of predominantly sulfate, sodium and chloride ions
(Griffith et al., 1997). Due to geologic and watershed influences, coastal dune lakes are
considered oligotrophic waterbodies with low biological productivity and low nutrient
levels. High concentrations of humus in the form of dissolved organic matter drains from
forests and wetlands throughout lake watersheds. These contributions cause the lake
waters to be slightly acidic (Hoyer and Canfield 2008). The dominant portions of colored
dissolved organic matter give the lakes water a dark, tea color with a mean true color
value of 99 Platinum-Cobalt units (Hoyer and Canfield 2008).
Coastal dune lakes are lentic water bodies without significant surface inflow (i.e.
fluvial inputs), and receive much of their freshwater from lateral groundwater seepage
through the surrounding well-drained coastal sands, minor streams and surface water
13
runoff (Hoyer and Canfield 2008). If seawater enters the lake, it is predominantly
through an outlet connection to the Gulf, however, storm overwash events, and potential
saltwater intrusion through the beach face can also be considered potential sources of
saltwater inputs (Hoyer and Canfield 2008; Browder and Dean 1998).
Seasonal climate conditions have a substantial role on lake water levels, outlet
connections and physicochemical variations. Rainfall varies throughout the State of
Florida, however, northwest Florida has one of the highest mean annual rainfall in the
state, with 60-64 inches a year (Borisova and Wade 2017). If groundwater is the
dominant source of water to coastal dune lakes, water table level fluctuations may
influence outlet connections and correlate with seasonal lake levels (Hoyer and Canfield
2008). During drier climate conditions, groundwater levels generally drop and lake
outlets do not typically connect to the Gulf of Mexico. Under these conditions lake
waters may experience stable lake levels and decreased water clarity due to saturation
of dissolved organic matter or increased clarity due low surface water inputs (Hoyer and
Canfield 2008). A wet climate can cause increased surface water runoff and potential
increases in groundwater levels, causing lake levels to rise and decreasing Secchi
depth with the addition of dissolved organic matter inputs (Hoyer and Canfield 2008).
Lake outlet connections to the Gulf of Mexico cause the lake level to drop, Secchi depth
measurements may increase due to seawater influx and stratified lake conditions can
occur within the water column (Hoyer and Canfield 2008).
The coastal dune lakes are also critical insect breeding sites which form the
basis of local food chains and are important habitat for fish, birds and mammals
inhabiting the lakes and surrounding coastal ecosystems (Griffith et al., 1997; FNAI
14
2010). Lake outlet habitats are important sources of food and habitat for many migratory
and native shorebirds. Threatened species, such as the Snowy plover and
Choctawhatchee Beach Mouse, utilize shorelines and sand dune habitat of coastal
dune lakes (de Villegas 2017).
Northwest Florida coastal dune lakes exhibit intermittent connections to the Gulf
of Mexico, known as outlets or outfalls, and open to the Gulf under a variety of
conditions based on hydraulic gradient, climate, and tide. Outlet openings are typically
the result of a shift in the hydraulic gradient between the beach face and the lake water
level. Often, when lake levels reach near flood level conditions, the hydraulic gradient
becomes reduced between the lake outlet and Gulf of Mexico beach face causing lake
water to breach the separating sand berm and flow into the Gulf (Browder and Dean
1998). Strong storms, tides, waves and wind can also cause an outlet connection to
occur in the reverse, Gulf water flowing into a lake through the outlet (Hoyer and
Canfield 2008). When lake water exchanges with seawater, system shifts from a
freshwater dominant state, 0-0.5 parts per thousand (ppt), to a brackish, estuarine-like
condition of 0.5-30 ppt. Variable outlet connection factors, along with the movement of
coastal sediments play a role in the frequency and duration of the outlet connections.
Each coastal dune lake outlet status (open or closed) varies in frequency and duration
of opening (Hoyer and Canfield 2008). Outlet status may not be the only mechanism by
which salinity changes in lakes. Groundwater flow, runoff, potential subaqueous
exchange through the beach face may also impact salinity in lakes.
For example, Campbell Lake experiences outlet connections to the Gulf, but
does not display changes in lake salinity under typical outlet conditions, i.e. flow from
15
the lake to the Gulf, and maintains a mean surface salinity of 0.06 ppt. This is likely the
result of lake elevation in comparison to the mean sea level (Griffith et al., 1997).
Conversely, Powell Lake has outlet connections lasting longer in duration, thus,
maintaining conditions more similar to a lagoon-state, with a mean surface salinity of
14.3 ppt. However, Powell Lake is manually opened during high water level conditions
that threaten residences and infrastructure.
All of the coastal dune lakes in Walton County have natural, unrestricted outlet
connections that move freely within their outlet sweep zone; meaning that they are not
stabilized by structural support or directed through artificial arrangements (Browder and
Dean 1998). Historically, four coastal dune lake outlets Western, Eastern, Deer and
Alligator Lake have permits to be manually maintained, or opened, by officials during
high water level events to protect infrastructure and residences.
It is hypothesized that coastal dune lakes of Northwest Florida developed from
various coastal processes, beginning as tidally influenced basins or lagoons that
became closed off by sand infilling its inlet buffering or eliminating a tidal influence
(Gross 2015; Liu and Fearn 2000; FNAI 2010). The U.S. Gulf of Mexico and Atlantic
coasts experienced a steady rise in sea level of approximately 6 millimeters/year,
relative to the land, causing sediment barriers to close off coastal water features from
the sea and a landward migration of water features (Bird 1994; Donoghue 2011).
Throughout its geological history, Northwest Florida experienced periods of flooding
from marine intrusions or exposed sandy coastlines (Coor 2013). In the Gulf of Mexico,
sea level rise gradually slowed in the middle to late Holocene epoch, approximately 6-
8,000 years ago (Blum et al., 2002: Coor 2013). Net longshore currents transported
16
sediment dominated by quartz sand erosion of the Appalachian Mountains to the
Apalachicola River delta, then west along the northwest Florida Panhandle region (Coor
2013). Radiocarbon dating of organic matter found in sediment cores from two
Northwest Florida coastal dune lakes showed the lake formed during a similar section of
the mid-Holocene, roughly 5,000 years ago (Liu and Fearn 2000).
Global Distribution
The shifting nature of coastal environments pose a challenge to global
classification and the definitions used to categorize coastal waterbodies. It can be
further complicated by regionally distinct waterbody functions, such as an estuary or
lagoon, and potential subtypes like the coastal dune lakes found in Northwest Florida.
Coastal waterbodies identified as coastal dune lakes have been reported throughout the
world, however often do not intermittently connect with seawater and therefore appear
to contrast the ones in Northwest Florida (Gross 2015; Timms 1986).
Coastal dune lakes have repeatedly been examined and classified based on
physicochemical and geomorphic characteristics (Timms 1986; Gross 2015; Coor
2013). In contemporary studies, similarly functioning systems have become recognized
as Intermittently Closed and Open Lakes and Lagoons (ICOLLs), but are
interchangeably used with references such as intermittently open and closed estuaries
(Gale et al., 2006; Haines et al., 2006). In South Africa, similarly functioning systems
exist, but are predominantly fluvial dominated systems that intermittently connect to
seawater based on climate conditions, known as Temporarily Open/Closed Estuaries
(TOCEs) (Perissinotto et al., 2010; Whitfield and Bate 2007; Cooper 2001). In New
Zealand, such waterbodies are referred to as coastal lagoons and have been
distinguished using a sub-classification of either coastal lakes known as ‘Waiatuna-
17
type,’ or river-mouth lagoons called ‘Hapua’ (Kirk and Lauder 2000). Within the United
States, the Pacific Northwest coast of Oregon and California, the Atlantic East coast of
North Carolina identify refer to intermittently open or closed coastal waterbodies as
simply lagoons, coastal ponds or coastal lakes (Kling 1986; Eilers et al., 1996; Klaus et
al., 2002). South American studies refer to these systems as lagoons or ICOLLs (Netto
and Fonseca 2017). The southern Baltic region has also reported comparable coastal
waterbodies as intermittently open/closed coastal lakes (Astel et al., 2016).
Internationally, studies on various coastal waterbodies that share parallels in hydrologic
functions to intermittently opened or closed waterbodies have occurred on every
continent (Tagliapietra et al., 2009). However, these waterbodies remain regionally
identified throughout the world due to the neglect of developing an internationally
recognized classification system.
On a global scale, wave dominated coastal waterbodies located on microtidal
coasts make up between 8% (Dürr et al., 2011) and 12% of the world’s coastlines
(Kennish 2015). Coastal environments are viewed as highly dynamic zones where
terrestrial ecosystems blend with marine systems (Dürr et al., 2011). It is due to these
forces, estuarine lagoons form in coastal environments and are a representation of an
ecotone between terrestrial and marine systems. Contrasting settings of these
waterbodies form on low lying coasts with limited tidal variation and have a tendency to
be wave dominated, versus tidally dominated (Kennish 2015; McSweeney et al., 2017).
Depending on the system’s dominant forces, i.e. catchment area, fluvial or tidal, a
connection to seawater exists with an intermittently opened or closed state and the
frequency and duration of its connection may be dependent on geomorphologic
18
characteristics. Generally, these systems are not permanently open to the sea
(McSweeney et al., 2017).
McSweeney et al. (2017) studied the global geomorphology of intermittently
open/closed coastal waterbodies referring to them collectively as intermittently
open/closed estuaries (IOCEs). Using estuaries as the largest classification order, after
oceanic waterbodies, McSweeney et al. (2017) contended that IOCEs are a rare sub-
class of wave dominated estuaries. Distinguishing features of ICOEs are their
geomorphic characteristics (catchment and surface area), physicochemical features and
intermittent and restricted connections to seawater (Tagliapietra et al., 2009;
McSweeney et al., 2017). IOCEs predominantly exist on coastlines that exhibit a
microtidal range, less than 2 meters, and are wave versus tide dominated (Tagliapietra
et al., 2009; McSweeney et al., 2017, Roy et al., 2001). Wave dominated coasts are
driven by wave energy and longshore sediment transport which control the morphology
of the coast versus tide dominated coasts that are driven by tidal currents redistribution
of sediments (Roy et al., 1994).
Using site specific characteristics to classify coastal dune lakes alleviates
misidentification errors and provides a foundation to compare similar functioning
waterbodies with. Estuarine lagoon subtypes tend to become regionally defined
because of the latter and non-existent or grouped with larger, worldwide coastal
waterbody classification systems. A variety of terms have been used to discuss and
identify these subtypes causing a general miscommunication of which systems are
being discussed and how to refer to these systems on an international scale.
19
To accurately depict the geomorphology and hydrology of coastal dune lakes in
Northwest Florida these lakes can be referred to as estuarine lagoons, encompassing
similar characteristics from both estuaries and lagoons. Estuarine lagoons have
parallels in their geologic formation, geographic locations, physicochemical
compositions, and relationship of morphometric variables.
Estuarine lagoons are referred to as geologically “young” formations with origins
establishing during the Holocene period of the Post-glacial Marine Transgression
approximately 8,000 years ago (Roy et al., 2001). Post-glacial Marine Transgression
induced episodic sea level rise that occurred worldwide (Donoghue 2011). Present day
sea levels were reached around 6,700 years ago, forming modern shorelines and
coastal features such as estuaries and lagoons (Blum et al., 2002). During this period,
the formation of coastal sand barriers at the mouth of flooded river valleys and coastal
inlets created isolated estuarine lagoon environments (Adlam 2014). Many modern
coastal waterbodies were formed during this time through sedimentation (Roy et al.,
2001). Typically, a coastal waters’ maturity is measured by its sedimentation, from an
immature unfilled state to a mature filled state (Adlam 2014). Estuarine lagoons are
considered geologically undeveloped and relatively short-lived formations with a strong
probability to disappear (FNAI 2010). However, Adlam (2014) hypothesizes that
estuarine lagoons’ geological evolutions are inhibited by local energy regimes.
Similarities to Estuaries
Estuarine lagoons can be grouped into many broad conceptual coastal
waterbody classes including estuaries, coastal lagoons, ICOLLs, TOCEs, tidal creeks,
river mouths, rias, fjords, fjards, and bahiras (Tagliapietra et al., 2009). Broad
20
classification for estuarine lagoons are used to show the arrow of unique subtypes of
estuary formations (McSweeney et al., 2017; Tagliapietra et al., 2009).
One of the original definitions of estuaries was created by Pritchard in 1967, but
more recently restructured by J. H. Day to include additional unique waterbodies found
in other regions that intermittently open or close to an ocean, as well as address
variations in salinity gradients found in estuary environments (Potter et al., 2010).
According to J. H. Day (1981), “an estuary is a partially enclosed coastal body of water
which is either permanently or periodically open to the sea and within which there is a
measurable variation of salinity due to the mixture of seawater with freshwater derived
from land drainage.” Defining and classifying estuaries follows a methodology organized
by the physical processes of a system and subsequent physicochemical and ecological
characteristics (Hume et al., 2007). Broadly, estuaries are formed by the physical
variables of latitude, oceanic basins and large landmasses (Hume et al., 2007). Their
formation and function is dominated by a single or many energy sources including
waves, river and/or tides (Heap et al., 2001). These foundational forces explain the
development of different estuaries and estuary subtypes, as well as their morphologic
and hydrologic characteristics which correlate with their global geographic region (Hume
et al., 2007). Morphologic characteristics can be explained by the processes that occur
within an estuaries’ catchment and its interactions with the marine and terrestrial
environments. Hydrologic conditions of an estuary are significantly influenced by salt
and freshwater inputs, precipitation, stratification, flushing, mixing and sedimentation
(Hume et al., 2007), and these physical processes drive an estuaries behavior and help
21
to distinguish them from other coastal waterbodies. However, there are always caveats
in science.
Recently, McSweeney et al., (2017) studied global documentation and reviewed
aerial imagery of estuarine lagoon subtypes which led to one of the first comprehensive
classifications of these unique systems. Their classification system bases all coastal
waterbody subtypes on an estuary classification scheme and further designates estuary
subtypes, terming them intermittently opened/closed estuaries (IOCE) (McSweeney et
al., 2017). Their observations deduced that these types of waterbodies occur in
microtidal coastal and are wave dominated, not river or tidally dominated. General
estuary classifications often exclude IOCEs or recognize them as a rare subset of wave-
dominated systems, lumping they all together into one class (McSweeney et al., 2017).
Unlike previous studies, a geomorphic organizational approach was applied, versus site
specific ecological or physicochemical (McSweeney et al., 2017). Through this
methodology, three variations of IOCEs (estuary subtypes) were determined: drowned
river valleys, barrier estuaries, and saline coastal lakes (McSweeney et al., 2017). Each
variation of IOCEs is distinguished by its morphologic sedimentary infill and entrance
condition, used to represent the age or level of progression of the IOCEs (McSweeney
et al., 2017). Young IOCEs have limited infill and deeper basins, while mature estuaries
are typically shallow due to restricted space for sediment accumulation (McSweeney et
al., 2017). The classification suggested by McSweeney et al., (2017) assists in
differentiating between IOCE subtypes on a global scale through the use of easily
measurable marine and fluvial variables: entrance channel width, estuary perimeter,
surface area, catchment area, estuary length, beach width and tidal prism (McSweeney
22
et al., 2017). As Tagliapietra et al., (2009) points out though, there can be various terms
and definitions used to describe estuaries, lagoons or similar coastal waterbodies, thus
creating an overlap in descriptions and the risk for confusion or disagreement.
The coastal waterbody subtypes found in South Africa, are referred to as
temporarily open/closed estuaries (TOCE) (Tagliapietra et al., 2009; Perissinotto et al.,
et al., 2010). Most of these estuary formations occupy a drowned river valley, yet some
have developed on coastal plains similar to the coastal dune lakes of Northwest Florida
(Whitfield 1992). TOCEs are classified into two subtypes based on the frequency of
their connection with seawater (McSweeney et al., 2017). The major drivers of the
South African TOCEs are their catchments and dominant influences on runoff and
streamflow (Perissinotto et al., et al., 2010). River inflow is reported as the most critical
factor influencing TOCE hydrology because of its role in the sedimentary and
hydrodynamic processes within individual systems (Whitfield 1992). Many TOCE are
considered perched estuaries with a minimum water level elevated above mean sea
level (MSL) (Perissinotto et al., 2010). The coastal dune lakes of Northwest Florida are
considered perched lakes in the context that they are perched over a confining surficial
freshwater layer slightly above mean sea level, creating a hydraulic gradient that exists
between the lakes and the Gulf of Mexico (Coor 2013). In both cases, a perched
estuarine lagoon allows for the system to sustain interval periods of an opened or
closed state with seawater. On a global scale, this aspect allows for unique outlet
behavior and creates dynamic hydrologic conditions that can vastly contrast other
similar systems. The subsequent physicochemical characteristics which are created
23
from estuarine lagoon connection behavior is an integral component in distinguishing
them from other coastal waterbodies that are permanently open (Lill et al., 2012).
