Upload
others
View
6
Download
0
Embed Size (px)
Citation preview
The Roles and Effects of Zebra Mussel (Dreissena
polymorpha) Distributions on Chlorophyll-a Concentrations
in the Near-bottom Regions of Eastern Lake Erie
Martin Chua Bao Jun
Supervisor: Dr. Anas Ghadouani
This dissertation is submitted in partial fulfillment for the degree of Bachelor of Engineering (Environmental) from
the School of Environmental Systems Engineering at The University of Western Australia
September 2007
I
Abstract The zebra mussel (Dreissena polymorpha) has recently invaded numerous rivers, reservoirs, and
lakes in North America. Lake Erie, of the five Laurentian Great Lakes has experienced various
ecological changes due to the invasion of the mussels. The grazing activities of the zebra mussels
since their introduction were found to have caused the declination of phytoplankton biomass and
the remarkable improvement to water clarity. However, in the past few decades, studies have
shown that the impacts of zebra mussels on the phytoplankton biomass in the deeper regions of
the central and eastern regions of Lake Erie were not well understood. This is where the present
study intends to investigate the roles of zebra mussels in depleting the chlorophyll-a
concentrations, focusing on the near-bottom regions in various locations within eastern Lake Erie.
The proposed method to achieve this objective was by utilizing a unique dataset gathered thorugh
the new spectrofluorometric method, from 14 stations sampled across eastern Lake Erie, during
the summer of 2002. The findings of the present study have suggested that depletion was found to
be of higher order in the offshore regions and in regions where mussels were gathered in high
densities. In conclusion, the zebra mussels were implied to be important agents in contributing to
the near-bottom depletion found across eastern Lake Erie and to the changes occurring within the
ecosystem. With regards to the findings, the present study hopes to have contributed as a
stepping-stone, in the broader aspects of understanding the effects of zebra mussels in depleting
chlorophyll-a concentrations in the near-bottom regions of eastern Lake Erie.
II
Acknowledgements I would like to take this grand opportunity to express my gratitude and acknowledgement to all
the people who have contributed directly and indirectly to this project. This dissertation could not
have been written without the influence of Dr. Anas Ghadouani who not only served as my
supervisor but also patiently encouraged and guided me throughout my project. His knowledge,
commitment and wisdom has inspired, and motivated me to constantly push forward. To the other
SESE faculty members who assisted and advised me through the dissertation process, I truly
thank them all.
The writing of this dissertation has been for me, one of the most significant academic challenges.
Without the support and understanding of the following people, this study would not have been
possible. It is to them that I find myself owing my deepest gratitude. To all my friends and
relatives, who spent time and effort providing support and words of encouragement, I thank you
all. To the most important people in my life; my family members and Iris who have shown
endless patience, support, and enthusiasm; who taught me about self-discipline, sacrifice and
compromise, my love goes out to all of you. Thank you for always caring and for believing in me.
III
Glossary of Terms and Acronyms Abiotic - Characterized by the absence of life or living organisms.
Algae - Simple one-celled or many-celled micro-organisms capable of carrying on photosynthesis
in aquatic ecosystems.
Anoxia/Anoxic - A condition where dissolved oxygen in the water column is totally depleted.
Basin - Geographic land area draining into a lake or river; also referred to as drainage basin or
watershed.
Benthic - Refers to being on the bottom of a lake.
Biofouling - The gradual accumulation of organisms such as algae, bacteria, barnacles, and
protozoa on underwater equipment, pipes, and surfaces, corroding and impairing
structures and systems.
Biomass - The weight of a living organism or assemblage of organisms.
Biotic - Relating to a live organism.
Chlorophyll-a - The green pigment that is responsible for a plant's ability to convert sunlight into
the chemical energy needed to fix carbon dioxide into carbohydrates.
Cyanobacteria - Bluegreen algae; phylum or organisms that are biochemically bacterial in nature
but perform plant photosynthesis.
DDT - Dichloro-diphenyl-trichloroethane - a widely used, very persistent pesticide in the
chlorinated hydrocarbon group now banned from production and use in many countries.
Diatom - Group of microscopic algae that have rigid cell walls composed of silica. They are an
important part of the food chain.
Dioecious - Having the male and female reproductive organs borne on separate individuals of the
same species.
Dissolved Oxygen (DO) - The amount of oxygen measured in the water.
EC - Environment Canada
Ecosystem - All of the interacting organisms in a defined space in association with their
interrelated physical and chemical environment.
Endocrine disruptors - Exogenous substances that interfere with the endocrine system and
disrupt the physiologic function of hormones.
EPA - Environmental Protection Agency
Epilimnion - The warm, upper layer of water that occurs in a lake during summer stratification.
IV
Eutrophication - The process by which a lake becomes rich in dissolved nutrients and deficient
in oxygen, occurring either as a natural stage in lake maturation or artificially induced by
human activities such as the addition of fertilizers and organic wastes from runoff.
Exotic Species - Species that are not native to the Great Lakes and have been intentionally
introduced or have inadvertently infiltrated the system.
Flow Rate - The rate at which water moves by a given point; in rivers it is usually measured in
cubic meters per second (m3/sec) or cubic feet per second (cfs).
Food-web / Food chain - The process by which organisms in higher trophic levels gain energy
by consuming organisms at lower trophic levels. Humans are at the highest level of many
food webs.
Forage Fish - Fish species utilized as principal food sources for major sport and commercial
fishes.
GLWQA - Great Lakes Water Quality Agreement
Heteromyaria - A division of bivalve shells, including the marine mussels, in which the two
adductor muscles are very unequal.
Hypolimnetic - Related to the layer of water below the thermocline.
Hypolimnion - The cooler, lower most layer of water in a thermally stratified lake.
Hypoxia/ Hypoxic - Deficiency in the amount of oxygen reaching body tissues.
LaMP - Lakewide Management Plan
Landuse - The primary or primary and secondary uses of land, such as cropland, woodland,
pastureland, forest, water (lakes, wetlands, streams), etc. The description of a particular
landuse should convey the dominant character of a geographic area and establish the
dominant types of human activities which are prevalent in each region.
Mesotrophic - The trophic state of a lake that is in between eutrophic and oligotrophic.
Metalimnion - The middle or transitional zone between the well mixed epilimnion and the colder
hypolimnion layers in a stratified lake. This layer contains the thermocline, but is loosely
defined depending on the shape of the temperature profile.
NALM - North American Lake Management Society
Nitrification - Bacterial metabolism in which ammonium ion (NH4+) is oxidized to nitrite (NO2-)
and then to nitrate (NO3-) in order to yield chemical energy that is used to fix carbon
dioxide into organic carbon. The process is a type of chemosynthesis which is comparable
to photosynthesis except that chemical energy rather than light energy is used. These
bacteria are aerobic and so require dissolved oxygen in order to survive.
V
NOAA - National Oceanic and Atmospheric Administration
Non-point Source - Source of pollution in which pollutants are discharged over a widespread
area or from a number of small inputs rather than from distinct, identifiable sources.
Nutrient Loading - Discharging of nutrients from the watershed (basin) into a receiving water
body (lake, stream, wetland); expressed usually as mass per unit area per unit time
(kg/ha/yr or lbs/acre/year).
Oligotrophic - The state of a poorly-nourished, unproductive lake that is commonly oxygen rich
and low in turbidity.
Oogenesis - The formation, development, and maturation of an ovum.
PAH - Polynuclear aromatic hydrocarbon
PCB - Polychlorinated biphenyl
Pelagic - Refers to the offshore open water, and often deep water, zone of a lake as opposed to
the nearshore fringe. Also referred to as the limnetic zone, in contrast to the (usually)
nearshore littoral zone which is usually defined either by the depth to which there is
sufficient light available for submersed aquatic plants to grow, or by the depth to which
light penetration is 1% of surface light irradiance.
Phytoplankton - Plant microorganisms that float in the water, such as certain algae.