Similar estuarine lagoon features exist on the coast of Tasmania, a small
Australian state island, and in New Zealand. These features are also identified as
estuary subtypes and subclassified by their acting physical features, geomorphology
and ecological communities (Edgar et al., 2000; Lill et al., 2012). In Tasmania, it was
determined that a variety of subgroups exist here which can be organized by their
geomorphic criteria, ranging from drowned river valleys, marine inlets, river estuaries,
permanently open barred estuaries, seasonally closed barred estuaries and even
included a lagoon subgroup (Edgar et al., 2000). Within this study, however, associated
ecological biota was strongly influenced by salinity and tidal range (Edgar et al., 2000).
This finding holds pertinent meaning to the ideology behind estuarine lagoon subtype
classification. Edgar et al., (2000) concluded that physical estuarine lagoon
classification based solely on physicochemical data is meaningless unless validated
with biological data. Lill et al., (2012) also determined signification correlations between
New Zealand intermittently closed estuaries and biological community structure. This
data can provide another aspect to the ecological distinctions between estuarine
lagoons and other coastal features, along with providing another edge in the
classification of these waterbodies globally.
Estuaries have been traditionally identified by their physical processes,
geomorphology, and connection to seawater. Some focus has been directed to an
estuaries tidal influence and subsequent salinity gradient. However, the estuary
subtypes discussed can range from having diurnal tidal influence to no tidal prism. The
24
unique aspects of estuarine lagoon subtypes cover a range of influencing processes
and may inspire a different perspective on subtype classification.
Similarities to Lagoons
The coastal dune lakes of Northwest Florida also share similarities with coastal
lagoons based on the nature of their outlet connection to seawater and potential
influence by tides. Similar to estuaries, coastal lagoons have historically had various
definitions used to define their attributes. One of the most widely accepted and
comprehensive definitions of coastal lagoons was developed by Kjerfve (1994). He
defines coastal lagoons as inland waterbodies, usually parallel to the coast, separated
from the ocean with one or more restricted inlets that can remain open or may be
intermittently open, and have shallow depths. Originally based on Phleger’s definition,
Kjerfve (1994) felt it was important to include a regard for the connection(s) of coastal
lagoons to the ocean and their potential to be closed off by sediment deposition from
wave action and littoral drift. Similar observations were made by McSweeney et al.,
(2017) in their research, confirming that a coast’s wave energy and sediment transport
have an important role to an estuarine lagoon’s connection with seawater. A coastal
lagoon seawater connection characteristics determine the influence of tides on the
system and subsequent salinity which can vary from a coastal freshwater lake to a
hypersaline lagoon (Kjerfve 1994).
Kjerfve distinguishes estuaries and coastal lagoons by establishing a
classification scheme based on the connection state and subsequent exchange of
seawater (Kjerfve 1994). Through Kjerfve’s classification, estuaries and coastal lagoons
were differentiated by freshwater inputs, tidal influence, dilution of saltwater and depth
(Kjerfve 1994). Focusing on coastal lagoons, he further subclassifies coastal lagoons by
25
the frequency and amount of seawater exchange which he believes are the dominant
forces and functions of lagoons (Kjerfve 1994). The lagoon’s connection channel to the
sea have certain characteristics which heavily influence the tidal influence and water
level fluctuations within the lagoon (Kjerfve 1994). Due to these channel characteristics,
three geomorphic subclasses of coastal lagoons were designated as choked, restricted
or leaky lagoons (Kjerfve 1994). Choked lagoons are made up of a series of connected
oval coastal waterbody features, connected to the sea by a long narrow channel
(Kjerfve 1994). The connection channel has a role in the flushing time of the lagoon and
its intermittent stratification which characterize the lagoon’s physicochemical
characteristics (Kjerfve 1994). A restricted lagoon has two or more connection channels
which allow for increased tidal influence and vertical mixing (Kjerfve 1994). Lastly, leaky
lagoons have many connection channels to seawater allowing for a strong tidal
presence in the lagoon and higher salinities which mirror the influencing seawater.
However, variations among coastal lagoons can also be attributed to their
geomorphologic history and the influence of geological, climatic, hydrological and
ecological factors, as well as humans (Kjerfve 1994).
In a study conducted by Haines et al., (2006), ICOLLs were classified by their
morphometric variables, such as surface area, volume, shape, tidal prism, catchment
area and Entrance Closure Index (ECI). Haines et al., (2006), based their analyses on
the ICOLLs morphometry and its direct influence on the hydrodynamics and subsequent
physicochemical nature (Haines et al., 2006). It is thought that these resemblances
would be observed in the coastal dune lakes of Northwest Florida if a comprehensive
morphometric analysis was conducted. Morphometric characteristics can also provide
26
an estimation of the evolutional state of an ICOLLs geomorphology which may correlate
with its sensitivity to external inputs (Haines et al., 2006). It was determined for New
South Wales, Australian ICOLLs that systems with smaller waterbody volume to
catchment area were more geomorphologically evolved and more vulnerable to external
activities (Haines et al., 2006).
According to Bird (2008), coastal lagoons with restricted or intermittently
opened/closed connections may resemble inland lakes versus traditional coastal
lagoons features. However, saline lakes are recognized largely as coastal lagoons
which can encompass a broad range of estuarine characteristics. It is challenging to
develop a comprehensive term that can be used to define and distinguish estuarine
lagoons throughout the world (Tagliapietra et al., 2009). However, the variability that is
observed within regions and throughout the world has some consistencies among
coastal lagoons in their functional operations.
Geomorphology
Geomorphologic variables have been referred to as the distinguishing features of
estuarine lagoons from other traditional coastal waterbodies. The morphometric
relationship between waterbody and catchment area are the two major drivers of
physical, chemical and biological aspects of an estuarine lagoon system (McSweeney
et al., 2017). These morphometric variables have a substantial control over the state of
the connection, or disconnection, to seawater (McSweeney et al., 2017). An estuarine
lagoon system with a large catchment area is directly correlated to seawater
connections on an annual basis (McSweeney et al., 2017). Geomorphologic
characteristics, such as connection channel width, tidal prism, waterbody area and
27
volume, and catchment size drives the frequency, duration, and state of estuarine
lagoon connection to seawater (McSweeney et al., 2017).
According to Elliott and Whitfield (2011), hydromorphology characteristics have
an influence on circulation, tidal prism and biota communities. An estuarine lagoon
system can have varying degrees of circulation and residence time. The influence of
freshwater and seawater inputs are the source of a system’s flushing rate and residence
time (Elliott and Whitfield 2011). These inputs impact the tidal prism of a system and
cause vertical and horizontal gradients within the system (Elliott and Whitfield 2011).
Salinity is the primary factor in ecosystem functions of estuarine lagoons as it
influences the presence and distribution of floral and faunal communities within a
system and impacts the ability to retain nutrients (Elliott and Whitfield 2011). Ecological
conditions, specifically salinity and temperature, are important in the geomorphological
evolution of coastal lagoons as they control the extent to which vegetation can colonize
shorelines, impeding erosion, promoting patterns of sedimentation and generating
organic deposits (Bird 1995). These conditions are then superimposed to the sediments
and geology of system. The dynamic nature of estuarine lagoon systems provides a
habitat only inhabitable to organisms that can tolerant a range of changes in
hydromorphologic variables, namely temperature and salinity (Elliott and Whitfield
2011). Consequently, the organisms of estuarine lagoons and other coastal waterbodies
are capable of absorbing natural and anthropogenic pressures more effectively than
other aquatic ecosystems (Elliott and Whitfield 2011).
Physicochemical Variables
Physicochemical variations occurring from interactions of freshwater and marine
waters have a strong correlation to an estuarine lagoon’s morphometry and subsequent
28
hydromorphologic conditions (Kennish 2015). These physical influences on the
estuarine lagoon generally control the connection state of the system and subsequently
have a large impact to the physicochemical conditions of the waterbody as an influx of
seawater can alter salinity, temperature, nutrients, primary productivity, species
presence and habitat availability (Perissinotto et al., 2010; Lill et al., 2011).
The physicochemical variables have been determined as a defining characteristic
of estuarine lagoons from other coastal waterbodies (Tagliapietra et al., 2009).
Physicochemical variables such as salinity, temperatures, dissolved oxygen, pH,
nutrients and organic matter are strongly influenced by inputs from both marine and
terrestrial environments (Tagliapietra et al., 2009). There are a number of functional
differences among the coastal dune lakes of Northwest Florida. Some coastal dune
lakes may be more true to traditional coastal lakes while others to more similar to
estuaries. However, limited literature on these systems exists demonstrating the many
information gaps that exist in our incomplete understanding of them, especially for the
coastal dune lakes of Northwest Florida.
29
Paleolimnology
Past environmental and hydrologic data can be reconstructed and interpreted
through the use of paleolimnology techniques applied to organic matter proxies (Birks
and Birks 2006; Gu et al., 1996). Aquatic environments, especially coastal systems, are
in a constant state of change due to internal and external influences, so long-term
observational data are critical to the best management practices of a waterbody.
However, many current observed data records are limited, intermittent or aren’t
available in an “ecologically relevant time frame” (Birks and Birks 2006; Smol 2008).
Water residence time and geomorphology (lake shape and bathymetry) can assist in
interpreting the physical historical changes of a waterbody (Cohen 2003). However,
sediments provide the most robust archives of historical changes since they persist
longer than the other archive sources (Cohen 2003). The composition of a waterbody’s
sediments can reveal many aspects about a waterbody and conditions of its watershed,
as well as insight to the environmental conditions that existed at the time of
accumulation (Routh et al., 2004). Paleolimnology is a multidisciplinary science that is
used to reconstruct past environmental conditions of inland waters through sediment
characteristics, known as proxies, and this application has been successful for fresh
and marine waters (Cooper 2001; Smol 2008; Tibby and Taffs 2011; Brenner et al.,
1993; Gu et al., 1996). Sediment proxies provide high-resolution chronologies of
environmental conditions within a waterbody, as well as its surrounding catchment.
They reveal patterns and changes that characterize variations in climate (temperature
and precipitation), hydrologic variability or anthropogenic influences (Birks and Birks
2006). Sediment proxies depict water conditions and catchment characteristics,
30
specifically changes in vegetation and land use (Birks and Birks 2006). Aquatic primary
productivity heavily influences or may control organic matter that accumulates in a
waterbody and offers implications to the historical conditions of biological productivity, or
trophic state conditions (Brenner et al., 1999; Gu et al., 1996). Environmental conditions
leave numerous geochemical signals in sediment records that can be used to interpret
paleoenvironmental changes (Routh et al., 2004), and these signals can be measured
through a variety of proxies, both biological and physical. Stable isotope signatures
(δ13C and δ15N) and total carbon and total nitrogen (C/N) ratios are some of the most
common and effective geochemical sediment proxies. Most methodologies suggest
using a variety of proxies to interpret environmental changes due to complex ecological
interactions that cannot be deduced by single proxy analyses (Brenner et al., 1999;
Birks and Birks 2006; Cohen 2003).
Preserved proxies found in aquatic sediments provide significant information on
the organic matter inputs, waterbody productivity dynamics and watershed land use
conditions (Das et al., 2008) as these aquatic systems are influenced by and exposed to
many external and internal forces (Cohen 2003). These stimuli range from macro
(climate, geology, vegetation) to micro (element cycles, biota) interactions, as well as,
anthropogenic impacts (Cohen 2003). The organic matter found in sediments
represents a small, but important fragment of sediments (Meyers and Teranes 2001).
Determining the origin and quantities of organic matter in sedimentary records can
provide critical information when reconstructing past conditions of a waterbody (Meyers
and Teranes 2001). Environmental changes in a watershed and the water affect how
much and what kind of organic matter is delivered to the aquatic system and initiate
31
series of geochemical processes that can leave a decipherable record in the sediments
(Routh et al., 2004; Das et al., 2008). Plants (aquatic and terrestrial) are the primary
source of organic matter to a waterbody (Meyers and Teranes 2001). Plants can be
divided into two distinct biochemical categories: vascular and non-vascular (Meyers and
Teranes 2001). Vascular plants (i.e. macrophytes, grasses, trees) contain carbon rich
cellulose and lignin, while non-vascular plants contain little to no carbon rich fractions
(i.e. phytoplankton) (Meyers and Teranes 2001). These biochemical and structural
differences leave behind different geochemical fossils in the sediment organic matter
which can be used to differentiate the origins of organic matter (Meyers and Teranes
2001).
Organic matter plays an important role in sediment processes and sources due
to its interactions with biota, nutrient cycles and geochemical processes as sources of
organic matter are strongly environment-dependent (Lamb et al., 2006; Routh et al.,
2004). Sediment organic matter reflects not only its sources but also the forming factors
and influences of processes that have altered and degraded the original material
(Meyers and Teranes 2001). Coastal and freshwater sediments receive organic matter
from both autochthonous, phytoplankton and other photosynthetic organism produced
organic matter, and allochthonous, primarily vascular plants and soil organic matter
transported from watershed, sources (Toming et al., 2013). Autochthonous sourced
organic matter is predominantly made up of nonhumic substances which are labile and
easily utilized or degraded by microorganisms (Toming et al., 2013). Allochthonous
derived organic matter consists of mostly humic substances that are resistant to
decomposition and have a brownish color (Toming et al., 2013). Aquatic systems that
32
do not endure regular flushing through fresh or marine water processes, such as
lagoons or isolation basins, are dominated by autochthonous sources of carbon and
control the supply of organic matter to the sediments (Lamb et al., 2006). For regularly
flushed systems, a balance is typically experienced between autochthonous and
allochthonous organic matter sources (Lamb et al., 2006). Changes in the organic
matter sources can influence primary productivity (autochthonous) and affect
sedimentation rates (Routh et al., 2004). Terrestrial vegetation contains more refractory
components, such as lignin, hemicellulose, cellulose, and less labile fractions, i.e.
protein, than aquatic plants (phytoplankton and photosynthetic micro-organisms) (Khan
et al., 2015; Meyers and Teranes 2001). Refractory structural components are nitrogen
poor and provide C3 terrestrial plants with high C/N (weight percent) ratios of greater
than 18 (Lamb et 2006; Khan et al., 2015; Meyers and Teranes 2001), but C4 vegetation
have C/N ratios greater than 30 (Lamb et al., 2006). Aquatic plants contain higher levels
of nitrogen and less lignin and cellulose portions, generally displaying lower C/N ratios
of 4 to 6 and less than 10, phytoplankton have a C/N range of 6 to 8 (Lamb et al., 2006;
Khan et al., 2015; Meyers and Teranes 2001). Elevated C/N ratios (> 20) indicate
terrestrial vegetation as the dominant organic matter source while lower C/N values (4
to 6, > 10) indicate dominant aquatic organic matter sources (Meyers and Teranes
2001; Lamb et al., 2006; Khan et al., 2015). C/N ratios of 13 to 14 suggest equal
contributions of aquatic and terrestrial plant organic matter, which is projected for most
lake systems (Meyers and Teranes 2001). Organic matter, from either aquatic or
terrestrial sources, undergoes diagenesis as it sinks through the water column and is
deposited into the sediments and may continue into sub-bottom depths (Meyers 1994).
33
However, various articles report that the organic matter source signatures are
accurately preserved and undergo little further change (Meyers 1994; Routh et al.,
2004). These general observations of organic matter C/N ratios can be utilized to
determine the terrestrial and aquatic organic matter source contributions to a waterbody
system.