Planktivores - Plankton feeding fish.
Point Source - A source of pollution that is distinct and identifiable, such as an outfall pipe from
an industrial plant.
Productivity - The conversion of sunlight and nutrients into plant material through
photosynthesis, and the subsequent conversion of this plant material into animal matter.
Regression analysis - examines the relation of a dependent variable (response) to specified
independent variables (explanatory).
Respiration - The metabolic process by which organic carbon molecules are oxidized to carbon
dioxide and water with a net release of energy. Aerobic respiration requires, and therefore
consumes, molecular oxygen (algae, weeds, zooplankton, benthic invertebrates, fish,
many bacteria, and people). Certain bacteria can use nitrate in place of oxygen
(denitrifiers) or sulfate (sulfate reducers), but only under anaerobic (anoxic) conditions -
typically present only in the sediments or in the hypolimnion after prolonged oxygen
depletion has occurred.
Re-suspension (of sediment) - The remixing of sediment particles and pollutants back into the
water by storms, currents, organisms and human activities such as dredging or shipping.
VI
Sedimentation - The removal, transport, and deposition of detached soil particles by flowing
water or wind. Accumulated organic and inorganic matter on the lake bottom. Sediment
includes decaying algae and weeds, precipitated calcium carbonate (marl), and soil and
organic matter eroded from the lake's watershed.
Sessile - Permanently attached or fixed; not free-moving.
Stratify/ Stratified/ Stratification - The tendency in deep lakes for distinct layers of water to
form as a result of vertical change in temperature and therefore in the density of water.
Substratum/ Susbtrate - Attachment surface or bottom material in which organisms can attach
or live-within; such as rock substrate or sand or muck substrate or woody debris or living
macrophytes.
Suspension - A heterogeneous mixture in which solute-like particles settle out of solvent-like
phase some time after their introduction.
t test - is any statistical hypothesis test in which the test static has a Student’s t distribution if the
null hypothesis is true.
Thermocline - A layer of water in deep lakes separating the cool hypolimnion (lower layer) from
the warm epilimnion (surface layer).
Turnover - Fall cooling and spring warming of surface water act to make density uniform
throughout the water column. This allows wind and wave action to mix the entire lake.
Mixing allows bottom waters to contact the atmosphere, raising the water's oxygen
content. However, warming may occur too rapidly in the spring for mixing to be
effective, especially in small sheltered kettle lakes.
USGS - United States Geological Survey
Water column - A conceptual column of water from lake surface to bottom sediments.
Zooplankton - animal microorganisms that float in the water.
All glossary terms and acronyms were obtained from the following sources: EPA 2007; NALM 2007; WOW 2007.
VII
Table of Contents
ABSTRACT I
ACKNOWLEDGEMENTS II
GLOSSARY OF TERMS AND ACRONYMS III
TABLE OF CONTENTS VII
LIST OF TABLES X
LIST OF FIGURES XI
CHAPTER 1: INTRODUCTION 1
1.1. DISSERTATION OBJECTIVES 2
1.2. RATIONALE 3
1.3. DISSERTATION OVERVIEW 4
CHAPTER 2: BACKGROUND AND LITERATURE REVIEW 5
2.1. STUDY SITE: LAKE ERIE 5
2.2. PHYSICAL CHARACTERISTICS 7
2.3. LAKE PROCESSES: STRATIFICATION 9
2.4. LAKE USE AND HISTORY OF ISSUES 10
2.4.1. Fish Harvest and Pollution 11
2.4.2. Eutrophication and Contaminants 11
2.4.3. Sediment Loading 12
2.4.4. Non-native Invasive Species 12
2.4.5. Other Issues Identified 13
2.5. THE INTRODUCTION OF ZEBRA MUSSELS 13
2.6. PHYSIOLOGY 15
2.7. DISPERSION OF THE MUSSELS 16
2.8. LIFE HISTORY AND MUSSEL REPRODUCTIVE BIOLOGY 16
2.9. FACTORS INFLUENCING LARVAL SETTLEMENT AND MORTALITY RATES 19
2.10. FILTER-FEEDING 20
2.11. IMPACTS OF ZEBRA MUSSELS 22
2.12. DISTRIBUTION AND ABUNDANCE OF MUSSELS IN EASTERN LAKE ERIE 25
VIII
CHAPTER 3: METHODOLOGY 29
3.1. SPECTROFLUOROMETRIC METHOD 29
3.2. DATA COLLECTION 30
3.3. EXTRACTION AND USE OF DATA 33
CHAPTER 4: RESULTS 35
4.1. THE DREISSENID DISTRIBUTION OF 2002 35
4.2. NEAR-BOTTOM CHLOROPHYLL-A DEPLETION AND TEMPERATURE DIFFERENCES 37
CHAPTER 5: DISCUSSION 45
5.1. CHLOROPHYLL-A DEPLETION 45
5.2. THE INCREASE IN BIOMASS 47
5.3. TEMPERATURE DATASET 47
5.4. LIMITATIONS AND RECOMMENDATIONS 49
5.4.1. Statistical Analysis 49
5.4.2. Sampling the Dataset 49
5.4.3. Data on Water Velocity 50
5.4.4. Phytoplankton Community Structure 50
5.4.5. Simple Modeling 50
5.4.6. Quagga Mussel Study 51
CHAPTER 6: CONCLUSION 52
REFERENCES 54
APPENDIX A: PROCESSED DATA SET 62
APPENDIX B: CHLOROPHYLL-A AND TEMPERATURE GRAPHS 69
B.1 STATION 23 69
B.2 STATION 439 70
B.3 STATION 440 71
B.4 STATION 442 73
B.5 STATION 443 74
B.6 STATION 444 76
B.7 STATION 445 77
B.8 STATION 448 78
IX
B.9 STATION 449 79
B.10 STATION 450 80
B.11 STATION 451 81
B.12 STATION 931 82
B.13 STATION 935 82
B.14 STATION 938 84
X
List of Tables Table 1: Physical characteristics and details of Lake Erie (Environment Canada & U.S. Environmental
Protection Agency 1995)................................................................................................................................ 9 Table 2: Dreissena total mean density and biomass in Lake Erie 2002 (Patterson et al. 2005)........................... 27 Table 3: Geographic detail of each of the 14 sampled sites .................................................................................... 31 Table 4: Indication of the Dreissena spp. densities in various locations across eastern Lake Erie. A rating
system was used for the indication of abundance - 5 indicating the highest and 1 being the lowest in
density. .......................................................................................................................................................... 36 Table 5: Summary of each of the sampled stations using the extracted data and preliminary findings ............ 38 Table 6: The maximum increase and decrease in chlorophyll-a concentrations found in each of the stations
including the period at which they occurred. The negative value indicates depletion while the positive
highlighted values indicate the increase in biomass.................................................................................. 46
XI
List of Figures Figure 1: A sketch showing the interaction of zebra mussels with the ecosystem in a shallow region. The solid
lines indicate the material flow and the dotted lines indicate the engineering effects (Vanderploeg et
al. 2002)........................................................................................................................................................... 1 Figure 2: A satellite image of the five Laurentian Great Lakes taken on April 2003 (Gumley 2006) .................. 6 Figure 3: A satellite image of Lake Erie taken on April 2005 (NOAA 2005) .......................................................... 6 Figure 4: Lake Erie and its three distinct basins separated by two boundaries that run from Point Pelee to
Lorain (Ohio) and from Long Point to Erie (Pennsylvania) (Conroy & Culver 2005)............................ 8 Figure 5: The bathymetry of Lake Erie showing the depth of the three main basins (Environment Canada &
U.S. Environmental Protection Agency 1995) ............................................................................................. 8 Figure 6: A sketch showing the three distinct layers in a thermally stratified lake; the epilimnion, metalimnion
and hypolimnion layers ............................................................................................................................... 10 Figure 7: A photo showing a sample of the zebra mussel (Dreissena polymorpha) (USGS 2007)........................ 14 Figure 8: A map of Northern America and the red dots on the map indicates the location of the zebra mussels
which were first discovered in Lake St. Clair back in 1988 (USGS 2007) .............................................. 14 Figure 9: By 2007 the spread of the zebra mussels has extended throughout Northern America (USGS 2007)15 Figure 10: A schematic staging of the life history events of a zebra mussel with each of the major life phases
presented in addition with the range in shell length (Ackerman 1995)................................................... 19 Figure 11: The three graphs show the recent changes of the phytoplankton community in the western region
of Lake Erie. Graph (a) gives the total phytoplankton biomass against the years. Graph (b) provides
data of the average cyanobacteria biomass over time. Graph (c) gives the percentage of cyanobacteria
over the years (Conroy & Culver 2005) ..................................................................................................... 25 Figure 12: A map showing the locations of the 107 stations sampled in Lake Erie 2002 (Patterson et al. 2005)26 Figure 13: A map showing the distribution of Dreissena in Lake Erie 2002; diagram A gives the mean density
while diagram B gives the dry tissue mass distribution (Patterson et al. 2005)...................................... 28 Figure 14: A sketch of fluoroprobe and its components (Beutler et al. 2002)....................................................... 30 Figure 15: Map of eastern Lake Erie and indication of some of the sites sampled .............................................. 32 Figure 16: A simplified sketch showing the difference between a near-bottom region with and without mussels.