Carbon isotopes are widely used as indicators of organic matter sources (Meyers
and Teranes 1999; Routh et al., 2004). Both algae (phytoplankton and photosynthetic
microorganisms) and vascular plants create organic matter with a carbon signature and
can be distinguished by their carbon compositions (δ13C) and carbon to nitrogen (C/N)
ratios (Meyers and Teranes 2001). Organic matter origins become easily visualized
when stable isotope δ13C measurements are plotted against C/N ratios (Routh et al.,
2004). Terrestrial plants can be categorized into C3, C4, and crassulacean acid
metabolisms (CAM) photosynthesizing groups based on the carbon fixation pathways
used during photosynthesis (Hobbie and Werner 2003). These pathways are reflected
in the δ13C isotopic concentrations and C/N ratios in plant tissues and translated into the
sediment organic matter they produce (Hobbie and Werner 2003; Khan et al., 2015;
Lamb et al., 2006). C3 pathway plants are the most common type of terrestrial
vegetation and dominate in temperate forests, freshwater aquatic and wetland
environments (Khan et al., 2015). During photosynthesis, C3 plants favor δ12C isotopes
and discriminate against heavier 13C isotopes, resulting in more negative δ13C
concentrations in organic matter, approximately -32‰ to -21‰ (Khan et al., 2015; Lamb
et al., 2006). C4 plants commonly discriminate less against heavier δ13C isotopes and
display low δ13C concentrations in organic matter, with a range of -17‰ to -9‰ (Khan et
34
al., 2015; Lamb et al., 2006). CAM plants have a wide range of δ13C concentrations, -
28‰ to -10‰, but tend to have similar ranges to that of C4 plants (Khan et al., 2015). C4
and CAM plants are common in water-stressed environments, usually occurring in arid
locations (Khan et al., 2015). These types of plants can also easily adapt to saline and
intertidal environments (Khan et al., 2015; Lamb et al., 2006). Algae, phytoplankton and
other aquatic photosynthetic microorganisms, utilize a C3 pathway distinguishable from
terrestrial C3 plants (Lamb et al., 2006; Meyers and Teranes 2001; Khan et al., 2015). In
aquatic environments algae display a range of δ13C concentrations, -30‰ to -18‰
(Khan et al., 2015; Lamb et al., 2006). Variations in δ13C occur between freshwater and
marine algae, generally freshwater algae have a δ13C range of -30‰ to -26‰ and
marine algae have a range of -23‰ to -16‰ (Khan et al., 2015; Lamb et al., 2006).
In addition to inferences of organic matter sources, stable isotope δ13C
measurements provide paleoproductivity and changes in nutrient sources to a
waterbody (Lamb et al., 2006; Meyers and Teranes 2001; Brenner et al., 1999). The
concentrations of sediment δ13C values are significantly determined by the rate of
carbon uptake during algal productivity and the composition of dissolved inorganic
carbon (DIC) (Brenner et al., 1999; Meyers and Teranes 2001). Aquatic plant δ13C is
controlled by whether the plant utilizes bicarbonate or dissolved carbon dioxide (Lamb
et al., 2006). Dissolved carbon dioxide has lower δ13C values than bicarbonate and
aquatic algae will preferentially take up carbon sources with lower concentrations until
the source is exhausted (Lamb et al., 2006). Algae preferentially uptake δ12C, a lighter
carbon isotope, causing the organic matter they produce to have higher δ12C
concentrations than the δ13C / δ12C ratio of DIC of the water column (Meyers and
35
Teranes 2001; Routh et al., 2004). Algal produced organic matter removes δ12C from
the water column DIC pool and during periods of increased productivity, the δ12C DIC
pools are depleted and the remaining δ13C values increase in the water column (Routh
et al., 2004; Brenner et al., 1999; Meyers and Teranes 2001). Due to limited labile forms
of δ12C, algae in these conditions consume the remaining δ13C isotopes, causing
elevated δ13C concentrations in sediment organic matter (Meyers and Teranes 2001).
When a waterbody moves towards a hypereutrophic state, its sediments may
experience longer periods of anoxia and increased methanogenic processes that
produce heavier δ13C carbon dioxide to the water column (Brenner et al., 1999). Shifts
in paleoproductivity can be reflected in the variations of δ13C changes, i.e. increases in
sediment organic matter δ13C can indicate periods of increased productivity due to
increases in nutrients (Meyers and Teranes 2001; Brenner et al., 1999; Gu et al., 1996).
This association of δ13C concentrations in sediment organic matter and
paleoproductivity is not always applicable. Variations in pH, temperature, nutrient
sources, productivity rates and the Suess effect can have an influence on the dominant
carbon isotope created in organic matter (Meyers and Teranes 2001; Routh et al.,
2004).
Stable isotope δ15N is another sediment proxy used to identify organic matter
sources and paleo-productivity, however, alone can be more challenging to interpret
than carbon isotopes (Routh et al., 2004). The nitrogen cycle is dynamic and nitrogen
isotope measurements may be more convoluted in their sources and influences (Talbot
2001; Meyers and Teranes 2001). In addition, other factors can influence nitrogen
concentrations in the natural environment, i.e. agricultural and industrial practices, as
36
well as anthropogenic and natural atmospheric nitrogen fallout (Talbot 2001). These
external influences can affect internal aquatic primary productivity and subsequent
nitrogen isotope composition of sediment organic matter (Talbot 2001).
Dissolved inorganic nitrogen (DIN) in the forms of ammonium, nitrate and nitrite
are important aquatic nitrogen pools used during primary production (Meyers and
Teranes 2001; Talbot 2001). The primary producers that utilize these labile forms of
nitrogen typically dominate aquatic systems. However, nitrogen fixing bacteria, most
commonly cyanobacteria, utilize atmospheric nitrogen (N2) when the latter forms of
nitrogen are exhausted (Talbot 2001). These two forms of primary producer’s utilization
of nitrogen and the nitrogen isotope signatures that subsequently remain can be used to
roughly distinguish between organic matter sources within an aquatic system (Talbot
2001; Meyers and Teranes 2001). Like carbon stable isotopes, nitrogen organic matter
sources can be inferred through the differences between δ15N and δ14N and the
availability of inorganic nitrogen pools to plants both in aquatic and terrestrial
environments (Meyers and Teranes 2001; Routh et al., 2004). Algae utilize the more
labile forms of DIN (ammonium, nitrite and nitrate) favoring the δ14N over the heavier
δ15N isotope and produce organic matter with a lower δ15N concentration (Meyers and
Teranes 2001). As DIN nitrogen pools are spent, δ15N isotopes gradually increase and
when they are utilized for primary production it is reflected in increased δ15N
concentrations of sediment organic matter. In general, studies show an average range
of δ15N concentrations in organic matter of approximately -5‰ to 20‰ (Talbot 2001;
Meyers and Teranes 2001). Typically, DIN display larger values of δ15N than
atmospheric nitrogen (N2) and are reflected in the aquatic sediment organic matter
37
(Meyers and Ishiwatari 1993; Meyers and Teranes 2001; Routh et al., 2004).
Autochthonous organic matter produced in an aquatic environment tends to have higher
δ15N signatures (Gu et al., 1996). Algae and aquatic C3 plants utilize DIN to produce an
approximate range of 7‰ and 10‰ of δ15N concentration in sediment organic matter,
whereas, allochthonous organic matter resulting from C3 terrestrial plants typically have
a δ15N concentration of roughly 0.4‰ (Meyers and Teranes 2001; Routh et al., 2004;
Gu et al., 1996). Terrestrial C3 plants and nitrogen fixing bacteria utilize atmospheric
nitrogen (N2), no nitrogen fractionation occurs, resulting in δ15N-depleted organic matter
with values near 0.4‰, but N-fixing bacteria have a range of -1‰ to 3‰ (Meyers and
Teranes 2001; Routh et al., 2004; Gu et al., 1996). A parallel exists in the variations of
δ15N and DIN availability and primary producer utilization of nitrogen pools (Talbot
2001). However, other external influences such as land use changes, changes in
organic matter sources and anthropogenic practices can affect the nitrogen isotopes
signatures in organic matter (Meyers and Teranes 2001; Routh et al., 2004; Talbot
2001).
The measurements of sediment δ15N can also provide an indication of the past
biological productivity, or trophic state, of a waterbody. In general, δ15N and δ13C
concentrations increase in autochthonous produced organic matter as the trophic state
increases from oligotrophic to eutrophic (Torres et al., 2012). Brenner et al., (1999)
found that δ15N concentrations decreased in hypereutrophic (extreme biological
productivity) Florida lakes and attributed this to nitrogen fixing bacteria. Yet, not all
eutrophic and hypereutrophic lakes are dominated by nitrogen-fixing microorganisms
(Gu et al., 1996). Therefore, low δ15N concentrations are not always attributed to
38
nitrogen-fixing bacteria but can be associated with low or high levels of aquatic
biological productivity (Gu et al., 1996). The conclusions of Gu et al., (1996)
demonstrates that primary productivity controls the δ15N concentrations through algal
consumption of labile nitrogen (nitrate, ammonium, nitrite) when there are sufficient
supplies of nitrogen to a waterbody and/or nitrogen-fixing bacteria in further eutrophic
lakes that are nitrogen limited. When stable isotope δ15N measurements are reviewed in
conjunction with other proxies, such as δ13C and C/N ratios, it can provide a more
comprehensive interpretation of past environmental conditions (Talbot 2001). Larger
δ15N values in sediment organic matter are displayed when there are more negative
δ13C values, indicating a decrease in productivity resulting from limited nitrate
availability (Routh et al., 2004). Reduced values of δ15N in sediment organic matter can
result from increased availability of DIN, usually stimulating primary productivity in lakes
(Routh et al., 2004).
Additional factors can play a role in the nitrogen isotopic signature of sediment
organic matter. The limiting nutrient of a system controls the level of productivity in a
waterbody and in lakes, phosphorus is often the limiting nutrient (Meyers and Teranes
2001). When phosphorus levels are depleted, only a small fragment of nitrogen is used
resulting in an insignificant alteration to the isotopic signature of DIN (Meyers and
Teranes 2001). Therefore, measurements of the nitrogen isotope signatures would
show no change and could adversely impact paleolimnological interpretations.
Fluctuations of algal species can affect the nitrogen cycle within the water column and
cause incorrect interpretations of nitrogen isotope concentrations and paleoproductivity
(Meyers and Teranes 2001; Routh et al., 2004). The use of stables isotope signatures
39
from bulk sediment organic matter to study past environmental conditions is based on
the assumptions that sediment organic matter originates from primary productivity in the
water column and that the isotopic ratios reflect the organic matter produced in the
water column (Gu et al., 2011). Therefore, stable isotopic composition of sediments can
be used as a function of nutrient driven productivity or trophic state of a waterbody (Gu
et al., 2011; Meyers and Teranes 2001; Lamb et al., 2006). Changes in the amounts
and kinds of biota (within lake and watershed) reflect changes in climate and other
environmental factors and can be inferred through stable isotope analyses (Meyers and
Teranes 2001).
Objectives
Kendall Tau statistical analyses of contemporary water quality data indicated that
two coastal dune lakes, Eastern and Big Redfish Lake, are shifting to more biologically
productive states. In order to examine these changes beyond contemporary data
collection, the historical environment of both lakes was reconstructed using
paleolimnological analyses of sediment organic matter collected from surface-sediment
interface cores. Geochemical analyses of 210Lead, total phosphorus (TP), total carbon
(TC), total nitrogen (TN), stable isotopes δ13C and δ15N were used to reconstruct and
study hydrologic variability within these two lakes.
The core objectives of this project were to i) determine historic lake nutrient
conditions through sediment accumulation of TP and TN, ii) assess sources of organic
matter by analyzing C/N ratios and stable isotope δ13C and δ15N values, iii) infer lake
paleoproductivity by assessing stable isotope δ13C and δ15N values and correlations to
40
nutrient changes and iv) infer periods of marine or freshwater dominant conditions
through analysis of C/N ratios and concurrent stable isotope δ13C and δ15N values.
41
Figure 1-1. Coastal Dune Lakes of Walton County (Walton Outdoors 2018).
42
Table 1-1. The 15 Recognized Coastal Dune Lakes of Walton County morphometric summary statistics (Hoyer and Canfield 2008).
Lake Watershed Area (km2)
Lake Surface Area (km2)
Mean Depth (meters)
Allen 0.71 0.07 1.1
Alligator 0.38 0.06 1.5
Big Redfish 1.19 0.09 1.6
Camp Creek 2.13 0.23 1.6
Campbell 0.08 0.44 3.5
Deer 1.38 0.17 2.8
Draper 1.93 0.11 1.4
Eastern 1.54 0.25 2.1
Eastern North* . . 1.5
Fuller 0.43 0.2 1.7
Grayton* . . .
Little Redfish . 0.05 1.8
Morris 0.87 0.32 3.1
Oyster 0.56 0.09 1.7
Powell 7.3 1.04 2
Stallworth 0.86 0.05 1.5
Western 2.75 0.69 5.2
Western Northeast* . . 1.7
Lakes listed with an asterisk symbol represent lakes portions that have data measurements collected independently of main lake body for water quality monitoring purposes. Eastern North is the northern portion of Eastern Lake and is delineated by Scenic Highway 30A; Grayton is the west lobe of Western Lake and Western Northeast is the upper northeast portion of Western Lake.
43
CHAPTER 2 MATERIALS AND METHODS
Site Description
Walton County coastal dune lakes are located along the coastal mainland of
Florida. This region of the Gulf Coast is unusual from surrounding coastlines in
Northwest Florida because it lacks protective offshore barrier islands (Gross 2015). This
portion of the Gulf of Mexico has one of the smallest tidal ranges, approximately 0.1 to
0.3 meters (U.S. Department of Interior 2001). The coastal dune lakes of Northwest
Florida reside in the Gulf Coast Lowlands Lake Region which characterizes waterbody
hydrology as acidic, soft water that contains high concentrations of dissolved organic
carbons, elevated levels of sulfate, sodium, and chloride ions and low nutrients (CBA
2017; Hoyer and Canfield 2008, Griffith et al., 1997). Coastal dune lake substrate is
primarily composed of sand with organic sediment deposits (FNAI 2010; Hoyer and
Canfield 2008). Previous research has identified some coastal dune lakes exhibiting a
microtidal influence during outlet connections (Coor 2013; Bhadha and Jawitz 2008).
Eastern Lake Eastern Lake (Figure 2-1) is one of the 15 recognized coastal dune lakes in Walton
County, Florida (N 30.312048, -86.093230) and has an irregular surface area of 25.4
hectares and a catchment area of 154 hectares (Hoyer and Canfield 2008). Eastern
lake is considered a shallow lake with an average depth of 2.1 meters (Hoyer and
Canfield 2008). Intermittently, the lake water naturally breaches through the beach sand
berm within its relic sweep area where the dunes are not as mature as surrounding
dunes and connects to the Gulf of Mexico through its outlet. The sweep area is the
historical measurement through aerial and survey data of the length of shoreline
44
identified as being influenced by all previous locations of the outlet channel (Browder
and Dean 1996). During outlet connections to seawater, the lake can potentially be
micro-tidally influenced (Bhadha and Jawitz 2008). Eastern Lake naturally breaches the
sand berm separating it from the Gulf of Mexico during high water levels or strong storm
events and can be manually opened by Walton County officials when it reaches a
designated flood threshold to protect residences and infrastructure. Eastern Lake is
intersected by Florida Scenic Highway 30-A subdividing the lake into two sections and
connecting them through a narrow open-water bridge.
Residential and commercial land use dominate Eastern Lake’s shoreline and
southern watershed area. Northern portions of the watershed consist of freshwater
forested/shrub wetlands and is preserved in the Point Washington State Forest,
consisting of Sandhill, basin swamps, Titi drains, wet flatwoods, wet prairies and
cypress swamps (Greene 2011). The lake’s intermittent connection with saltwater forms
an estuary-like environment in the lake, where fresh and marine flora and fauna coexist.
Emergent salt- and freshwater marsh plants exist interspersed along the lake shoreline,
and relic sand dunes and dune vegetation occur along the southern portion of the lake
near the outlet.
Big Redfish Lake
Big Redfish Lake (Figure 2-2) is one of 15 recognized coastal dune lakes in
Walton County, Florida (N 30.338853, W -86.195428) and has a surface area of 9.22
hectares and a watershed of 119 hectares (Hoyer and Canfield 2008), with an average
lake depth of 1.58 meters (Hoyer and Canfield 2008). Big Redfish Lake has a natural,
intermittent connection to the Gulf of Mexico through a restricted outlet. The lake is
45
intersected by Florida Scenic Highway 30-A at its northern section and is connected to
the upper portion through culverts under the highway.