The grazing effects of the mussels were expected to deplete the chlorophyll-a concentrations. Chl2
represents the chlorophyll-a concentrations at two to three meters above the mussel bed and Chl1
represents the chlorophyll-a concentrations directly above the mussel bed (roughly one meter from
the seabed). ................................................................................................................................................... 34 Figure 17: Graph shows the mean density of individuals per meter squared by substrate of Dreissena across
Lake Erie, 2002 and the error bars indicate the standard deviation (Patterson et al. 2005)................. 36 Figure 18: The chlorophyll-a differences over time at station 451. Depletion was found during June, July and
September. .................................................................................................................................................... 39 Figure 19: The chlorophyll-a differences over time at station 450. Depletion was found in July and in
September..................................................................................................................................................... 40
XII
Figure 20: The chlorophyll-a differences over time at station 449. Depletion in chlorophyll-a was found
starting May until the end of June ............................................................................................................. 40 Figure 21: The chlorophyll-a differences over time at station 448. Starting May, there were signs of depletion
in the chlorophyll-a concentrations but the levels decreased to low depletion from July to August.
Biomass was found to increase in September ............................................................................................ 41 Figure 22: The chlorophyll-a differences over time at station 938. Depletion in chlorophyll-a was evident in
June and slight depletions were found in August to September .............................................................. 41 Figure 23: The chlorophyll-a differences over time at station 444......................................................................... 42 Figure 24: The chlorophyll-a differences over time at station 931. No evidence of depletion were found and
biomass has slightly increased from May to July...................................................................................... 43 Figure 25: The temperature differences over time at station 931. Although June had temperature difference
that may indicate stratification, the lake was considered well-mixed from July to October................. 43 Figure 26: The chlorophyll-a differences over time at station 445. Other than the slight depletion detected in
May, no other significant changes were found .......................................................................................... 44 Figure 27: The chlorophyll-a differences over time at station 23........................................................................... 44
1
Chapter 1: Introduction In the recent decades, the bivalve Dreissena polymorpha (zebra mussels) have invaded many
freshwater ecosystems worldwide (USGS 2007). Their potential to reproduce at high rates and
their ability to settle on almost any solid substratum has given them an edge in invading these
systems (Mackie 1991). Zebra mussels usually out-compete the native species and cause severe
damage to waterworks (USGS 2007). There has been significant amount of concern regarding the
invasion of Dreissena polymorpha into North American waters during these past 30 years. Ever
since their introduction to the lakes and waterways of North America, a large number of studies
were performed to better understand their ecological and economical impacts. It has been widely
agreed upon and proven in some cases that the introduction of zebra mussels has brought about
various changes to the ecosystems of the Laurentian Great Lakes and their surrounding
waterways (Nalepa & Schloesser 1993; Riccardi et al. 1996; Bastviken et al. 1998; Sakai et al.
2001; Karatayev et al. 2006). Figure 1 shows an example of the interaction of the zebra mussels
with the ecosystem through their feeding mechanisms, habitat modifications, and nutrient
excretion (faeces or pseudofaeces) (Vanderploeg et al. 2002). These zebra mussels are notorious
for their biofouling capabilities and are of large ecological concern due to their filter-feeding
habits, their ability to adapt and colonize areas in high densities (USGS 2007).
Figure 1: A sketch showing the interaction of zebra mussels with the ecosystem in a shallow region. The solid
lines indicate the material flow and the dotted lines indicate the engineering effects (Vanderploeg et al. 2002)
2
Particularly in Lake Erie, one of the five Laurentian Great Lakes, the grazing activities of the
zebra mussels since their introduction were found to have caused the declination of
phytoplankton biomass (estimated by chlorophyll-a concentrations) and the remarkable
improvement to water clarity (Nicholls et al. 1999). This however, was mostly found to be true in
the shallow to nearshore regions of the western basin of Lake Erie where researchers have even
speculated that zebra mussels could strip the entire basin of algae in less than 24 hours (Bunt et
al. 1993). Also, the depletion of phytoplankton in western Lake Erie was found to be most severe
directly over the mussel beds (MacIsaac et al. 1992). Charlton et al. (1999) concluded in their
study that the impacts of zebra mussels on the phytoplankton biomass in the deeper regions of the
central and eastern regions of Lake Erie were not well understood. This is where the present study
intends to investigate the relationship between the distribution densities of zebra mussels and
chlorophyll-a depletion in the near-bottom regions of eastern Lake Erie.
In the past, techniques such as remote sensing methods were employed to study the
phytoplankton biomass across the lake (Ghadouani & Smith 2005). Each technique used had their
share of advantages and disadvantages. Most traditional methods were often found to be costly in
terms of man-hours (Beutler et al. 2002). A recent study by Ghadouani & Smith (2005), utilized a
new in situ spectrofluorometric technique to study the link between zebra mussels and the
plankton communities within Lake Erie. This method, which may lack synoptic spatial coverage
available through the remote sensing method, has the advantage of providing high spatial and
temporal resolution to phytoplankton communities (Beutler et al. 2002; Ghadouani & Smith
2005).
1.1. Dissertation Objectives
The objectives of the present study aims to utilize the data gathered from the spectrofluorometric
method to assess its reliability and to establish an understanding of the roles of zebra mussels in
depleting the chlorophyll-a concentrations, focusing on the near-bottom regions in various
locations within eastern Lake Erie. The study will dwell into the possibility of zebra mussels
being one of the main causes and not as the only reason for the depletion of the chlorophyll-a
concentrations. The other objective of the dissertation would be that if this method were to show
successful application, we can possibly predict the dynamics of particles in the water column, and
3
apply the use of this method to other regions of Northern America, namely the other Great Lakes
and various waterways.
1.2. Rationale
The study will be based on a few assumptions, one of which is that high densities of zebra
mussels will cause high depletion rates of chlorophyll-a due to their intensive filter-feeding at the
near-bottom regions of the lake (over the mussel bed). The study will look into finding
indications of depletion in chlorophyll-a by examining the difference in concentrations just above
the mussel bed (0.5 to one meter) and concentrations between two to three meters above the
mussel bed. More information on this subject can be found in the Methodology chapter of the
present report.
The results and findings from Patterson et al. (2005) will be employed into the present study to
determine the lake-wide dreissenid density distribution across Lake Erie. Zebra mussels would
also need to depend on various transport mechanisms, such as re-suspension, vertical transport,
and horizontal advection to provide them with food, due to their habitat and lack of mobility
(Edwards et al. 2005). A literature (Prins et al. 1995) found that phytoplankton depletion, due to
filter feeding activities, was influenced by water column mixing. Hence, the replenishment of
filtered water (by water currents) at a rate slower rate than the consumption of the phytoplankton
can result in the depletion of phytoplankton biomass near the mussel bed.