Its southern watershed region consists of residential development, while the
northern region is made up of freshwater forested/shrub wetlands, basin swamps, wet
flatwoods, wet prairies and cypress swamps in Point Washington State Forest and
Florida State Park (Greene 2011). Fringing and emergent salt- and freshwater
vegetation, such as S. alterniflora and Phragmites australis, occur along the lake’s
shoreline (Hoyer and Canfield 2008). Barrier sand dunes and dune vegetation occur
along the southern portion of the lake watershed.
Surface Water Chemistry Collection
Big Redfish Lake and Eastern Lake have three water quality monitoring stations
on each lake, sampled monthly by CBA citizen scientists. At each station, two water
samples were collected, and Secchi disk depth measurements and datasonde
physicochemical variables recorded. Open water surface samples were collected at 0.5
m with a 250-mL, acid-cleaned, triple-rinsed Nalgene bottle. The large, 1000-mL
Nalgene bottle is filtered within 48 hours post field collection through a Gelman 47mm
Type A-E glass fiber filter (CBA 2017). The filter is preserved in a filter label and frozen
until collected by Florida LAKEWATCH coordinators quarterly (CBA 2017). The 250-mL
Nalgene bottle is frozen on site and collected quarterly by the Florida LAKEWATCH
personnel. Florida LAKEWATCH laboratory in Gainesville, Florida analyzed water
samples and filters for total phosphorus (TP) (µg/L) and total nitrogen (TN) (µg/L), True
Color (Platinum-Cobalt Units), conductivity (µS/cm) and total chlorophyll (Canfield et al.,
2002; CBA 2017). All water samples and filters were analyzed within three months and
data results provided to CBA and state agencies. Secchi disk depth (water clarity) and
46
water column depth are collected at each station and data is submitted to
LAKEWATCH. Datasonde equipment collected surface and bottom chemical variables
at each station with variables including temperature, dissolved oxygen, pH, salinity, and
turbidity.
Statistical Analyses
Data was compiled into an annual water chemistry report by CBA and the Mattie
Kelly Environmental Institute. Using the Kendall-Tau trend analysis on monthly water
quality data collection (CBA 2017). All surface water quality variables have a grand
mean calculated in JMP software, with means processed in R software using Kendall
Tau trend analysis. The results from these analyses show annual and intra-annual
variance of each coastal dune lake (CBA 2017). In addition, geochemical variables were
assessed using Kendall-Tau trend analysis and processed in R software to determine if
significant trend occurred within the data set.
Collection and Preparation of Sediment Samples
Surface sediment-water interface cores collected from two coastal dune lakes in
Walton County, Florida were dated using 210Pb,137Cs, and 226Ra isotope signatures and
analyzed for total phosphorus (TP), total carbon (TC) and total nitrogen (TN)
concentrations, stable isotopes (δ13C and δ15N). The site locations were chosen to
represent depositional environments in the deep, centrally located sites using
LAKEWATCH bathymetric maps as reference (Hoyer and Canfield 2008).
Sediment-water interface cores were recovered with a large-volume (~1 meter)
manual piston corer to avoid disturbing lake sediment. Polycarbonate tubes with a 3-
inch inner diameter were used to collect the cores which were sealed on both ends and
held in a vertical position to maintain sediment integrity and avoid mixing of sediments.
47
A 75-centimeter surface sediment-water interface core (N 30.310550, W -86.092667)
was collected from Eastern Lake on May 4, 2017. A 95-centimeter surface sediment-
water interface core (N 30.337585, W -86.191956) was recovered from Big Redfish
Lake. Sediment core locations were stored with a Garmin etrex 10 geographic
positioning system unit. The sediment cores were extruded vertically and subsampled at
2-cm intervals using clean, plastic spatulas. A total of 38 sediment samples were
obtained from the Eastern Lake core and 46 sediment samples were obtained from Big
Redfish Lake. Core subsections were labeled and kept in individual one-gallon ziplock
sealed bags in a refrigerated cooler at 2 to 7 degrees Celsius. Preparation of
subsamples were conducted at the Mattie Kelly Environmental Institute of Northwest
Florida State College laboratory. Subsamples were weighed using a Unibloc top-loading
balance and dried in a gravity convection oven. Dried samples were sent to University
of Florida laboratories for further analysis.
Geochronology
Subsamples of Eastern and Big Redfish Lake sediment cores were dated using
210Pb, 137Cs, and 226Ra isotope signatures. These isotopes are common variables used
to date sediments of the late Holocene epoch, approximately 100 to 200 years before
present (BP), and provide an estimation of a waterbody’s sedimentation rate. Lead-210
is a naturally occurring radioactive element that forms as part of the radioactive decay of
Uranium-238 (Jeter 2000; de Souza et al., 2012). Uranium occurs at an “unchanging
concentration” over time and is present in all soils and sediments (Jeter 2000; de Souza
et al., 2012). Eventually, it decays into radium-226 which exhibits the same static
concentration (Jeter 2000; de Souza et al., 2012). Then, 226Ra is transformed into 210Pb
which maintain secular equilibrium (production rate is equal to decay rate). Generally,
48
210Pb is formed from the radioactive decay of Radon-222 (Jeter 2000; de Souza et al.,
2012). It can either be deposited through natural atmospheric fallout (“unsupported
210Pb”) or produced by radioactive decay of 226Ra into 222Rn (“supported 210Pb”) in
sediments (de Souza et al., 2012). 210Pb dating is calculated by the excess of
“unsupported 210Pb” activities, subtract the “supported 210Pb” from 210Pb activities in
every sediment subsample (de Souza et al., 2012; O’Rielly et al., 2011). The Constant
Rate of Supply model (CRS) assumed the excess “unsupported 210Pb” is supplied at a
constant rate over time and was the method used when determining the age of the
sediments (de Souza et al., 2012). The CRS model calculates the sedimentation rates
of materials deposited over time through the sediment core (de Souza et al., 2012).
Subsamples from entire sediment core were prepared in pre-weighed ceramic
crucibles and a wet weight was recorded using an electronic top-loading balance.
Sediments were dried at 105° Celsius in a convection oven for 48 hours and dried
subsamples were ground with a mortar and pestle and re-weighed to determine dry bulk
density weight.
Sediment dry bulk density data and dried samples were shipped to the University
of Florida Institute of Paleoenvironmental Research (FLIPER) laboratory for analysis. At
FLIPER laboratory, the radiometric measurements (210Pb, 226Ra and 137Cs) were made
using low-background gamma counting with well-type intrinsic germanium detectors
(Schelske et al., 1994). Sediment ages were calculated using the constant rate of
supply (CRS) model (Appleby and Oldfield 1983; Oldfield and Appleby 1984) with age
errors propagated using first-order approximations and calculated according to Binford
(1990).
49
Total Phosphorus
A total of 85 subsamples were analyzed for total phosphorus (TP) at the
University of Florida’s Institute of Fisheries and Aquatic Sciences Analytical Research
Laboratory (ARL). Subsamples from entire sediment cores were prepared for ARL
analysis at the Mattie Kelly Environmental Institute of Northwest Florida State College
Laboratory. Subsamples were dried in crucibles dried at 105° Celsius in a convection
oven in ceramic crucibles for 48 hours, followed by grinding with a mortar and pestle,
passage through a 2mm mesh sieve, and weighing to obtain dry weight. Samples were
then placed in sterile plastic Whirl-Paks and shipped to ARL for analysis.
The ARL follows the semi-automated colorimetry protocol of the United States
Environmental Protection Agency (EPA) Method 365.1, Revision 2.0: Determination of
Phosphorus by Semi-Automated Colorimetry (1992). Subsamples were placed into 250
mL high density polyethylene (HDPE) containers and preserved with sulfuric acid, then
gently boiled at approximately 100° Celsius to convert all the phosphorous in the
sample to the orthophosphate form. A portion, 50 mL, of the sample was measured into
a digestion vessel and ammonium molybdate or antimony potassium tartrate solution
added to each sample portion and boiled. This allows for the organophosphorus
compounds to digest and formmolybdenum blue, the intensity of which is proportional to
the amount of phosphorous in solution.
Geochemistry
Eastern and Big Redfish Lake surface sediment-water interface cores were
analyzed for bulk organic carbon (%C) and nitrogen (%N) percent contents and stable
isotopes signatures of δ13C and δ15N of sediment organic matter. The sediment core
subsamples were weighed in pre-weighed ceramic crucibles. Then, dried at 105°C in a
50
convection oven in crucibles for 48 hours. Subsamples were then ground with a mortar
and pestle, passed through a 2 mm sieve and weighed again. Dry sediment bulk density
were calculated using these measurements and weighed to 100 to 250 milligrams,
packed in sterile plastic Whirl-Paks, and shipped to the University of Florida’s
Department of Geological Sciences Light Stable Isotope Mass Spectrometry Laboratory
for analysis.
Subsamples were loaded into tin capsules and placed in a 50-position automated
Zero Blank sample carousel on a Carlo Erba NA1500 CNHS elemental analyzer. After
flash combustion in a quartz column containing chromium oxide and silvered
cobaltous/cobaltic oxide at 1020°C in an oxygen-rich atmosphere, the sample gas was
transported in a He carrier stream and passes through a hot reduction column (650°C)
consisting of reduced elemental copper to remove oxygen. The effluent stream then
passes through a chemical (magnesium perchlorate) trap to remove water. The stream
then passes through a 0.7 meter GC column at 125°C that separates the nitrogen gas
(N2) and carbon dioxide (CO2) gases. Finally, the gases pass through a
thermal conductivity detector that measures the size of the pulses of N2 and CO2.
The sample gas next passed into a ConFlo II interface and into the inlet of a Thermo
Electron Delta V Advantage isotope ratio mass spectrometer running in continuous flow
mode where the sample gas was measured relative to laboratory reference N2 and CO2
gases. All carbon isotopic results were expressed in standard delta notation relative
to VPDB, and all nitrogen isotopic results were expressed in standard delta
notation relative to air.
51
Figure 2-1. Eastern Lake and catchment area
This image demonstrates the variety of land use within the Eastern Lake catchment. Development dominates the southern region, while wetlands and upland forests comprise the northern regions of the catchment area.
52
Figure 2-2. Big Redfish Lake and catchment area
Big Redfish Lake and its catchment area is presented in this aerial image demonstrating the variety of land use within it. Coastal land use in the southwest area is made up of predominantly residential development, while the northern portion consists of wetlands and upland forests.
53
CHAPTER 3 RESULTS
Surface Water Chemistry
Nutrients
Eastern Lake
The 2016 Coastal Dune Lake Water Chemistry Summary Report produced by
CBA and MKEI of Northwest Florida State College reported that Eastern Lake has
observed statistically significant positive trends in TP, TN, and total chlorophyll with a
statistically significant negative trend in water transparency over a 19 year period,1997-
2016 (Figure 3-1) (CBA 2017). Long term trophic state variable trend analysis statistics
were calculated on an annual basis using monthly water quality data from three
monitoring stations within Eastern Lake for TP (µg/L), TN (µg/L), total chlorophyll (µg/L),
water transparency (m). The results show the inter-annual (monthly variability within the
year) and intra-annual variance (variability among years) (CBA 2017). Throughout the
19-year monitoring period, TP had a mean concentration of 12 µg/L, a maximum of 17.8
µg/L and a minimum of 9.14 µg/L (CBA 2017). TN concentration during this time frame
had a mean of 286 µg/L, a maximum of 399 µg/L and a minimum of 214 µg/L (CBA
2017). Total chlorophyll had a mean value of 3.55 µg/L, a maximum of 6.71 µg/L and a
minimum of 2.14 µg/L (CBA 2017). Secchi water transparency depth had a mean of
1.45 meters, a maximum of 1.97 meters and a minimum of 0.94 meters (CBA 2017).
The Kendall’s Tau coefficient, a statistical measure of rank between variables, was
applied to determine the long term trophic state trend analysis in TP, TN, total
chlorophyll and water transparency (CBA 2017). Statistical results display significant
54
positive trends in nutrients (TP, TN) and total chlorophyll with a significant negative
trend in water transparency (CBA 2017).
Big Redfish Lake
The 2016 Coastal Dune Lake Water Chemistry Summary Report produced by
CBA and MKEI of Northwest Florida State College reported that Big Redfish Lake has
observed statistically significant positive trends in TP, TN, and total chlorophyll with a
statistically significant negative trend in water transparency. Long term trophic state
variable trend analysis statistics were calculated on an annual basis using monthly
water quality data from three monitoring stations within Big Redfish Lake TP (µg/L), TN
(µg/L), total chlorophyll (µg/L), water transparency (m) over an 18-year period, 1998
through 2016 (CBA 2017). The results show the inter-annual (monthly variability within
the year) and intra-annual variance (variability among years) (CBA 2017). Long term TP
(µg/L) concentrations had a mean of 13 µg/L, with a max of 22.6 µg/L and minimum of
4.67 µg/L based on sampling events throughout the 18-year monitoring period (CBA
2017). Long term summary statistics of TN (µg/L) concentrations measured a mean of
364 µg/L, a max of 657 µg/L and a minimum of 149 µg/L. Results of long term summary
statistics of total chlorophyll (µg/L) concentrations measured a mean of 5.39 µg/L, a
max of 10.5 µg/L and a minimum of 1.67 µg/L (CBA 2017). Long term summary
statistics of water transparency, measured with a Secchi disk in meters, had results of a
mean transparency of 0.98 (m), max transparency of 1.5 meters and a minimum of 0.68
meters (CBA 2017). The Kendall’s Tau coefficient analysis, a statistical measure of rank
between variables, was used to statistically analyze the data of long-term trophic state
trends of TP, TN, algal biomass (total chlorophyll) and water transparency within Big
55
Redfish Lake (CBA 2017). Statistical analysis results (Figure 3-2) over an 18-year
period show positive trends in nutrients (TP, TN) and total chlorophyll with a negative
trend in water transparency (CBA 2017).
Physicochemical Variables
Eastern Lake
Due to the coastal dune lake’s interactions with the Gulf of Mexico and its high
color status, its dissolved oxygen, pH and salinity long term summary statistics will be
described. As reported in the Coastal Dune Lake Water Chemistry Summary Report
(2017), Eastern Lake surface measurements (Table 3-1) of dissolved oxygen display a
range of dissolved oxygen from a minimum of 5.55 to a maximum of 7.79 mg/L with a
mean of 6.77 mg/L (CBA 2017). Eastern lake’s surface pH measurements vary between
a minimum of 7.12 and a maximum of 7.97 pH, with a mean of 7.53 pH (CBA 2017).
The surface salinity is measured in parts per thousand (ppt) have a minimum of 2.01
and a maximum of 18.5 ppt, with a mean of 8.23 ppt (CBA 2017). Eastern lake’s true
color (Platinum-Cobalt units) measurements range from a minimum of 45 to a maximum
of 173 Pt-Co units and mean of 96.9 Pt-CO Units (CBA 2017). Bottom measurements of
all variables except for TN, TP and total chlorophyll (~0.5 meters above bottom) are also
collected at each water quality monitoring station, however, only surface water
measurements are analyzed for the report Eastern Lake Long Term Summary Statistics
(CBA 2017)
Big Redfish Lake
Big Redfish lake’s long-term summary statistics (Table 3-2) of dissolved oxygen,
pH and salinity will be described because of its relativity to this research. As reported in
the Coastal Dune Lake Water Chemistry Summary Report (2017), Big Redfish Lake’s
56
surface dissolved oxygen has minimum of 4.49 mg/L and maximum of 7.36 mg/L, with a
mean of 5.98 mg/L (CBA 2017). The surface water pH ranges from a minimum of 6.6 to
a maximum of 7.72 pH, with a mean of 7.2 pH (CBA 2017). Surface salinity (ppt)
measurements range from minimum of 0.66 to 16 ppt, with a mean of 5.14 ppt (CBA
2017). The true color (Platinum-Cobalt Units) range from a minimum of 43.3 to a
maximum of 326 Pt-Co Units, with a mean of 150 Pt-Co Units (CBA 2017). Bottom
measurements are recorded during field collections but are not analyzed or reported in
the annual Coastal Dune Lake Water Chemistry Summary Report.