Therefore, another assumption is that the grazing influence of the mussels and their food particle
availability would depend on whether the water column is stratified or well-mixed. A proposed
method to determine this would be to analyze the water temperature data. If the difference in
temperature between the surface and the bottom of the water column is negligible, it would
suggest that the region is well-mixed. In this case, low levels of wind energy would be needed to
mix the lake completely. As the temperature increases, the surface water becomes less dense than
the water below. While the wind may still be able to mix the lake, eventually the upper water
layer would become too warm to mix completely with the deeper and denser water. If there is a
distinct difference in temperature, which usually occurs during warmer periods (i.e. summer), it
would then suggest that the water column is stratified and the difference in temperature has
prevented mixing from occurring.
4
1.3. Dissertation Overview
This part of the report gives the summary for each of the six main chapters. Chapter 1 begins with
the introduction and description of the project’s importance, the main objectives, and rationale.
Chapter 2 presents the literature review and background information of the project; detailing the
study site, the zebra mussel invasion history, impacts, implications, and past research. Chapter 3
describes the methods undertaken including a preliminary dataset which was chosen to
supplement the initial dataset. This section presents details on how the extracted dataset was
planned to be utilized in conjunction with the preliminary data to evaluate the objectives. Chapter
4 then presents the results which were made using the methods from the previous chapter.
Noteworthy results were presented to provide understanding to the findings. Next, the fifth
chapter is the discussion stage. This chapter aims to discuss about the findings from the previous
chapter. This chapter provides the data analysis of the present study and explanations were given
for the findings. The limitations found in the study were outlined along with the findings.
Recommendations were examined and future research works were proposed accordingly. Chapter
6 is the concluding and closing chapter of the dissertation which explains implications of the
findings, their relevance and importance to the broader research field of zebra mussels in Lake
Erie.
5
Chapter 2: Background and Literature Review In the past century, more than one hundred and fifty non-native species have been introduced
directly and indirectly into the Laurentian Great Lakes in North America (USGS 2007). Thirteen
of them have distinctively altered the lakes’ ecosystems and the Dreissena spp. (both zebra and
quagga mussels) were among the ones responsible (MacIsaac et al. 1999). The following chapter
presents vital background information and description of the study site, history of the zebra
mussel invasion into North American waters, and their subsequent implications. The primary area
of interest, as described in the Chapter 1, would be the eastern region of Lake Erie. This chapter
also reviews a range of literature (books, online resources, and journal articles) to cover and
better understand past research and significant information regarding the zebra mussels. Most of
the materials reviewed and analyzed, were aimed to aid the present report in demonstrating the
short-term and long-term influence of the mussels to their surrounding environment.
2.1. Study Site: Lake Erie
The five Laurentian Great Lakes – Superior, Michigan, Erie, Huron, and Ontario are the largest
group of freshwater lakes in the world, containing approximately 18 percent of the world
freshwater supply. They are held with great importance as physical, cultural and historical
heritage to parts of North America and Canada as they provide water for various uses including
daily consumption, transportation, power, economical purposes and more (Environment Canada
& U.S. Environmental Protection Agency 1995). However, the condition and the ecosystem of
the lakes have been deteriorating in the past few decades due to agricultural activities,
urbanization, and industrialization. In addition, due to the size of the watershed, the physical
characteristics such as soil, climate, and topography tend to differ across the basin (Environment
Canada & U.S. Environmental Protection Agency 1995). The area of particular interest in the
present study is Lake Erie which is located in the southeast section of the five lakes (Figure 2 and
Figure 3).
6
Figure 2: A satellite image of the five Laurentian Great Lakes taken on April 2003 (Gumley 2006)
Figure 3: A satellite image of Lake Erie taken on April 2005 (NOAA 2005)
Lake Erie
7
2.2. Physical Characteristics
Lake Erie is the eleventh largest lake in the world with an approximate area of 25,700 squared
kilometers (Environment Canada & U.S. Environmental Protection Agency 1995). Lake Erie is
also the warmest, shallowest, and most productive of the five Laurentian Great Lakes (Bolsenga
& Herdendorf 1993). The average depth throughout the whole lake is roughly 19 meters and
because it is the shallowest of the five Great Lakes (maximum depth of 64 meters), it warms
rapidly in summer and spring, and during long winter seasons it freezes over occasionally
(Environment Canada & U.S. Environmental Protection Agency 1995). Lake Erie, being 388
kilometers long with maximum width of 92 kilometers has the shortest retention time of the Great
Lakes (2.6 years). The eastern and central basins stratify each year and this stratification causes
impacts to the internal dynamics of the lake (Environment Canada & U.S. Environmental
Protection Agency 1995). These characteristics cause the lake to practically function as three
distinct basins separated by two ridges (Figure 4) (Bolsenga & Herdendorf 1993). The western
basin is the shallowest part, comprising about one-fifth of the lake with an average depth of seven
meters and a maximum depth of 19 meters (Richards & Baker 2002). The central section of the
lake (central basin) is a deeper region with an average depth of 18 meters. Finally the eastern
basin, which happens to be the deepest basin of the three (maximum depth of 64 meters), has an
average depth of 24 meters (Conroy & Culver 2005). Strong northeast and southwest winds set
up seiches (usually from west to east), which create the major differences in the water depth
between both ends of the lake (Toledo and Buffalo) (Hamblin, 1979). The current and wave
trends that occur in Lake Erie are highly changeable and are often related to the direction of the
wind blowing (Bolsenga & Herdendorf 1993). A major percentage of the lake’s total inflow
comes from the Detroit River, and the remainder from precipitation and other tributaries flowing
through from Ohio, Pennsylvania, New York and Michigan (Bolsenga & Herdendorf 1993). A
general comparison between the bathymetry of the entire lake can be observed in Figure 5 and a
hydrographic account of the lake can be found in Table 1.
8
Figure 4: Lake Erie and its three distinct basins separated by two boundaries that run from Point Pelee to
Lorain (Ohio) and from Long Point to Erie (Pennsylvania) (Conroy & Culver 2005)
Figure 5: The bathymetry of Lake Erie showing the depth of the three main basins (Environment Canada &
U.S. Environmental Protection Agency 1995)
9
Table 1: Physical characteristics and details of Lake Erie (Environment Canada & U.S. Environmental
Protection Agency 1995)
2.3. Lake Processes: Stratification
It was proposed in this study to investigate the state of the lake being stratified or well-mixed by
examining the temperature differences between the upper and lower water layers. This
temperature and density difference usually becomes more apparent during summer. If mixing
were to occur in a lake, the depth of which it mixes would depend on the exposure of the lake to
wind and its size. Larger lakes like Lake Superior may be well-mixed at depths ranging from 10
to 14 meters during summer, as opposed to three to seven meters in smaller or moderately-sized
lakes (Environment Canada & U.S. Environmental Protection Agency 1995).
During summer in the deeper lake regions, they generally become stratified into three identifiable
layers (refer to Figure 6). These layers are known as the epilimnion, metalimnion and
hypolimnion layers (Environment Canada & U.S. Environmental Protection Agency 1995). The
Length: 388 km Elevation: 173 m Breadth: 92 km Mean Depth: 19 m Maximum Depth: 64 m Volume: 484 km3 Water Surface Area: 25,700 km2 Total Drainage Area: 78,000 km2 Total Area: 103,700 km2
Shoreline Length (including islands): 1,402 km Outlet: Niagara River/Welland Canal Replacement Time: 2.6 years
10
upper layer, epilimnion, is usually found to be well-mixed. The highly productive epilimnion
layer is well known for its warm temperatures due to direct light penetration, and richness in
oxygen and nutrient levels. This is followed by the next layer called the metalimnion layer; also
know as the thermocline region, where the temperature declines rapidly with increasing depth.