Geochronology and Sedimentation Rates
Geochronology results provided reliable dating for sediment core samples from
both Eastern and Big Redfish Lake sediment-water interface cores. A combination of
210Pb, 137Cs and 226Ra isotopes were used to measure the isotopic activity levels and
provide high-resolution age-depth relationships (Kenney et al., 2016). These are
commonly applied paleolimnological methods which offer an approximate chronology to
the sediment core samples. The age of the sediment core subsamples were calculated
using the constant rate of supply (CRS) model (Appleby and Oldfield 1983; Oldfield and
Appleby 1984). The approximate measurements of sedimentation accumulation rates
(mg/cm2/yr) for the sediment core profiles were also determined through the CRS
model.
Eastern Lake
A successful total of 14 samples were measured for 210Pb, and 137Cs on Eastern
Lake (Figure 3-3). These results provided a sediment record of 151 years at a down
core depth of 28 cm. The date at 28 cm was approximately 1866 with a surface date of
close to 2017. Total 210Pb decreased down core until it reached supported levels at 28
57
centimeters. 137Cs activity remained near-constant, with an area of slight activity at
approximately 14 cm. 226Ra displayed relatively constant activity with a slight increase at
roughly 18 cm. Eastern Lake sedimentation accumulation rates (Figure 3-4) display a
decreasing down core trend with one significant peak and a less significant peak within
core. The largest measurement of accretion was in the uppermost 2 cm, at 161
(mg/cm2/yr). Decreasing down core to 18.5 (mg/cm2/yr) at 2 cm until 8 cm where rates
increase to a small peak of 20 (mg/cm2/yr). A gradual decreasing trend continues down
core until 16 cm where a significant accumulation rate increases from 14 cm depth of
15.1 to 31.4 (mg/cm2/yr). A down core decrease continued to 24 cm where the lowest
accumulation rate occurrs at 9.5 (mg/cm2/yr). Sedimentation rates peaked again at 26
cm at 14.1 (mg/cm2/yr) then decreased to the last measurement at 28 cm to 13.0
(mg/cm2/yr). The average accumulation rate of the sediment core from Eastern Lake
was 27.15 (mg/cm2/yr).
Big Redfish Lake
A total of 11 samples were measured for 210Pb, and 137Cs from Big Redfish Lake.
The successful measurement of these isotopes provided a record of 122 years at a core
depth of 22 centimeters, 1889 through 2011 (Figure 3-5). 210Pb was constant for the
upper 6 cm, peaked at 8 cm and then decreased with depth. At 24 cm, it increased and
then decreased until 26 cm. 137Cs activity remained constant with little measured activity
near 8 cm. 226Ra activity was constant for the upper 20 cm, then decreased until 22 cm
where a small peak occurred at 26 cm. Big Redfish lake sedimentation accumulation
rates (Figure 3-6) decrease down-core with two peaks. The largest accumulation rate
occurred at the surface 2 cm depth with 22.1 (mg/cm2/yr), then decreased to 16.6
(mg/cm2/yr) at 4 cm depth, and continuing to decrease to 7.2 (mg/cm2/yr) at 10 cm. A
58
peak in sedimentation occurred at 12 cm and increasing to 8.0 (mg/cm2/yr). A decrease
in accumulation continued for the next six centimeters (depth 14, 16, 18 cm) with no
change in rates between 16 and 18 cm, 4.5 (mg/cm2/yr). An increase to a peak of 5.4
(mg/cm2/yr) at 20 cm occurred and then a decrease occurred at the last depth
measurement of 22 cm to 4.0 (mg/cm2/yr). The average accumulation rate of the core
from Big Redfish Lake was 9.41 mg/cm/yr2.
Total Phosphorus
Eastern Lake
The Eastern Lake surface sediment-water interface core TP (Figure 3-7)
concentrations had a mean of 0.37 mg g-1 within the 75-cm sample depth. There was an
increasing trend in TP from the base of the core to the surface, modern day sediments.
A significant positive trend occurs between 32 cm and 30 cm, just outside of the Pb-210
date range of 1866.2. The TP concentration between 30 to 2 cm, had a pronounced
increase with a mean of 0.62 mg g-1. A decrease occurred between max depth 4 and 2
cm, correlating with Pb-210 dates of approximately 2011 and 2017. The mean of TP
concentrations between 32 cm and 75 cm depth, prior to the positive trend mentioned
above, was 0.21 mg g-1. The TP concentrations in this interval (32 to 75 cm) were
somewhat variable with four distinguished peaks near 38, 44, 56 and 70 cm. The range
of TP between 32 and 75 cm had a minimum of 0.13 mg g-1 and maximum of 0.37 mg g-
1. Kendall Tau significance analysis determined sediment core results exhibited a
significant positive increase up core in TP values, p-value< 0.05.
59
Big Redfish Lake
Big Redfish Lake surface sediment-water interface core measured a TP mean of
0.31 mg g-1 within the 95 cm sediment depth (Figure 3-8). The TP concentrations
showed an increasing trend to the core surface, along with some variability within core.
A positive significant trend occurs towards the sediment surface. An increase is
observed at approximately 36 cm and continues into the uppermost portion of the
sediment. The mean TP concentration between 36 cm and the upper portion of the core
was 0.50 mg g-1. This portion of the core had a minimum concentration of 0.09 mg g-1
and a maximum concentration of 0.77 mg g-1. There was a steep decrease in TP
concentrations between 32 and 36 cm. The second lowest concentration of the whole
sediment core, 0.09 mg g-1, occurred at 36 cm. Between depths 38 and 95 cm, there
was more variability than in the Eastern lake core. The mean TP concentration between
38 and 95 cm was 0.21 mg g-1, with a minimum value of 0.08 mg g-1 occurring at 95 cm
and a maximum value of 0.45 mg g-1 at 92 cm. Within this section, there were five
significant peaks at 46, 58, 66, 78 and 92 cm. Kendall Tau significance analysis of
sediment core data determined a significant positive increase, p-value<0.05, in TP
values with depth.
Geochemistry
Eastern Lake
Eastern and Big Redfish Lake surface sediment-water interface cores were
analyzed for bulk organic carbon (%C) and nitrogen (%N), (C/N) weight percent (wt%)
contents and stable isotopes signatures of δ13C and δ15N of sediment organic matter.
The Eastern Lake core weight percent of carbon and nitrogen displayed an increasing
trend up core but varied with multiple peaks and troughs (Figure 3-9). The core %C
60
mean was 3.24%, with a minimum of 1.08% and a maximum of 7.04%. The average
core %N was 0.25% with a minimum of 0.08% and a maximum of 0.60%. The ratio of
Eastern Lake presented a decreasing trend towards the sediment surface. The mean
C/N ratio of Eastern Lake sediment core was 13.21 (% weight).
The stable isotope measurements of δ13C and δ15N varied within and among
sediment cores of Eastern and Big Redfish Lake. Carbon isotope ratios (δ13C/ δ12C)
were presented as per mille (‰) deviations (δ13C) from Vienna Pee Dee Belemnite
(VPDB) (Lamb et al., 2006). Nitrogen stable isotope ratios (δ15/ δ14N) were presented as
per mille deviations (δ15N) from atmospheric air (vs. air). Eastern Lake sediment stable
isotope δ13C values display a variable trend with depth (Figure 3-10). The minimum
value was -26.39‰ and the maximum was 24.07‰ (per mil, vs VPDB), with a mean
δ13C value of -24.92‰, respectively. Sediment δ15N of the Eastern Lake core displayed
a slight increasing trend (Figure 3-11). The minimum δ15N value was 2.80‰ and the
maximum was 4.40‰ (per mil, vs AIR), respectively. The mean δ15N value of Eastern
Lake core was 3.67‰, respectively.
Big Redfish Lake
Big Redfish Lake’s core weight percent carbon and nitrogen measurements also
exhibited a decreasing trend up the sediment profile (Figure 3-12). The core %C had a
mean of 5.27% with a minimum of 0.73% and a maximum of 12.61%. Big Redfish Lake
had a mean %N of 0.36%, a minimum of 0.04% and a maximum of 0.79%. The average
C/N ratio of Big Redfish Lake sediment core was 14.95 (% weight).
Big Redfish Lake sediment core stable isotope values displayed more variation
than Eastern Lake with a mid-depth decrease for both δ13C and δ15N. The mean δ13C
61
value for the Big Redfish Lake core was -25.95‰ (per mil, vs VPDB), with a minimum
value of -26.39‰ and a maximum value of -24.75‰, respectively (Figure 3-13). The
mean of δ15N was 2.90‰ (per mil, vs AIR), with a minimum of 1.96‰ and a maximum of
3.82‰, respectively (Figure 3-14). Stable isotope δ13C displayed a negative, decreasing
trend through the Big Redfish Lake core and a slight increasing trend of stable isotope
δ15N.
62
Table 3-1. Eastern Lake long term summary statistics (CBA 2017)
Mean Max Min Median Std Error
TP (µg/L) 12 17.8 9.14 11.7 0.56
TN (µg/L) 286 399 214 288 10.8
CHL (µg/L) 3.55 6.71 2.14 3.48 0.3
Secchi Depth (m) 1.45 1.97 0.94 1.5 0.07
Temperature (C) 22.5 25.4 17.5 23 0.45
Dissolved Oxygen (mg/L) 6.77 7.79 5.55 6.76 0.17
pH 7.53 7.97 7.12 7.5 0.05
Salinity (ppt) 8.23 18.5 2.01 8.47 1.41
Turbidity (NTU) 2.48 8.97 0.7 2.6 0.55
Color (Pt-Co Units) 96.9 173 45 95.5 19.1
Specific Conductance (µS/cm) 3230 3950 2610 3190 201
Surface water chemistry was collected on Eastern Lake since 1998, this data was statistically analyzed to determine long term analysis of surface water variables.
63
Table 3-2. Big Redfish Lake long term summary statistics (CBA 2017).
Surface water chemistry data was collected on Big Redfish Lake since 1999, this data was statistically analyzed to determine long term analysis of surface water chemistry variables.
Mean Max Min Median Std Error
TP (µg/L) 13 22.6 4.67 13.5 0.95
TN (µg/L) 364 657 149 448 32.6
CHL (µg/L) 5.39 10.5 1.67 5.97 0.55
Secchi Depth (m) 0.98 1.5 0.68 0.91 0.06
Temperature (°C) 23.9 30.5 21.6 23.4 0.52
Dissolved Oxygen (mg/L) 5.98 7.36 4.49 5.88 0.23
pH 7.2 7.72 6.6 7.22 0.07
Salinity (ppt) 5.14 16 0.66 8.32 1.31
Turbidity (NTU) 2.27 8.86 0.74 1.97 0.6
Color (Pt-Co Units) 150 326 43.3 156 47.5
Specific Conductance (µS/cm) 1520 4550 360 1840 694
64
Figure 3-1. Eastern Lake long term trends analysis (CBA 2017) Eastern Lake water chemistry summaries indicate statistically significant positive trends in total phosphorus, total nitrogen and total chlorophyll, while water transparency exhibited a significant negative trend. The inter-annual (monthly variability) within 2016 is indicated by colored circles and intra-annual variance (variability among years) is indicated by black triangles.
65
Figure 3-2. Big Redfish Lake long term trend analysis (CBA 2017)
Big Redfish Lake water chemistry summaries indicate statistically significant positive trends in total phosphorus, total nitrogen and total chlorophyll, while water transparency exhibited a significant negative trend. The inter-annual (monthly variability) within 2016 is indicated by colored circles and intra-annual variance (variability among years) is indicated by black triangles.
66
Figure 3-3. Eastern Lake Lead-210 geochronology
Eastern Lake geochronological analysis provided 210Pb activity dating that resulted in a 151 year period of record.
67
Figure 3-4. Eastern Lake sedimentation rate
Eastern Lake sedimentation rate displayed relatively unchanged depositional rates until 16 cm (1959) where a peak of 31 mg/cm2/yr occurred and then again at depth 2 cm (2017) where an increase of 161 mg/cm2/yr occured.
68
Figure 3-5. Big Redfish Lake geochronology
Big Redfish Lake geochronological analysis of 210Pb activity resulted in a 127 year period of record.
69
Figure 3-6. Big Redfish Lake sedimentation rate
Big Redfish Lake sedimentation rate displayed low depositional rates until 6 to cm (1997) where a peak of 16 mg/cm2/yr occurred. Sedimentation rates increased up core reaching a maximum of 22 mg/cm2/yr at 2 cm depth (2012).
70
Figure 3-7. Eastern Lake sediment total phosphorus
Eastern Lake sediment total phosphorus concentrations (mg g-1) exhibited a significant increase toward the sediment surface, p<0.05.
71
Figure 3-8. Big Redfish Lake sediment total phosphorus
Big Redfish Lake sediment total phosphorus concentrations mg g-1 exhibited a significant increase toward the sediment surface, p<0.05.
72
Figure 3-9. Eastern Lake C/N ratio
Eastern Lake sediment organic matter C/N (wt%) ratios demonstrated a significant decrease with surface sediment core depths, p<0.05.
73
Figure 3-10. Eastern Lake 13C
Eastern Lake sediment δ13C indicated organic matter sources are dominated by
terrestrial and aquatic C3 plants.
74
Figure 3-11. Eastern Lake δ15N
Eastern Lake sediment geochemical proxy δ15N exhibited a significant decrease
towards sediment core surface, p<0.05.
75
Figure 3-12. Big Redfish Lake C/N ratio
Big Redfish Lake sediment organic matter C/N (wt%) ratios exhibited a significant decrease towards sediment core surface, p<0.05.
76
Figure 3-13. Big Redfish Lake δ13C
Big Redfish Lake sediment geochemical proxy δ13C demonstrated a significant
decrease towards sediment core surface, p<0.05.
77
Figure 3-14. Big Redfish Lake δ15N
Big Redfish Lake sediment geochemical proxy δ15N exhibited an increase towards
sediment core surface.
78
CHAPTER 4 DISCUSSION
Geochronology using 210Pb of sediment organic matter provided a reference
period of 151 years for Eastern Lake and 122 years for Big Redfish Lake. Results
indicated parallel changes throughout both Eastern and Big Redfish Lake sediment
cores. Lakes demonstrated significant increases in sediment TP from approximately 30
cm to the sediment core surface demonstrating that these lakes began to experience
changes caused by nutrients prior to the oldest 210Pb date of 1866. Throughout both
sediment cores, sediment C/N ratios exhibited a decrease towards sediment core
surface indicating that these waterbodies have historically become more influenced by
autochthonous-produced organic matter rather than allochthonous. Stable isotopes δ13C
and δ15N assisted in estimating the origin of organic matter sources, however terrestrial
and aquatic C3 plants demonstrated overlapping δ13C values. However, marine organic
matter origins can be distinguished from terrestrial sources by elevated δ13C and δ15N
values. These inferences assisted in the determination of seawater influences on lake
hydrology. Studies conducted on characteristic Florida lakes associated increases in
both stable isotope δ13C and δ15N values during periods of elevated primary productivity
(Brenner et al., 1999; Gu et al., 1996). Eastern Lake demonstrated a decrease in δ13C
and a significant increase in δ15N values toward surface sediments, while Big Redfish
Lake exhibited a significant decrease in δ13C values and an increase in δ15N values.
Inferred lake productivity through sediment TP and δ13C trends did not show strong
correlations in either Big Redfish or Eastern Lake. The absence of correlation between
TP and δ13C values indicates that primary productivity may not be the dominant source
79
of organic matter to the lake systems or that bacteria has a coupled or dominant
influence on carbon cycling within the waterbody (Teranes and Bernasconi 2005).
Stable isotope values were also used to infer periods of marine or freshwater
dominance in the lakes. Although the research results from this study investigated
historical and modern trends within the lake study sites, the complexity of each coastal
dune lake’s ecosystems is evident and should involve comprehensive research
investigation.
Surface Water Chemistry
Surface water quality data Kendall-Tau analyses presented statistically
significant increases in nutrient concentrations and total chlorophyll for both coastal
dune lakes. Increasing trends in nutrient concentrations (TN and TP) and total
chlorophyll over a period of time indicates that a waterbody may be undergoing
eutrophication, experiencing an increase in nutrients and biological productivity moving
toward a higher trophic state. Collectively, these water quality variables provide insight
and understanding to the conditions of lake study sites over the last 20 years.