The third layer, the hypolimnion is the bottom layer of colder water. The metalimnion acts as a
barrier that prevents mixing of the upper and lower layers for months during the summer season.
Eventually when late fall arrives, the surface waters cool, become denser and begin to descend.
This displaces deep waters and causes a mixing (turnover) of the entire lake (Environment
Canada & U.S. Environmental Protection Agency 1995). In winter, the temperature of the lower
parts of the lake approaches 4°C, while surface waters are cooled to the freezing point and
occasionally ice forms. In most cases the lakes remain mixed throughout the winter. As
temperatures and densities of deep and shallow waters change with the warming of spring,
another turnover may occur (Environment Canada & U.S. Environmental Protection Agency
1995).
Figure 6: A sketch showing the three distinct layers in a thermally stratified lake; the epilimnion, metalimnion
and hypolimnion layers
2.4. Lake Use and History of Issues
Among the five lakes, Lake Erie is the smallest lake in terms of volume (484 cubic kilometers)
and is exposed to effects from agriculture and urbanization due to the fertile soils surrounding the
lake (Environment Canada & U.S. Environmental Protection Agency 1995). The lake provides
drinking water for 17 metropolitan areas surrounding it (a total of 11.6 million) (Environment
0 10 20 30Temperature (°C)
Epilimnion
Metalimnion
Hypolimnion
Depth (m)
11
Canada & U.S. Environmental Protection Agency 1995). The lake is also used as a multi-purpose
resource for recreational activities, boating, commercial fishing, cooling water extraction, and
transport. As the use of the lake and the land surrounding the basin changed over the years, so did
the issues which concerned Lake Erie (Beeton 2002). Although certain issues were able to be
controlled or resolved in the years after their discovery, some of them re-occurred due to different
reasons (Dolan 1993). This goes to show that the significant modifications to the lake’s
ecosystem will continue with the increase in population growth and demand for the lake’s uses.
2.4.1. Fish Harvest and Pollution
Commercial over-fishing and habitat destruction due to pollution began in the late 1800s
(Environment Canada & U.S. Environmental Protection Agency 1995). By the 1880s, popular
commercial fish populations in the Lake Erie had declined. The golden days of the commercial
fishery were over by 1950s (Burns 1985). Since then, the value of the commercial fishery has
declined drastically. Due to the raw sewage discharge polluting the shoreline waters, various
drinking water intakes for the highly populated areas were relocated offshore to avoid waterborne
diseases (Environment Canada & U.S. Environmental Protection Agency 1995).
2.4.2. Eutrophication and Contaminants
With the increase of the watershed’s population (largest among the lakes) since the influence of
the European settlers in the last century, agricultural, sewage and water treatment activities has
primarily caused excessive amounts of phosphorus to be inputted into the lake (Burns 1985;
Conroy & Culver 2005). The excessive amounts of phosphorus entering the lake through the
point and non-point sources were first discovered in the 1950s (Burns 1985). The nutrient
loadings peaked during the 1960s to the 1970s (Sly 1976). Eutrophication, which started to occur
due to the extreme input of the phosphorus, has caused harmful algal blooms and hypolimnetic
hypoxia to develop in certain parts of the lake (Burns 1985; Beeton 2002). The Maumee River
contributes to most of the nutrients received in the western basin (Richards and Baker 2002).
While the central basin has a propensity to develop a hypoxic condition (dead zones due to
decomposition of organic matter) during summer seasons, the eastern basin being much deeper
than the central basin has less chance of experiencing oxygen depletion (Charlton 1980). In
addition to the stressing of the biological communities, extreme hypoxia has the ability to change
chemical processes on the bottom of the lake (Burns 1985). For example, phosphorus from the
sediments can be regenerated and recycled back into the water column at a faster rate.
12
2.4.3. Sediment Loading
Lake Erie is also the lake most subjected to sediment loading especially in the southwest Ontario
and northwest Ohio region due to intensive agricultural activities (Environment Canada & U.S.
Environmental Protection Agency 1995). Fine sediment particles which become suspended from
the bottom of the lake, due to the winds, caused ecological changes to the basin and the river
mouths of most of the tributaries (Burns 1985). During heavy storm periods, long stretches of the
Lake Erie shoreline undergo episodes of erosion. The western basin contributes most of its
sediment load into the central and eastern basins due its high turbidity (Environment Canada &
U.S. Environmental Protection Agency 1995). With raising concerns for the nutrient loadings
into the lake, an agreement between the Canadian and United States governments in the early
1970s was made to improve the water quality. After the implementation of the Great Lakes Water
Quality Agreement of 1978, phosphorus input decreased to target levels placed in the agreement
with the help of the extensive pollution control regulations, stringent water quality standards,
bans and controls of certain chemical use, improved treatment techniques, and pollution
prevention awareness (Dolan 1993). By 1980s, algal blooms, phytoplankton and zooplankton
biomass decreased. The oxygen depletion rates in the central basin went down and these changes
indicated that Lake Erie was shifting towards a more mesotrophic state (Bertram 1993).
2.4.4. Non-native Invasive Species
In the late 1970s and early 1980s, efforts to restore lake trout were interrupted with the arrival of
the non-native invasive sea lamprey. Despite the control efforts implemented in 1986, the sea
lamprey invaded most of Lake Erie and the upper Great Lakes by the early 1900s (Eshenroder &
Burnham-Curtis 1999). The arrival of zebra mussels in the late 1980s caused ecological change
by altering food web dynamics, nutrient and contaminant cycling, and energy transfer within the
lake ecosystem. With the arrival of the quagga mussel, goby, ruffe, and several zooplankton
species, the ecosystem was further complicated (Environment Canada & U.S. Environmental
Protection Agency 1995). Even though the effects of eutrophication were kept at bay with the
establishment of the Great Lakes Water Quality Agreement, the zebra mussels has caused major
disruption to the phosphorus levels. For example, at some point the phosphorus concentrations in
the eastern basin improved well below the target set, whereas, some nearshore areas had
phosphorus concentrations high enough to cause extensive algal blooms again (Environment
Canada & U.S. Environmental Protection Agency 1995). Therefore, attempts to manage the
system now by controlling the phosphorus loads was no longer a viable option. All of these
13
disturbances have resulted in changes in aquatic and terrestrial habitat. The combined result has
been the disruption of the complex communities of plants and animals resulting in loss of
biodiversity.
2.4.5. Other Issues Identified
In the 1990s changes in land use and construction of nearshore structures to protect property from
high water levels, have altered the natural habitat and beach flows along the shoreline. Wetlands
in the area have also been drained, filled and changed according to their uses until they no longer
function naturally (Conroy & Culver 2005). Another issue that was brought up in the 1990s was
the effects of endocrine disruptors such as Polychlorinated biphenyls (PCBs), Dichloro-Diphenyl-
Trichloroethane (DDT), and mercury on both the aquatic and human health (Environment Canada
& U.S. Environmental Protection Agency 1995). As a result, the research and understanding of
the internal dynamics of the ecosystem, and phosphorus cycling within the lake, are constantly
under surveillance and monitoring. Since the issues concerning the lake’s health will continue to
change over time, management decisions need to be based on the potential impacts on the overall
system. Hence, the future remedial, restoration and management actions would be provided by
the Lake Erie Lakewide Management Plan (LaMP), coordinated by federal, state and provincial
government agencies in the two countries (Environment Canada & U.S. Environmental
Protection Agency 1995).