A waterbodies’ trophic state is evaluated using nutrients (TN and TP), chlorophyll
(algal response to nutrients) and Secchi disk clarity measurements to either calculate a
cumulative score, known as a trophic state index, or individual variable assessment
(Hoyer and Canfield 2008). Waterbodies with low nutrient concentrations and low
biological productivity are referred to as oligotrophic, and waterbodies with moderate
concentrations of nutrients and mid-biological activity are mesotrophic, while high
nutrient concentrations and biological activity are considered eutrophic conditions and
excessive nutrients and biological productivity are hypereutrophic conditions
(LAKEWATCH 2000). The trophic state variables had a strong correlation to amounts of
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algae, aquatic vegetation and wildlife that a waterbody can sustain (Hoyer and Canfield
2008). The Forsberg and Ryding (1980) trophic state classification (Table 4-1) provides
a range of values for each variable to assess trophic states based on individual
measurements of TN, TP, chlorophyll and Secchi depth. However, due to coastal dune
lake high true color value, use of Secchi depth measurements is not encouraged as
dark tannic waters can cause shallow Secchi depth measurements and would
deceptively indicate a eutrophic state (Hoyer and Canfield 2008).
Eastern and Big Redfish Lake study sites are classified as predominantly
oligotrophic with nutrient poor conditions (Hoyer and Canfield 2008). As such, they are
considered unimpaired by cultural eutrophication in Florida (Hoyer and Canfield 2008).
An examination of surface water data reveals that Eastern Lake nutrients and
chlorophyll values fall within range of an oligotrophic waterbody, however the long-term
range of TP and chlorophyll cross into mesotrophic lake conditions (Table 4-2) (CBA
2017). Eastern Lake appears to be shifting towards a more biologically productive
waterbody based on the significant positive increases in nutrients and total chlorophyll.
Notable physicochemical variables analyzed in the Coastal Dune Lake Water
Chemistry Summary Report (2017) were dissolved oxygen, pH, salinity and
temperature. Both study lakes displayed a wide range of values between the surface
and bottom measurements. These variables were influenced by physical, chemical and
biological interactions of coastal dune lakes and should be examined as additional
indices of hydrologic conditions and external influences. The differences recorded
between the surface and bottom measurements depict the variability in lake conditions
and reflects external influences on the lake hydrology (Table 4-3).
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Eastern Lake exhibited a range of surface and bottom measurements of
dissolved oxygen, as its surface range indicated a generally well oxygenated system.
This would be expected in a surface water because oxygen is more easily mixed into
the water column through aeration, diffusion and photosynthetic activities. Bottom
dissolved oxygen measurements exhibited a wider range with low values near hypoxic
conditions.
Dissolved oxygen concentrations can be suggestive of algal and nutrient cycling,
as well as stratification within a waterbody. Dissolved oxygen is utilized throughout the
water column by organismal functions for energy or nutrients. Organisms engaging in
photosynthesis influence dissolved oxygen concentrations which fluctuate with the time
of day, along with aeration and mixing (Wetzel 2001; Boehrer and Schultze 2008).
Generally, dissolved oxygen values are lower in benthic layers due to upper layer
consumption by organisms and slow diffusion of oxygen from the surface. Lower
dissolved oxygen concentrations are influenced by a variety of physical and biological
functions. A strong influence on dissolved oxygen concentrations of the benthic layers is
the presence of saline water formed by a connection to seawater. During times of
intermittent outlet connections, salinity variations can change the amount of oxygen that
is able to diffuse to the bottom lake layers. Temperature can also impact the solubility of
oxygen in water, and oxygen and temperature are inversely related and can be strongly
influenced by seasonality and by changes in climate conditions (Wetzel 2001). Both
variables, salinity and temperature, can create stratification of lake waters and
potentially anoxic conditions which may trigger sediment nutrient release (Dittrich et al.,
2013). Furthermore, bacterial decomposition utilizes dissolved oxygen, lowering
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concentrations that are already limited and not easily replenished (Boehrer and
Schultze 2008).
It can be theorized that coastal dune lake study sites, Eastern and Big Redfish
Lake, are also influenced by these variables. Eastern Lake exhibited standard mean
surface dissolved oxygen concentrations but displayed with highly variable bottom
measurements. Benthic measurements displayed a range of measurements from
hypoxic to well oxygenated, but overall had a mean concentration slightly lower than
surface measurements. Due to Eastern Lake’s morphological and physicochemical
features it is susceptible stratification and lower benthic dissolved oxygen
concentrations. In addition, intermittent connections to seawater could play a critical role
in accentuating stratification within the lake water column, especially during closed
conditions. Long term trends of increasing nutrients and total chlorophyll may also
influence water column and benthic dissolved oxygen, however, studies to investigate
these occurrences have not been conducted.
Big Redfish Lake also displayed well oxygenated conditions of dissolved oxygen
concentrations and a wider range of bottom dissolved oxygen. Surface dissolved
oxygen means displayed slightly lower ranges of concentrations compared to Eastern
Lake. Surface dissolved oxygen levels are likely correlated with aeration, diffusion and
photosynthetic activities. Big Redfish Lake displayed bottom dissolved oxygen levels
ranging from anoxic to well oxygenated benthic conditions. Due to the nature of the
coastal dune lakes and their intermittent connection to seawater, the variation between
surface and bottom dissolved oxygen measurements are hypothetically correlated with
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lake outlet connections, freshwater inputs, periods of low circulation, nutrient inputs and
primary productivity within the lake.
Perissinotto et al., (2010) attributes low dissolved oxygen levels in South African
Temporarily Open or Closed Estuaries (TOCEs) to the absence of tidal influence and
strong freshwater inflows. Without strong influences on circulation water, column
stratification becomes more distinct in the TOCE systems (Perissinotto et al., et al.,
2010). Deteriorating dissolved oxygen levels in benthic waters of estuarine lagoons is
believed to be controlled by a combination of rainfall, solar insulation, wind stress and
tidal mixing (Gale et al., 2006). The coastal dune lakes exhibit a range of dissolve
oxygen levels, particularly in the bottom layers of the lake, and at times, dissolved
oxygen levels are anoxic on the lake bottom. There are likely many factors that
influence this phenomenon, such as seasonal lake temperatures, climate, and
intermittent saltwater connections. Current literature on the coastal dune lakes has not
examined lake stratification events or if they are associated with seasonal changes.
Stratification within coastal dune lake waterbodies is probable to occur after an outlet
connection occurs and seawater settles at the bottom of the lake creating a chemocline.
A lake could become further stratified if the outlet connection becomes disconnected
again and mixing decreases.
The pH measurements from the coastal dune lakes are informative to lake
system functions, physicochemical interactions and the subsequent influence on water
column conditions. Freshwater typically has a pH of 6-8 and measurements can reflect
photosynthetic and respirative activities of microorganisms and algae, determine the
solubility of nutrients and heavy metals, and influence the distribution of aquatic
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organisms in addition to other waterbody characteristics (Suwanee River Water
Management District 2018). Changes in pH can impact the solubility of nutrients and
one of the main drivers of phosphorus release is pH change in the water column and
sediments (Dittrich et al., 2013).
The coastal dune lakes are characterized by their dark, tea colored waters which
are a product of watershed inputs of dissolved organic matter. This dissolved organic
matter can have an influence on the water pH, typically lowering water pH (Hoyer and
Canfield 2008). An average lake pH is 6-8 but is ultimately dependent on the local
geology and inputs from surrounding lands. When freshwater inputs are low, coastal
dune lake water is clearer, whereas increased freshwater inputs cause dissolved
organic matter to increase (Hoyer and Canfield 2008). In addition, marine waters may
have an influence on water pH. Generally, estuary systems have pH averages near 7.0
to 7.5 in sections closer to freshwater inputs and a pH range of 8.0 to 8.6 near areas
with higher salinity influences (EPA 2006).
Similar variability in surface water pH was observed in lake study sites. Eastern
Lake surface pH had a mean of pH of 7.53 with little variability, however, benthic pH
measurements exhibited a wider range and a mean of 7.5. Big Redfish Lake
demonstrated alkaline pH water conditions in surface measurements and variable
bottom pH. Benthic pH levels have a wider range of variability but exhibited a mean
close to traditional fresh waterbodies. The pH variability in both lake study sites
indicates that other physical and biological factors may influence water column pH.
The salinity of the coastal dune lakes of Northwest Florida is directly driven by
the outlet connection frequency and duration which is based on a lake’s morphometry,
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watershed and hydrology (Hoyer and Canfield 2008). Storm surges that push saltwater
from the Gulf of Mexico can influence geochemical signatures in these lakes and impact
lake salinity (Das et al., 2013; Lambert et al., 2008). Limited literature exists of the
hydrologic exchanges between the intertidal zone of the Gulf of Mexico, lake water and
groundwater of the coastal dune lakes impact salinity. Other studies in Florida have
discussed this zone as an area of exchange between seawater and fresh groundwater
or aquifer sources (Hoyer and Canfield 2008; Heiss and Michael 2014). Salt spray from
the Gulf of Mexico can also potentially influence the salinity of the lakes as it effects
vegetation and soils further inland. Strong storms were identified to have large
influences on the coastal lake’s hydrology during seawater overwash events (Liu and
Fearn 1993; Liu and Fearn 2000; Lambert et al., 2008; Das et al., 2013). According to
water chemistry results, a vertical halocline of the coastal dune lakes water column is
commonly measured in lakes with outlet connections. The variable range of surface and
bottom salinity measurements exhibit the dynamic nature of the coastal dune lakes and
can resemble the salinity characteristics of estuaries influencing lake hydrology.
Eastern Lake exhibited a wide range of surface and bottom salinity
measurements, typically with lower surface and higher benthic salinities. Its surface
salinity exhibited a wide range of measurements from 2.0 to 18.5 ppt and bottom
measurements were 1.21 to 33.13 ppt (CBA 2017). Such a range demonstrates the
variability and influence of both freshwater and saltwater inputs which are ultimately
driven by a lake’s morphometry. Eastern Lake has a slightly higher surface and bottom
salinity compared to Big Redfish Lake. This may indicate that Eastern Lake salinity is
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correlated with more frequent or longer outlet opening events, however lake outlet data
is limited.
Big Redfish Lake also exhibited variable surface and bottom salinities, but
exhibited slightly lower measurements than Eastern Lake. Big Redfish Lake has a mean
surface salinity range of 0.66 to 16 ppt and a bottom range of 0.17 to 33.04 ppt (CBA
2017). The range of bottom salinities are, at their max, near measurements of the Gulf
of Mexico. Its slightly lower mean salinity could potentially indicate it receives more
freshwater inputs, less frequent or shorter outlet connections to seawater, and may be
more influenced by watershed hydrology.
Surface temperatures of the coastal dune lakes are likely influenced by
morphometric characteristics, fresh and saltwater inflows and climate. The size, shape
and volume of a waterbody influences circulation and vertical mixing within the water
column. Physical forces, such as wind or inflows, impact the temperature of a
waterbody. The coastal dune lakes are considered shallow lakes with an average
maximum depth of 2 meters. A lake’s depth is can have a role in its water temperature
and thermogradients which can develop into chemoclines. Shallow lakes heat up faster
in spring and summer months (Wetzel 2001). Colored waters, like that of the coastal
dune lakes, alter the light entering a waterbody by increasing the vertical light
attenuation and absorbing heat radiation (Wetzel 2001). Cooler ground and surface
water inflows can influence lake water temperatures and correlate with changes in
climate conditions. Air temperature can also have an influence on water temperatures.
Eastern Lake exhibited a mean surface water temperature that was lower than
bottom measurements. It had a mean surface temperature of 22.5°C and bottom mean
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of 23.9°C with the benthic temperatures exhibiting more variability (CBA 2017). Warmer
bottom temperatures may be influenced by seawater presence and lake depth, and
deeper saltwater layers could be more inclined to warm from solar radiation and reflect
higher mean benthic water temperatures. Warmer water temperatures are inversely
correlated with dissolved oxygen concentrations and could lead to additional lake
stratification.
Big Redfish Lake demonstrates a similar pattern of slightly warmer bottom
temperatures compared to its surface. Overall it exhibited a larger range of both surface
and bottom temperatures, with higher maximum values 9.5 to 33.7°C (CBA 2017). Big
Redfish Lake has an average depth of 1.58 meters, approximately 0.5 meters more
shallow than Eastern Lake. The slightly more shallow water depth may correlate with its
slightly warmer average surface and bottom temperatures.
Saltwater can influence water temperatures and subsequent circulation. As water
densities change between fresh and saltwater mixing, the denser saltwater settles to the
bottom. The heat capacity of saltwater is lower than freshwater and can heat up faster.
Estuarine lagoons can be described as waterbodies that routinely receive seawater
from an external source, known as ectogenic meromixis, in which more dense saltwater
sinks below the less dense freshwater (Wetzel 2001). Estuarine lakes often exhibit
heliothermic characteristics where a layer of dense saltwater becomes thermally
stratified from the less dense layer and may become heated influencing the
development of a chemocline (Wetzel 2001). The lake study sites may also
demonstrate these conditions by exhibiting slightly warmer mean bottom temperatures
due to heliothermic properties.
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Changes in water temperature stimulate a range of biological, physical and
chemical functioning within an aquatic system. Light intensity affects photosynthesis
and growth rates of algal communities which have a positive correlation to water
temperatures (Wetzel 2001). Increased growth and respiration of microorganisms can
cause higher oxygen consumption and could eventually lead to oxygen depletion in the
bottom layer of the lakes. Increased water temperatures can also impact the solubility of
compounds and bound elements (Wu et al., 2014). Temperature is one of the top three
contributing factors to phosphorus release from sediments (Wu et al., 2014), and these
changes could also cause a shift in microorganism communities within these layers and
at the sediment-water interface (Zu et al., 2014).
Coastal dune lake water chemistry summaries demonstrate that the coastal dune
lakes surface and bottom physicochemical variable measurements can be highly
variable and susceptible to seasonal climate influences causing lake stratification. The
variations in coastal dune lake waters are likely influenced by lake morphology and its
dynamic interactions with external environments. Outlet connections to saltwater,
freshwater inputs, dissolved oxygen, pH, nutrients and morphometric features of each
lake can all have an influence on the degree of stratification that occurs within the lakes.
Almost all the coastal dune lakes in Northwest Florida have highly colored tannic
waters that drain from the watershed into surface and groundwaters which feed into the
lakes as the true color of a lake is influenced by the dissolved organic matter in its
waters. Lakes with true color values of 51-100 (Pt-Co Units) are considered highly
colored waterbodies (LAKEWATCH 2004). Most coastal dune lakes, including Eastern
and Big Redfish Lake, exhibit highly colored true color values (Hoyer and Canfield
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2008). Eastern Lake has a mean true color of 96.9 Pt-Co Units and range of 45 to 173
Pt-Co Units (CBA 2017). Big Redfish Lake exhibits a wider range of true color values
from 43.3 to 326 Pt-Co Units and an elevated mean of 150 Pt-Co Units (CBA 2017). As
previously demonstrated in physicochemical variable ranges, Big Redfish Lake is
potentially influenced more by its watershed hydrology. This is also depicted through its
higher mean and range of true color values. True color is associated with climate
conditions, mainly precipitation and drought, which can affect the amount of runoff that
seeps into a waterbody or lack of inflow to transport dissolved organic matter
(LAKEWATCH 2004).
Water Column Nitrogen to Phosphorus Ratios
As freshwater lakes, the coastal dune lakes are presumed to be phosphorus
limited, however variations in water column nitrogen and phosphorus ratios indicate that
the study lakes may shift between nutrient limiting states or are nutrient co-limited.
Variations between nutrient limitations have been observed in other lakes and estuaries,
however the estuarine features of the coastal dune lakes may be have a key role in
nutrient limitations and co-limitations of the lakes.
According to the Florida LAKEWATCH Nutrient Circular (2001), phosphorus and
nitrogen are the most common limiting nutrients in Florida waterbodies due to the
regional geology. Determining the limiting nutrient to a lake is often analyzed by
reviewing the TN/TP ratios and the relationship of TP to TN/TP, increasing TP typically
causes a decrease in TN/TP (Downing et al., 1992). The TN/TP ratio can often be
correlated to the trophic state of a waterbody (Downing et al., 1992). Most literature
discuss phosphorus (P) as the limiting nutrient in freshwater lakes and nitrogen (N) as
the limiting nutrient in coastal marine waterbodies (Taylor et al., 1995). Discrepancies to
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this standard can be observed in marine environments that receive large seasonal
inflows of freshwater and exhibit shifts between P- and N-limitations, as well as marine
waterbodies with restricted or low freshwater inputs and long residence time that can
become P-limited (Taylor et al., 1995). These types of conditions may also be reflected
in the coastal dune lakes that have intermittent outlet connections. Lakes can display
more variability than marine environments in total nitrogen and total phosphorus levels,
with some of the lowest and highest concentrations of TN and TP (Guildford and Hecky
2000). These examples of contrasting nutrient limitations and co-limitation were
discovered to be similar among key types of ecosystems and have been observed
nearly many studies (Ptacnik et al., 2010). Thus, indicating that many systems exhibit
these conditions and therefore it would not be unlikely to see these trends in the coastal
dune lakes.
Florida lakes show three general ranges of TN/TP (μg/L) ratios which can provide
some insight into which nutrient has the most significant role (LAKEWATCH 2000). A
TN/TP ratio that falls in the range less than 10, indicates that phosphorus may not be
the only factor affect productivity in a lake and nitrogen limitation may have a large role
as limiting nutrient (LAKEWATCH 2000; Guildford and Hecky 2000). Lakes that have a
TN/TP range between 10 and 17 can be interpreted as having nutrient limitation by
either nitrogen or phosphorus or potentially co-limitation of these nutrients
(LAKEWATCH 2000). Lakes that exhibit a TN/TP range greater than 17 would be
expected to be phosphorus limited (LAKEWATCH 20000; Guildford and Hecky 2000).
There may be some caveats to these nutrient generalizations, a potential exists for
nitrogen to be the limiting factor when the TP value is greater than 100 μg/L due to its
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overabundance (LAKEWATCH 2000). In addition, lakes with TN/TP values less than 10
may still be phosphorus limited when TP values are less than 50 μg/L (LAKEWATCH
2000). Downing et al., (1992) associated a lake’s TN/TP ratio with its trophic status:
oligotrophic lakes ranged from 21 to 240, mesotrophic lakes ranged 17 to 96, eutrophic
lakes ranged 4 to 71, and hypereutrophic lakes have a range of 0.5 to 9.
Over a 19 year period of surface water samples, Eastern Lake demonstrated a
typical phosphorus limitation. Its average TN/TP value was 26 μg/L, but displayed a
range of 7 to 75 (μg/L) (CBA 2017). Generally, Eastern Lake would place into a
category of an oligotrophic phosphorus limited system. However, its range of TN/TP
values also indicate that it potentially experiences intermittent nitrogen limitations
(TN/TP < 9) or co-nutrient limitation and may classify as a mesotrophic stat according to
Downing et al., (1999).
Big Redfish Lake demonstrated an average phosphorus limitation, but also
exhibited a range of TN/TP ratio values that indicate it may also fall into nitrogen or co-
nutrient limitation. It has a mean TN/TP value of 31 μg/L and exhibits a range of 8 to
133 (μg/L) indicating a potentially mesotrophic (17 to 96) to eutrophic system (4 to 71).
The variable nature of the coastal dune lakes makes it challenging to determine a
single limiting nutrient, but it seem evident that they can change between nutrient
limitations and may be impacted by factors other than just nutrients and productivity.
Supplementary information is necessary to make a comprehensive assessment of the
nutrient dynamics of the coastal dune lakes. The dominant sources of new nutrients to
lakes are terrestrial runoff and atmospheric input (Guildford and Hecky 2000). As
coastal development continues to increase along the Florida Panhandle corridor,
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development along the shorelines and within the watersheds of the coastal dune lakes
may change the dynamics of nutrient limitations within the coastal dune lakes.
Geochronology
Eastern and Big Redfish Lake sediment cores functioned properly as 210Pb,
137Cs, and 226Ra isotopic dating mediums and provided roughly 150-year periods for
both lake cores. Collectively, these radioactive isotopes are known as Lead-210 (210Pb)
dating methods but are often used together to obtain maximum chronological data
(Jeter 2000). The 137Cs dating methodology is based on its atmospheric fallout from
nuclear weapons testing events (Jeter 2000). Initial 137Cs measurements mark the date
of 1954 or after (Jeter 2000). Isotope 210Pb enters the environment through the fallout
and decay of 222Rn, referred to as unsupported 210Pb, as well as from natural 226Ra in
sediments known as supported 210Pb (Jeter 2000; de Souza et al., 2012). It may also
enter the environment through anthropogenic wastes enriched with radionuclide (de
Souza et al., 2012). Whichever pathway occurs, it mixes and accumulates or interacts
with sediments of a waterbody.
The 210Pb dating method is based on the measurement of unsupported 210Pb
activities (de Souza et al., 2012). The Constant Rate of Supply model (CRS) assumes a
constant unsupported flux of 210Pb to the waterbody sediments and allows for the
sediment supply to vary (de Souza et al., 2012). The oldest date from the Eastern Lake
sediment core determined through the 210Pb dating was 1866 at a depth of 30 cm, 151
years before present, with the most current date of 2017 at 2 cm. The 137Cs peaked at
depth 14 and 16cm indicating a time frame close to 1954 (nuclear weapons testing
events).
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The Big Redfish Lake sediment core dating determined 1890 to be the oldest
collected date at 22 cm. The most recent date 2012 was observed at 2 cm. The 137Cs
record was blurred more, however indicated a minor increase at 8 cm being associated
with the 1954 weapons testing time frame. The CRS model also provides the
sedimentation rates from core sediments analyses and can be indicative of external and
internal variations within waterbody. Both, Eastern and Big Redfish Lake sediment core
data displayed increasing up core trends in sedimentation rates. In addition to
chronological reconstructions, Bennett and Buck (2016) argue that the pattern of
sediment rates and accumulation can be used to interpret environmental change.
Eastern Lake’s mean sedimentation rate is 27 mg/cm2/yr. At depth 2 cm, a large
increase of 161 mg/cm2/yr occurs at a date of 2017. The other sedimentation rates
resemble stable increases in deposition. This large increase in sedimentation rates at
the surface sediment could indicate increasing sediment input from either
autochthonous or allochthonous sources (Bennett and Buck 2016). Eastern Lake’s
sediment C/N ratio results confirm a transition from allochthonous to autochthonous
throughout the core. However, C/N ratios increase around 2 cm and TP decreases at
this depth potentially indicating an increase in terrestrial input. If this peak value was
excluded the mean sedimentation rate it would be 16.9 mg/cm2/yr. However, Eastern
Lake would still exhibit an increasing rate of sedimentation which also indicates that
environmental changes are occurring, but cannot be determined through these
variables.
Big Redfish Lake exhibited a lower mean sedimentation rate of 9.4 mg/cm2/yr. A
notable increase from 8.7 to 15.7 mg/cm2/yr was observed at depth 6 cm at the 210Pb
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date of 1997. An environment with constant sedimentation will typically display
decreasing rate of sediment deposition (Bennett and Buck 2016). Sediment rates that
exhibit increasing accumulation rates, like Big Redfish Lake, indicate environmental
changes are occurring (Bennett and Buck 2016; Sommerfield 2006). However,
accumulation rates cannot provide data on frequency, duration or origin of sediments
and may also be influenced by other factors such as compaction and biological mixing
(Sommerfield 2006).
Sediment Total Phosphorus
Surface water data displayed significant increases in total phosphorus (TP)
concentrations in Eastern and Big Redfish Lake. This information was confirmed
through an R-software Kendall Correlation Rank analysis of sediment TP concentrations
from both study sites, Eastern Lake p-value <0.05 and Big Redfish Lake p-value <0.05.
Eastern Lake displayed relatively steady TP variations up core until 34 cm, then
exhibited a dramatic increase from 32 cm to 30 cm and steeply increased to the surface
(2 cm). Depth 34 is just outside of the geochronological dating depth, however depth 28
cm represents a date of approximately 1886. Big Redfish Lake displayed variable TP
values, but also experienced a steep increase at 36 cm. It continued to increase with
some variable positive trends until 2 cm where is drops slightly. The oldest available
lead-210 dating for Big Redfish Lake is depth 22 cm, representing approximately 1889 -
shortly after this dramatic increase in TP.
It is challenging to speculate what caused both lake cores to exhibit TP increases
near the same depth in the late 1800s. Historical records note this region of Florida
being used for timber and turpentine plantations. Various studies verify a strong
correlation between chlorophyll, representing algal biomass, and TP (Canfield 1983;
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Brenner et a 1999; Ptacnik et al., 2010; LAKEWATCH 2000). This association forms TP
into one of the main limiting nutrients in freshwater systems because of its effect on
biological activity and is often used as the trophic state variables (LAKEWATCH 2000;
Carey and Rydin 2011). Increases in surface water TP were correlated with significant
increases in surface water chlorophyll, indicating that both lakes could be moving
towards a eutrophic system. Internal and external TP inputs impact phytoplankton
growth which influence various other physical, chemical and biological aspects of the
aquatic environment. Phosphorus entering a waterbody via watershed drainage is
closely associated with particulates with iron, aluminum, calcium and humic substances
or can exist in dissolved inorganic forms of orthophosphates. (Wilson et al., 2008).
Sediments also play an important role in the source of nutrients to waterbodies
(Hou et al., 2013). Various processes allow for phosphorous to be released from
sediments into the water volume, some of the common interactions being dissolution
and desorption of bound phosphorus, dissolution of P in sediment pore water, and
mineralization of organic matter (Hou et al., 2013). Wu et al., (2014) state that P release
from sediments is one of the most important factors in the aquatic phosphorus cycle.
Remobilization of sediment P causes water column P concentration to increase (Carey
and Rydin 2011). This can occur when bottom anoxia exists, and P is released from
sediments (Wilson et al., 2008). Many of the coastal dune lakes do experience seasonal
anoxia due to water and climate temperatures, dissolved oxygen concentration and
bottom salinity. The potential exists that their sediments are releasing P back into the
water column. Sediments also act as sinks for phosphorus, removing it from the water
column through precipitation and storing it in sediments. Increasing trends in TP in
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Eastern and Big Redfish Lakes could indicate signs of increased nutrient loading to the
lakes and could lead to increased phytoplankton growth.
Organic Matter
Sediment organic matter C/N ratios are often analyzed in paleolimnological
studies to determine the dominant source of organic matter to a waterbody (Brenner et
al., 1999; Meyers and Teranes 2001; Lamb et al., 2006). The coastal dune lake study
sites, Eastern and Redfish Lake, display a decreasing trend of C/N ratios towards
sediment surface which suggest that both lakes are transitioning from dominantly
allochthonous, externally transported organic material, to autochthonous, internal in suit
origins, organic matter sources. These findings may potentially be correlated to changes
in the lake watersheds or development encroachment. Additionally, anthropogenic
nonpoint source pollution inputs may affect algal productivity altering its organic matter
sources from traditionally terrestrial to autochthonous. However, since these trends are
not significantly negative it is difficult to conclude a specific cause of change and is
more likely a cumulative impact from a variety of sources.
Determining the dominant source of organic matter to a waterbody can provide
an understanding to what the internal and external influences exist and how they may
affect the physical, chemical and biological qualities of a waterbody. Organic matter
consists mainly of carbon-based compounds and contains approximately 50% of carbon
(Meyers and Teranes 2001). It originates from a combination of organism components,
both living and dead, that are made up of an assortment of lipids, carbohydrates, and
proteins along with other portions produced from organisms and geochemical reactions
in and around a lake (Meyers and Teranes 2001).
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Organic matter is a source of nutrients, energy and storage of compounds for
both terrestrial and aquatic environments for both microorganisms and plants. As
organic matter moves through a landscape and into aquatic system it is altered with
only a small fraction surviving degradation (Lamb et al., 2006; Meyers 1994). Any
remaining portion of organic matter successfully escaped remineralization and is
incorporated into a waterbody’s sediments (Meyers 1994). Despite continued
diagenesis of sediment organic matter, geochemical markers are capable of being
preserved and utilized to identify organic matter sources (Meyers 1994). In particular,
sediment C/N ratio and stable isotope ratio of δ13C/ δ12C and δ15N/ δ14N have been
proven to be critical in identifying these sources (Meyers 1994; Meyers and Teranes
2001; Lamb et al., 2006; Gu et al., 1996; Brenner et al., 1999; Brenner et al., 2006).
The biogeochemical components of organic matter can be used to distinguish
between its origin of source, dominantly terrestrial and/or aquatic. Terrestrial vegetation
has greater refractory components and less labile portions, while aquatic contain more
labile fractions and less refractory leaving distinct differences in their organic matter
signatures (Khan et al., 2015; Meyers and Teranes 2001). Approximately 90% of
terrestrial vegetation are C3 photosynthetic plants and have high C/N (wt %) ratio
greater than 14, due to their high carbon and low nitrogen fractions (Khan et al., 2015;
Meyers and Teranes 2001). The coastal dune lakes are potentially largely influenced by
C3 terrestrial vegetation since many have watersheds that are partially preserved in
state parks or forest lands. Aquatic plants are also C3 plants, including macrophytes and
microscopic algae, and are composed of greater nitrogen, labile, fractions (Lamb et al.,
2006; Meyers and Teranes 2001; Meyers 1994; Khan et al., 2015). Due to a higher
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nitrogen content, C/N ratios decrease to between 5 and 7 or less than 12 (Lamb et al.,
2006; Meyers and Teranes 2001; Meyers 1994). C4 pathway plants, such as emergent
vegetation like S. alterniflora and J. roemerianus, are regularly observed along coastal
dune lake shorelines offering an additional source of organic matter input. C4 plants are
well adapted to saline intertidal areas and will increase C/N ratios to influenced
waterbodies and their sediments. C4 plants contain more carbon components and
create a larger C/N ratio of greater than 35 (wt %) (Lamb et al., 2006; Meyers 1994).
Variations in the C/N ratio can provide inferences to changes in organic matter sources.
However, post depositional and significant fractions of inorganic nitrogen can limit the
utility of C/N ratios in a marine setting (Hu et al., 2006).
C/N ratios are more effective when they are analyzed with concurrent stable
isotope (δ13C and δ15N) concentrations. The addition of these two stable isotopes in a
sediment analysis provide a more detailed indication of the organic matter contributions
from contrasting sources (Figure 4-1) (Lamb et al., 2006). The photosynthetic pathways
of plants produce distinct ranges between terrestrial and aquatic plants and even
between fresh and marine contributions (Lamb et al., 2006).
The range of δ13C in terrestrial plants is typically lower (more negative) than
aquatic C3 plants, approximately ranging from -32‰ to -21‰ (Lamb et al., 2006). These
ranges can overlap with aquatic C3 plants as well. Typically, freshwater C3 algae have
δ13C values from -30‰ to -26‰, a smaller range compared to terrestrial plants, while
marine algae have higher (less negative) δ13C values of -23‰ to -16‰ (Lamb et al.,
2006; Khan et al., 2015; Sampaio et al., 2010). C4 dominant vegetation can drive δ13C
values higher (less negative) having approximate values of less than or equal to -16‰
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(Lamb et al., 2006). Stable nitrogen isotope δ15N assist in identifying source specific
signatures of organic matter when analyzed with δ13C but is not as reliable as δ13C
values alone (Hu et al., 2006; Brenner et al., 2006). In terrestrial environments, C3
plants have a lower approximate δ15N range than algal values of organic matter; the
terrestrial range is approximately -5 to +18‰ with a mean of 3‰ (Hu et al., 2006) or
0.5‰ (Meyers 2003), while algae have higher range of δ15N values, 4 to 9‰ (Sampaio
et al., 2010; Meyers 2003).
These values can change due to physical, chemical and biological influences.
Das et al., (2013) investigated sand dune systems in the surrounding coastal dune lake
watersheds and found that dune organic matter inputs influenced a decrease in C/N and
stable isotope measurements. Bacteria also have the potential to influence these
variables but are typically minute and have a minimal effect on the sediment organic
matter (Lamb et al., 2006). Das et al., (2013) found that terrestrial plants including some
C4 plants around the coastal dune lakes have a range of δ13C values from -30.8 to
-13.5‰ and a δ15N range of -9.1 to 1.6‰. Submerged vegetation displayed δ13C values
of -25.6 to -23.2 ‰, with δ15N values of 4.4 to 8.7‰; while emergent plants displayed a
range of δ13C from -28.2 to -14.3 and δ15N values of 2.5 to 8.9 (Das et al., 2013).
Determining that the coastal dune lakes in their study, Eastern and Western Lake, were
influenced by predominantly C3 plants (Das et al., 2013).