2.5. The Introduction of Zebra Mussels
The zebra mussel, Dreissena polymoprha (Figure 7) is a freshwater bivalve native to the Caspian
and Black Seas region (Turgeon et al. 1998). Since the late 18th century, zebra mussels had been
identified as an aquatic pest in Europe (Mackie et al. 1989). The mussels, being native to Eastern
Europe, existed during the pre-industrialization period. By the early 1900s, they had extended
their population to most of the major drainages of Europe through the various canal systems
constructed back then (Mackie et al. 1989). These freshwater zebra mussels were first discovered
in North American waters in the year 1988 (Figure 8). The colonization of the Laurentian Great
Lakes started off from an established population in Lake St. Clair, which connects to Lake Huron
and Lake Erie (Mackie et al. 1989; Nalepa & Schloesser 1993). The mussels were discovered in
various eastern states in the following years since their dispersion, through man-made canals, into
the Illinois and Hudson rivers (Idrisi et al. 2001). Their rapid progression and wide-spread in the
14
past 20 years can be noted from the zebra mussel distribution maps produced by the U.S.
Geological Survey (see Figure 8 and Figure 9). The successful colonization of the North
American waters was attributed to their unique physiology, reproductive potential and adaptive
nature (Nalepa & Schloesser 1993).
Figure 7: A photo showing a sample of the zebra mussel (Dreissena polymorpha) (USGS 2007)
Figure 8: A map of Northern America and the red dots on the map indicates the location of the zebra mussels
which were first discovered in Lake St. Clair back in 1988 (USGS 2007)
15
Figure 9: By 2007 the spread of the zebra mussels has extended throughout Northern America (USGS 2007)
2.6. Physiology
Among the many evolutionary features of the zebra mussel, the retention of the byssal glands in
adult mussels has been the main reason for their heteromyarian form (their mussel-like form)
(Nuttall 1990). The tapered shape of the shell allows the mussel to attach themselves tightly to
suitable substrates providing a secure anchorage such as rocks, stones, gravel, shingle, dead
shells, man-made structures or even debris using the aforementioned byssal apparatus (Mackie
1991). Zebra mussels being highly adapted to life on hard substrates were even found attached to
living invertebrates such as crayfish and clams (Mackie 1991). The mussels were even found in
areas where 99 percent of the actual substratum is soft or muddy sediment (Berkmann et al.
1995). The vertical stability attained through the attachment to the substrate provides the mussel
with protection against natural predators and shear produced from wave actions. A study by
Clarke & McMahon (1995), found that the byssal thread production increased with higher current
velocities. However, the findings also suggested that flows above a certain velocity would
suppress the byssal thread production and in turn compromise the mussel’s ability to attach firmly
to a substrate. These findings partially account for the observation that zebra mussels in lakes are
16
most abundant in between depths of three to seven meters and relatively sparse in depths above
three meters, where they may be subjected to agitation from wave actions (Claudi & Mackie
1994).
2.7. Dispersion of the Mussels
The zebra mussels were first found to have invaded the North America region through a release
of larval mussels during the ballast exchange of transoceanic commercial ships traveling from the
Black Sea to the Great Lakes (Nichols & Kollar 1991). Their effectiveness in spreading
throughout the Great Lakes involved both the planktonic and benthic phases of the mussel life
history. Most of the dispersal methods were due to the passive drifting of the larval stage termed
‘pelagic veliger’ (Ackerman et al. 1994). The planktonic veliger stage allows the mussel to be
transported by water currents whereas the adult mussel utilizes the byssal apparatus to attach
itself to drifting materials or to boats navigating lakes and rivers. Since the invasion of the Great
Lakes, it was theorized that the mussels managed to extend their reach to connected waterways
due to barge traffic (Reed-Anderson et al. 2000). The adult zebra mussels, being hardy
organisms, are able to stay alive under humid conditions even after several days out of the water
(Ackerman et al. 1994). This gives them the edge in rapidly spreading towards many inland lakes
which are not connected to the waterways in North America through the overland trailering of
boats (recreational boating) from infested regions (USGS 2007).
2.8. Life History and Mussel Reproductive Biology
Dreissena polymorpha are dioecious, meaning that they have unisexual reproductive units with
male and female sexes, and usually they indicate a typical sex ratio of three females to two males
(Sprung 1987; Nalepa & Schloesser 1993). Zebra mussels exist as sequential, broadcast spawners
where fertilization usually occurs externally in the water column (Ackerman et al. 1994). The
number of eggs carried by an individual female was estimated to range from 30,000 to 40,000
eggs per year (Stanczykowska 1977). An adult mussel usually becomes sexually mature when it
reaches shell lengths of five to twelve millimeters, which takes about two years to mature (Nalepa
& Schloesser 1993).
17
The growth and time to maturity varies with the influence of temperature and physical
environment (Nichols 1996). Initial reports gathered from Russia, Europe and parts of Northern
America indicated that reproduction initiated when water temperatures were above 12°C (Nalepa
& Schloesser 1993). The temperatures at which the mussels grow or reproduce vary from location
to location. For example, in Poland most of the colonies experience rapid growth rates at 11°C,
whereas, populations in the Netherlands experience growth at 6°C (Stanczykowska 1977). In the
western parts of Lake Erie, populations were found to be producing larvae at temperatures of
22°C (Nichols & Kollar 1991). In the central basin, reproduction of larvae was found at
temperatures ranging from 16 to 18°C and at 12°C in the eastern basin (Nalepa & Schloesser
1993). These values indicated that reproduction began during the July to August period when
temperatures were measured to be at 15°C on average (Nichols 1996). Warmer waters provide for
a higher growth rate (development from egg to juvenile) in comparison to mussels found in
cooler waters (Nichols 1996). Since the temperature gradient of the lakes and most of the
waterways in North America are within the same region, it would be effortless for the zebra
mussels to continue to flourish and spread given the opportunity. Under natural thermal
conditions, female zebra mussels undergo oogenesis (creation of ovum) in autumn (USGS 2007).
The eggs continue developing until release and fertilization, outside the body by the males. This
process usually occurs in the spring or summer, and is dependant on water temperature (USGS
2007).
The zebra mussel larvae were first identified in the 1890s and it was not until 1901 when their
entire developmental cycle was first documented (Ackerman et al. 1994). The life cycle of a
zebra mussel involves five development stages (see Figure 10) (Ackerman et al. 1994). It begins
with the larval stage (trochophore phase) and progresses to the first veliger larval stage (D-
shaped veliger) with the development of a ciliated velum and secretion of shell material. The
velum is an organ used for feeding and swimming during the larval life. The second veliger stage
(velioconcha phase) begins after increased growth and secretion of the second larval shell. Then,
the next stage would be known as the post-veliger stage (pediveliger phase) with the development
of a foot and byssal apparatus which allows for the settling of the pediveliger onto a firm
substratum. Once the pediveliger has attached itself firmly, it undergoes a transformation from
the planktonic state to the benthis state. During this phase (plantigrade mussel phase) the mussel
would secrete an adult shell. After the metamorphosis has been completed, the mussel is usually
less than one millimeter long and is referred to as the juvenile mussel stage. During the juvenile
18
mussel stage, the mussel does not permanently attach itself to a substratum (Stanczykowska
1977). At the base of the foot that forms the byssal threads, there is a gland which produces an
enzyme used to dissolve the threads. This then allows the mussel to migrate or re-locate into
shallower waters during spring and deeper waters during fall (Stanczykowska 1977).
Growth of the mussels is generally ranging from one to two centimeters per year. Adult zebra
mussels can grow up till two to three centimeters in shell length (Mackie 1991). Figure 10 gives
the representation of the ecological location (by depth) of each stage, represented by its location
on the figure. The pelagic stages occur near the top and benthic stages on the bottom of the
figure. The larval life cycle is usually completed within a timeframe of three to four weeks and
the average adult mussel life expectancy is approximately two years (Mackie 1991). A recent
study actually showed that the zebra mussel's lifespan may vary from a minimum of 2 years to a
maximum of 8 years (Karatayev et al. 2006). The planktonic stage of a mussel also depends on
temperature, food supply and suitability of the settlement substratum. Hence, it can even take up
to ten weeks or more for the mussel to settle. Zebra mussels usually occur in high densities due to
their physical characteristics and high reproductive potential (Mackie 1991). A colony of zebra
mussels can achieve densities averaging from 50,000 to 200,000 mussels per meter squared
(MacIsaac et al. 1991). Zebra mussels also possess an exclusive ability to accumulate layer after
layer in three-dimensional colonies with thickness ranging from 20-30 individuals (Burk et al.
2002).
19
Figure 10: A schematic staging of the life history events of a zebra mussel with each of the major life phases
presented in addition with the range in shell length (Ackerman 1995)
2.9. Factors Influencing Larval Settlement and Mortality Rates
The settlement pattern which involves the distribution and density of a zebra mussel colony is
mainly tied to the quality and suitability of the substrate, and the calcium concentration (calcium
levels below 50 milligrams per liter) in the water column (Ramcharan et al. 1992). A study by
Stanczykowska (1977) concluded that the mortality rates of larvae were limited by water
currents, amount of sediment, calcium and oxygen levels in the water column. The study by
Nichols (1996) suggested that mussel larvae are even capable to adapting to conditions outside of
their usual conditions. For example, zebra mussels have shown adaptability in response to local
conditions and are reproducing rapidly in North America. The mussel larvae were able to develop
and settle in lower calcium concentrations and temperatures of less than 12°C, as compared to
their original native environment.
20
Another factor that would add to the mortality rates of the mussel larvae and sometimes even
adult zebra mussels would be predation (Kornobis 1977; Nalepa & Schloesser 1993). However,
Mackie (1991) concluded that predation is not a large factor to larvae due to the fact that their
transition stage from the pelagic stage to a safer benthic stage is rather short. A model produced
and tested in the laboratory by MacIsaac et al. (1991) suggested that cannibalism (adult mussels
predating on veligers) may be sufficient to control the population numbers. However, the
predation control would not apply to Lake Erie because some of these veligers were found to be
alive in the faeces of the adult mussels even after ingestion (Nalepa & Schloesser 1993).
2.10. Filter-feeding
The greatest difference between zebra mussels and native clams occurs in the mantle cavity
(Nichols et al. 1996). Native clams exhibit an active planktonic community within their mantle
cavity. Most of these organisms readily travel between the water column and the mantle cavity
through the siphons. On the other hand, the zebra mussels do not hold such a community. This
difference was found to be due to their filtration capacity in comparison to native clams (Nichols
et al. 1996).
Zebra mussels exist as filter feeders which have both inhalant and exhalant siphons. Veligers
were also found to filter materials, although, their impact was far less than that of sessile adults.
For example in the western region of Lake Erie, adult mussels have a grazing rate of a hundred
and three times more than that of the veligers (MacIsaac et al. 1992). Adult zebra mussels were
discovered to be capable of filtering up to one liter of water per day while feeding primarily on
algae and phytoplankton (USGS 2007). Mussels were able to filter particles smaller than one
micrometer in diameter, although they preferentially select larger particles (Sprung & Rose
1988). Other suspended materials which are filtered from the water column include, bacteria,
protozoans, other micro-zooplankton and silt. This is one of the reasons why mussels can be
found lodged in pipes, because of the constant supply of food flowing through the water columns.
One of the major effects of filter-feeding Dreissena polymorpha, is their ability to divert energy
from primary productivity. This can in turn cause a declination in grazing zooplankton, and
consequently declinations in planktivorous forage fish including fish larvae (USGS 2007). A
laboratory experiment was conducted to study and demonstrate the filter-feeding impacts of the
21
mussels (Nalepa & Schloesser 1993). The results reported a pronounced decline of approximately
46 percent in phytoplankton biomass in a pond stocked with Dreissena, in comparison to a
control pond lacking mussels.
During the first years of invasion in Lake Erie, the diatom abundance declined by 82-91 percent
(Holland 1993). Large populations of zebra mussels in the Great Lakes reduced the biomass of
phytoplankton significantly following invasion with their intensive feeding habits, particularly in
well-mixed systems where re-filtration of water is reduced. Depletion has also been observed in
various inland lakes and rivers in North America. In Oneida Lake, the chlorophyll-a
concentrations declined sharply for three years beginning the year 1990 (Idrisi et al. 2001).
During both 1991 and 1992, the Hudson River showed a decline of 90 percent corresponding
with dramatic growth of Dreissena populations (Nalepa & Schloesser 1993). Another example of
declination of phytoplankton biomass following the invasion of the zebra mussels were the
samples taken from stations in Saginaw Bay, where there was a 70 percent drop in chlorophyll-a
(Fahnenstiel et al. 1993). The consumption of phytoplankton by zebra mussels in inland lakes
was found to have low dissolved organic carbon (DOC) concentrations (Raikow 2002). A study
by Roditi et al. (2001) has shown that mussels were able to directly assimilate DOC. Therefore,
bacteria would be an important source of food.
The filtration of suspended organic and inorganic material by the mussel corresponds to the
percentage (or volume) of algal biomass removed from the water column over time (Bunt et al.
1993). This was termed as ‘clearance rate’ in various literatures (Nalepa & Schloesser 1993;
USGS 2007). Factors that influence the clearance rates are such as particle types being filtered,
time at which the mussels filter, size of the mussels, and concentration of the mussel colony
(Jorgensen 1990; USGS 2007). Their existence in high densities gives them the capacity to
drastically improve water clarity in invaded ecosystems (USGS 2007). The rate of bio-
sedimentation due to the production of pseudofaeces was very high under turbid conditions in
Lake Erie thus, supporting the fact that the introduction of the mussels has indeed increased water
clarity (Klerks et al. 1996).
Studies have also shown that water temperature generally affects the filtration rate of mussels.
For example, filtration rates in mussel colonies were always found to be low at high temperatures
(26°C) than lower ones (20°C) (Nalepa & Schloesser 1993; Payne 1997). Past research also made
22
discoveries that a fraction of phytoplankton ejected, after ingestion by the mussels, become re-
suspended in the water column (Horgan & Mills 1997). Complications arise when measurements
of clearance rates were made because only a portion of the organic particle filtered were digested.
Although so, the clearance rates can still provide a rough indicator of disturbance and impacts
caused by the filter-feeding mussels (O’Riordan et al. 1995). In conclusion, the feeding activity of
a mussel can be described from the clearance rate and faeces or pseudofaeces production after
ingestion.
2.11. Impacts of Zebra Mussels
The zebra mussels have the potential to cause various economical, and ecological related impacts
for many reasons. According to the research and data compiled by the U. S. Fish and Wildlife
Service, the mussels have the potential to cause damages and complications. This in turn, may
cost the population relying on the water use around the Great Lakes, up to five billion U. S.
dollars in the next decade (USGS 2007). An example would be that power plant and water users
around the Great Lakes have spent up to millions of dollars cleaning out and retrofitting devices
to prevent the zebra mussels from disrupting their facilities. At one point, a power plant in
Michigan was reported to have zebra mussel densities as high as 700,000 individuals per squared
meter, clogging the pipes and reducing the output of the facilities to one-third its optimum rate
(USGS 2007). Since the early 1900s, a large number of studies were also focused on
understanding the ecological impacts and spread of the mussel invasion in North American lakes
and waterways (MacIsaac et al. 1992; Bunt et al. 1993; Nalepa & Schloesser 1993; MacIsaac
1996; MacIsaac et al. 1999; Idrisi et al. 2001; Sakai et al. 2001). It was the general consensus of
the studies that the implications of the zebra mussels will not be fully understood unless
understanding of specific processes were first made.
The introduction of mussels affected the environment, and altered the benthic habitat complexity.