In the present study, Eastern Lake displayed a mean C/N ratio of 13 (wt %) and a
range of 11 to 15 (wt %), indicating that aquatic C3 plants dominated organic matter
sources dominated. However, stable isotope analyses indicate that both terrestrial and
freshwater algae dominated the organic matter input. Eastern Lake (Figure 4-2)
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displayed a mean δ13C of -25‰ and a range of -26‰ to -24‰ with δ15N values of 2.8 to
4.4‰ and mean 3.7‰. These results demonstrate that these coastal dune lakes are
influenced by mainly terrestrial and freshwater C3 plants, with potentially some marine
organic matter sources.
Big Redfish Lake displayed similar variations in stable isotopes and a slightly
higher range of C/N values than Eastern Lake. Its C/N ratio mean was 15 (wt %) with a
range of 12 to 18 (wt %), indicating a stronger terrestrial dominance (Figure 4-3). Its
δ15N values were lower with a mean of 3.01 (wt %) and a range of 1.96 to 3.82‰,
indicating more terrestrial sources. Its δ13C values were more negative, with mean of -
26‰ and range of -28 to -25‰, than Eastern Lake’s measurements, also indicative of
more terrestrial dominance. Mackie et al., (2005) correlated high C/N ratios (14 to 26)
and ẟ13C values between -27 to -25‰ as freshwater aquatic and terrestrial organic
matter dominance (Mackie et al., 2005). This indicates that Big Redfish Lake may be
more influenced by its watershed and terrestrial organic matter inputs. Both lakes
display a decreasing C/N ratio trend that can be attributed to a change in organic matter
sources from terrestrial to aquatic (Das et al., 2013).
As exemplified by the literature and previous data, the coastal dune lakes are
influenced by all of the mentioned organic matter sources (terrestrial, freshwater and
marine algae, and C4 dominant vegetation) due to their dynamic nature and close
proximity to coastal interactions. This includes the influence of intermittent connections
to the Gulf, groundwater, and terrestrial vegetation along with saltwater and freshwater
marsh vegetation. Temporal variations in geochemical signatures of coastal dune lake
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sediments are believed to represent and correlate with climatic and lake hydrologic
variations (Das et al., 2013).
Paleoproductivity
In addition to organic matter sources and origins, geochemical signatures
indicate paleoproductivity of a waterbody. Stable isotope δ13C values are associated
with nutrient concentrations in a lake and correlate to nutrient increases (Brenner et al.,
1999). Increases in the heavier carbon isotope δ13C (versus δ12C) indicate the shift from
algae preference of δ12C due to its depletion during periods of increased productivity.
The depletion of δ12C forces C3 aquatic plants to rely on δ13C for carbon fixation
(Brenner et al., 1999). In addition, when water carbon dioxide levels are low, C3 aquatic
vegetation may rely more on bicarbonate as a carbon source which is enriched with
δ13C (Brenner et al., 1999).
Under eutrophic lake conditions, long periods of anoxia may occur causing
methanogenic bacteria to become active producing isotopically heavy carbon dioxide,
subsequently enriching produced organic matter with δ13C (Brenner et al., 1999). Gu et
al., (1996) provided results to support the use of δ13C as an indication of
paleoproductivity. Their study showed a positive correlation between δ13C and water
column chlorophyll-a in 83 Florida lakes (Gu et al., 1996). In addition, their study
showed that δ15N can be correlated to sediment TP and used to infer past productivity in
a lake (Gu et al., 1996).
According to Talbot and Laerdal (2000), increasing 15N values in an East African
Lake were a result of intense utilization of its DIN reservoir due to increases in lake
productivity caused by released nutrients from the flooded land surface. Interpretations
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using δ5N as an indicator alone can be limited by nitrification and nitrogen-fixing
bacteria, it was determined to not be as successful as δ13C (Gu et al., 1996).
Sediment δ13C can be influenced by physical, chemical and biological processes
within the lake as well as from its watershed (Brenner et al., 1999). Both study sites
demonstrate variations of increasing and decreasing δ13C values. These periods of
increases can potentially be correlated to periods of marine inundation where a greater
input of nutrient occurs and algal productivity peaks.
Paleosalinity
Stable isotopes, δ13C and δ15N, have previously been utilized to reconstruct
marine and freshwater inundation (Das et al., 2013; Lamb et al., 2006; Lambert et al.,
2008). This information can provide interesting insight into coastal formations such as
the coastal dune lakes. In fact, two separate studies have utilized some coastal dune
lakes of Northwest Florida sediment cores to reconstruct paleotempestology records in
the Gulf of Mexico coastal region (Das et al., 2013; Liu and Fearn 2000). These studies
determined that concurrent increases in both δ13C and δ15N indicate a marine dominant
state, likely caused by seawater inundation from major storm events, whereas a
decrease indicates freshwater dominance (Das et al., 2013; Liu and Fearn 2000; Lamb
et al., 2006; Lambert et al., 2008).
Organic matter of marine origins is generally more enriched with stable isotopes
δ13C and δ15N, than that of freshwater or terrestrial origins (Lambert et al., 2008; Das et
al., 2013). The Gulf of Mexico has more elevated nutrient concentrations and dissolved
inorganic carbon than coastal lake waters (Das et al., 2013; Lambert et al., 2008).
Coastal dune lake TN levels off at 300 µg/L, while the Gulf of Mexico averages 1600
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µg/L with higher levels near 6000 µg/L (Das et al., 2013). The geochemical signatures
that remain in the lake sediment are believed to represent strong storm events, in
addition to hurricanes, that caused seawater to enter the lakes (Lambert et al., 2008).
In the Lambert et al., (2008) model, coastal lakes have two states: isolated and
flooded. In isolated state, the lake is not connected to seawater and is characterized by
a dominant organic matter source of terrestrial input, indicated by low nutrients and
stable isotopes (Lambert et al., 2008). A flooded state is when the lake is subject to
seawater flooding produced by strong winds which force seawater into the lake causing
marine-like conditions with concurrent increases in stable isotope values (Lambert et al.,
2008). Marine water inundation would provide labile nitrogen components leading to
increased algal productivity and causing sediments to be further enriched with heavy
isotopes (Das et al., 2013). These increases or decreases in stable isotopes interpreted
with concurrent decreasing or no significant change of C/N ratios supports the Lambert
et al., (2008) model.
The study sites of the present research displayed variations that align with the
Lambert et al., (2008) model and Das et al., (2013) determinations. Eastern Lake
exhibited many variations in the stable isotope measurements with an increasing up
core trend of δ13C values and decreasing trend in δ15N measurements and C/N ratios.
Big Redfish Lake displayed less intermittent variations in stable isotopes and appears to
exhibit longer temporal ranges of increasing or decreasing isotope trends with an overall
decreasing C/N ratio. Extended increasing trends occur from 95 to 70 cm, 55 to 35 cm
with a slight decrease in both isotopes at 45 cm. These increasing events seem to cover
depth ranges of 15 to 20 cm, potentially over decades. Eastern Lake does display
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concurrent increases in isotopes with decreasing C/N ratios which are over smaller
depth intervals compared to Big Redfish Lake.
Perhaps these temporal variations are indicative of outlet openings of traditional
occurrences and not only associated with strong storms. These “traditional” openings
would be driven by climatic influences and not just strong storm events, and more
associated with dry or wet conditions over longer periods of time along with each lakes
morphometry. Since peaks and troughs of the data tend to cover multiple depth
intervals, this could represent decades of changes not induced by single storm events.
Oscillations in climate driven by events such as El Nino and La Nina variations could
potentially have a strong influence on the wet or dry periods of this region and may have
a strong influence on outlet opening events also influencing organic matter sources,
nutrient inputs and aquatic productivity (Elliott and Whitfield 2011).
This research project provides stimulating information to the past and present
functioning of the coastal dune lake study sites and has created a foundation for other
research endeavors to build and expand upon. However, additional research is
necessary to create a comprehensive library on these unique coastal features.
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Table 4-1. Trophic state classification (Forsberg and Ryding 1980).
Trophic State Total nitrogen (μg/L)
Total phosphorus (μg/L)
Chlorophyll (μg/L)
Secchi (m)
Oligotrophic <400 < 15 <3 >4.0
Mesotrophic 400-600 15-25 3-7 2.5-4.0 Eutrophic 600-1500 26-100 7-40 1.0-2.5 Hypereutrophic >1500 >100 >40 <1.0
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Table 4-2. Eastern and Big Redfish Lake mean long term trophic variables (CBA 2017).
Lake Total Nitrogen (μg/L)
Total Phosphorus (μg/L)
Chlorophyll (μg/L)
Secchi (m)
Big Redfish Lake 364 13 5.39 0.98 Eastern Lake 286 12 3.55 1.45
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Table 4-3. Surface and bottom physicochemical variables between Eastern and Big Redfish Lake
Variables Eastern Lake Big Redfish Lake
Dissolved Oxygen (mg/L) surface 6.77 7.79 5.55 5.98 7.36 4.49 bottom 6.33 9.5 2.4 3.91 10.12 0.13 pH Surface 7.53 7.97 7.12 7.2 7.72 6.6 Bottom 7.50 11.83 4.30 7.21 9.88 5.07 Salinity (ppt) Surface 8.23 18.5 2.0 5.14 16 0.66 Bottom 16.11 33.13 1.21 14.36 33.04 0.17 Temperature (C) Surface 22.5 25.4 17.5 23.9 30.5 21.6 Bottom 23.89 32.94 10.72 24.20 33.72 9.5
This table demonstrates water column stratification that can occur within Eastern and Big Redfish Lake’s surface and bottom measurements of water chemistry variables.
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Figure 4-1. Organic matter origins relationships between δ13C and C/N ratio (Lamb et
al., 2006).
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Figure 4-2. Eastern Lake sediment δ13C and C/N ratio relationship.
Eastern Lake δ13C and C/N ratio ranges demonstrate a lake system that is
predominantly influenced by C3 terrestrial plants, but also influenced by C3 freshwater aquatic plants and marine organic matter sources.
110
Figure 4-3. Big Redfish Lake δ13C and C/N ratio relationship.
Big Redfish Lake δ13C and C/N ratio ranges demonstrate a lake system predominantly
influenced by C3 terrestrial plants, but also by C3 freshwater aquatic plants and marine organic matter sources.
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CHAPTER 5 CONCLUSION
Through sediment core analyses, coastal dune lake environments have been
reconstructed to infer what changes they have undergone and if these changes are
significant. In addition, knowledge of what previous conditions were provided insight to
what the future conditions of these lakes may hold, as well as indicate cycles or
variations that have occurred or may occur in the future. The study site coastal dune
lakes exhibit similar trends in organic matter sources over the last 150-year period. Both
lakes displayed decreases in C/N ratios due to increases in %C and %N. This trend
indicates organic matter sources may be transitioning from mainly terrestrial sources to
aquatic organic matter sources. Higher nitrogen in C3 aquatic plants caused the C/N
ratio to decrease as its N value increased. Big Redfish Lake exhibited slightly higher
C/N ratios than Eastern Lake leading us to believe that it was more influenced by
terrestrial vegetation inputs from its watershed. Further investigations into the origins of
organic matter using stable isotopes δ13C and δ15N allowed for a more detailed
perspective of the initial results from C/N ratio organic matter sources. Eastern Lake
displayed mean stable isotope ranges within the dominant range of terrestrial and
freshwater C3 plants organic matter sources. Big Redfish Lake displayed geochemical
signatures more closely aligned with terrestrial and freshwater algae dominance. The
sediment geochemical signatures are guidelines to the influences of terrestrial and
aquatic organic matter and show variation within the sediment core for all variables. It
seems likely that the coastal dune lakes would have some marine algae organic matter
sources at times, however it would be variable, and be expected to correlate with outlet
openings or strong storm events, demonstrating the complexity of these coastal
112
systems and their interactions with both marine and freshwater and terrestrial
influences.
The paleolimnological methods utilized in this study provided information on
nutrients, organic matter sources, productivity and paleosalinity conditions of two
coastal dune lakes in Northwest Florida. These lakes are unique coastal features that
are dynamic and ever changing coastal environments that are significant on a global
and regional scale as subsets of coastal estuary-lagoon like formations. Gaining an
understanding of how environmental factors influence their natural conditions is
imperative to their future health and protection, as well as understanding such
anomalies.
Bolstering knowledge on the coastal dune lakes through reconstructing past
environmental conditions has proven to be an effective tool to gaining insight to
changes in environmental conditions and deducing potential causes. The present study
has helped to describe water chemistry, organic matter sources and origins, and the
hydrology of two coastal dune lakes found. Results provided background information on
historical changes within these systems, but due to limited published literature, there
remains a limited understanding on how these lakes function and interact with their
environment.
Both lakes exhibited surface water sample data indicating statistically significant
changes in nutrients, chlorophyll, and Secchi depth over the last two decades indicating
eutrophication. These variables are traditionally used to determine what trophic state a
lake exists in and if changes to its trophic state indicate a severe imbalance likely
caused from external sources. The surface water sample data indicates that both lakes
113
have high levels of nutrients (TN and TP) and increased levels of algal biomass
correlated to amplified available sustenance, as well as decreased water clarity. Surface
water data has only been collected for roughly 20 years and in the scale of
environmental change this is only a fraction of the lakes existence. Furthermore,
determining if these changes may be due to environmental changes with only this
dataset is subjective and limits the scale of changes incorporated into an examination of
coastal dune lake variations. Nonetheless, any consistent data is important to long term
assessments of these lakes and has led us to the paleolimnology methodology we used
in this research.
Geochemical sediment signatures have also been used to illustrate
paleoproductivity trends within a waterbody. Correlations between δ13C and algal
biomass have been used in previous studies to demonstrate inclinations in
paleoproductivity. In some cases, δ15N was used as an indicator of productivity, but can
have many limitations. Using δ13C as an indicator for the two coastal dune lake study
sites past changes in aquatic productivity were inferred. Stable isotope measurements
did not correlate with sediment TP trends and indicate that primary productivity may not
be the dominant source of organic matter to these lake systems. Furthermore,
geochemical signatures are influenced by a variety of physical, chemical and biological
factors that can alter its sediment concentration.
Paleotempestology work from previous studies on the coastal dune lakes
indicated that variations in geochemical signatures could be used to determine storm
over wash events and subsequent marine inundation. These results were used to
estimate storm events but were discovered to be useful tools to evaluate marine and
114
freshwater dominance within the coastal dune lakes. Utilizing the standards they
provide, this research projects coastal dune lake study sites were analyzed to show
variations between marine and freshwater conditions. Eastern Lake displayed shorter,
but more frequent occurrences of marine intrusions and Big Redfish Lake appears to
exhibit longer and less periods of marine dominance. These insights into coastal dune
lake hydrology and interactions with seawater are important aspects to understand and
enhance future research endeavors on them.
This research project provided stimulating information to the past and present
functioning of the coastal dune lake study sites and created a foundation of background
conditions of both study lakes for other research endeavors to build and expand upon.
Establishing historical trends in TP provides insight to nutrient conditions and can be a
platform for investigating other aspects of the lake community, such as diatom-inferred
TP values. Understanding primary producer communities within the coastal dune lakes
could help our understanding of how these lakes respond to nutrient inputs. In addition,
knowledge of the coastal dune lake outlet influences can help to fill data gaps that exist,
such as physicochemical variability and impacts on microbial communities within the
lake waters and sediments. Understanding outlet dynamics can also help in our
understanding on the influence of land use changes within their watershed and how it
may impact organic matter sources to the lakes.
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BIOGRAPHICAL SKETCH
Brandy Foley was born in Little Rock, Arkansas. After graduating from Cotter
High School in Cotter, Arkansas Brandy attended the University of West Florida in
Pensacola, Florida. She received a Bachelor of Science with a major in environmental
studies and a specialization in environmental policy in December 2009. In 2011, she
was employed as an AmeriCorps at the Northwest Florida State College in partnership
with the Choctawhatchee Basin Alliance (CBA) in Santa Rosa Beach, Florida. A year
later she was employed as a CBA program assistant and later became CBA’s
Monitoring Coordinator. In January 2016, she completed the Soil and Water Science
Department Wetland and Water Resource Management graduate certificate at the
University of Florida in Gainesville, Florida. In May 2018, she graduated with a Master
of Science in environmental science at the University of Florida.