The changes included the transformation of abundance and assortment of the native benthic
species (Mackie 1991; Nalepa & Schloesser 1993). For example, the endemic unionid mussels
(native species), which are an important component of North American freshwater biodiversity,
has been on a decline. Structural complexity of the mussel beds also caused changes to the
substrate roughness by increasing the mixing in the benthic boundary layers (Commito &
Rusignuolo 2000). Since the invasion of the zebra mussels into the Great Lakes, most research
23
has focused on their impact on the phytoplankton communities due to their high filter-feeding
capabilities (MacIsaac et al. 1992; Bunt et al. 1993). The transport of food from the pelagic to
benthic zone is the key factor to the ability of the zebra mussels to consume the phytoplankton in
the water column (Noonburg et al. 2003). Conroy & Culver (2005) suggested that filter-feeding
and nutrient excretion provides for a more direct impact unto Lake Erie’s ecosystem. Mussels can
enhance a benthic community by providing a more complex habitat substratum or by organically
enriching sediment microhabitat. On the flip side, the mussel can degrade a community by
forcing competition, anoxia, and destabilization of the sediment through filter-feeding (USGS
2007).
There were also various other research performed on the role of the mussels in affecting the
nutrient budgets (mainly phosphorus and nitrogen) with high turnover rates and contaminant
cycling (Arnott & Vanni 1996; MacIsaac 1996; Conroy et al. 2005). Mussel beds can process
nutrients through their own metabolism and through bacterial processes. Bacteria in sediments
can cause mineralization of organic nitrogen into ammonia (nitrification to nitrate) (Dame et al.
1991). It was found in a study (Bruner et al. 1994) that zebra mussels were able to negatively
impact the food-web (of other benthic organisms) by accumulating and cycling concentrations of
polycyclic aromatic hydrocarbons (PAH), PCBs and contaminant metals into the sediments. They
deposit these pollutants as loose pellets of mucous mixed with particulate matter (pseudofaeces)
that they filter from the water. Upon consumption of these pseudofaeces, the animals may pass
these harmful pollutants up the food-web. Other studies provided data that the invasive mussels
made food-web interactions between groups of species, not usually connected, possible (Holland
1993; Strayer et al. 1999).
Studies related to the nutrient budgets, have concluded that cyanobacteria growth and blooms
were favorable in low nitrogen to phosphorus ratios, both of which were found to be of
significant amounts in mussel excrement (Smith & Benett 1999; Conroy et al. 2005). When the
nutrient controls were introduced back in the late 1970s, the frequency and duration of the
cyanobacteria blooms, and the phytoplankton biomass decreased (see Figure 11 graph (a)).
However, recently both the biomass started to increase even with the controlled nutrient input
(see Figure 11 graph (a) & (b)). Graph (c) in Figure 11 distinctively indicated that in the recent
years, algal blooms re-occurred and the water quality in the western basin began to decline. This
indicated that a nutrient cycling process was coming from within the lake and the invasion of the
24
zebra mussels was a possible cause (Conroy et al. 2005). Thus, zebra mussels are also considered
a source of nutrients, particularly ammonia-nitrogen and phosphate-phosphorus, because the
excreted waste materials are taken in as food products by algae.
In the past few decades, various models were made to attempt in some way to predict the benthic-
pelagic coupling, hydrodynamics, nutrient cycling, and food-web interactions affected by zebra
mussels (Ramcharan et al. 1992; MacIsaac et al. 1999; Reed-Anderson et al. 2000; Ackerman et
al. 2001; Conroy & Culver 2005; Strayer & Malcom 2006). Most of these models were used to
mathematically examine specific locations, mussel population, and were governed by precise
applications, limiting them to only applicable in unique situations. For example, biological
models which utilize bioenergetics modeling would have to assume that zebra mussels feed
throughout the water column due to the lack of a hydrodynamic role. This, as mentioned by
Ackerman et al. (2001), would hold true in well-mixed systems but in the majority cases, zebra
mussels will have limited access to the total water column (MacIsaac et al. 1999). This supports
the argument that vertical density gradients do impact the ecosystem in the water column.
Therefore, models utilizing the bioenegetic approach would be suitable in unstratified lakes such
as Lake Mendota (well-mixed lakes).
Even with all the research and models performed in the past century, determining and pin-
pointing the impacts of zebra mussels on the Lake Erie or any large lake ecosystem is a difficult
task (Sakai et al. 2001). The Lake Erie ecosystem, even before the introduction of zebra mussels,
was already extremely complex to begin with (Conroy & Culver 2005). Consequently, predicting
the short-term state of the lake in the future would be possible, as there is every hint that
ecological impacts will continue to be forced by zebra mussels, but predicting the long-term state
would remain as a complex task.
25
Figure 11: The three graphs show the recent changes of the phytoplankton community in the western region
of Lake Erie. Graph (a) gives the total phytoplankton biomass against the years. Graph (b) provides data of
the average cyanobacteria biomass over time. Graph (c) gives the percentage of cyanobacteria over the years
(Conroy & Culver 2005)
2.12. Distribution and Abundance of Mussels in Eastern Lake Erie
Dreissenids can attain high densities in aquatic systems. Biomass values for dreissenids in the
Great Lakes vary depending on the habitat (Wilson & Sarnelle 2002). There has been rapid
colonization of zebra mussels on hard substrates in the Great Lakes followed by slower
colonization on softer substrates (Wilson & Sarnelle 2002). It was also discovered that zebra
mussel distribution on soft sediments can be affected by wave action and depth-related
production. For example in Lake Michigan zebra mussels on sandy substrates between depths of
15 to 30 meters and between 30 to 50 meters, benefit from low wave energy and the deep
chlorophyll layer that intersects the bottom at these depths (Wilson & Sarnelle 2002).
26
Due to the nature of the present study, it was important to gather records on the abundance of
invasive dreissenids in the eastern region of Lake Erie to supplement the available dataset. The
study by Patterson et al. (2005) was deemed the most suitable resource as they managed to
perform a lake-wide benthic survey of Lake Erie during summer 2002 (Figure 12).
Figure 12: A map showing the locations of the 107 stations sampled in Lake Erie 2002 (Patterson et al. 2005)
Dreissenids were found to be present in 57 of the 107 sites surveyed in 2002. The total lake-wide
density and dry biomass of the dreissenids, were found to be 2,025 ± 5,665 individuals per meter
squared and 24.7 ± 71.3 grams of dry tissue per meter squared respectively. The data of the total
mean density and the total mean biomass for each of the basins was tabulated into Table 2.
Densities of zebra mussels were greatest at depths of four to eight meters lake-wide. Dreissenids
were found to be of highest abundance in the eastern region (almost 90 percent of total mean
density) as compared to the west and central basins (see Figure 13). A reason for the difference in
mussel abundance among basins would be due to differences in substratum (Patterson et al.
2005). All sites sampled in the east basin at depths equal or less than 10 meters were on bedrock,
boulders, or cobbles (suitable substratum for colonies to flourish). In conclusion, the mean
densities of dreissenids were found to be much greater in the northern region as compared to the
southern region of the eastern basin (Figure 13 – diagram A). The results of the surveys in 2002
27
also suggested that the distribution and size structure of dreissenid populations in Lake Erie is
still in an evolving stage.
Table 2: Dreissena total mean density and biomass in Lake Erie 2002 (Patterson et al. 2005)
Basin Location
Depth (m)
Number of sites
Sampled
Total Mean Density
(Individuals m2)
Standard Deviation
(Individuals m2)
Total Mean Biomass
(Grams m2)
Standard Deviation
(Grams m2)
0 - 4 4 180.0 303.6 2.4 4.8
4 - 8 15 1,253.2 3564.2 11.1 28.2
8 -15 30 331.2 953.7 3.6 14.4
15 - 24 - - - - -
West
> 24 - - - - -
0 - 4 - - - - -
4 - 8 11 983.1 1628.4 18.8 34.5
8 -15 13 1,047.2 1613.6 28.6 48.8
15 - 24 7 230.7 249.5 0.7 1.3
Central
> 24 - - - - -
0 - 4 3 11,002.6 4,309.1 58.1 44.1
4 - 8 3 3,816.1 1,103.4 37.4 24.9
8 -15 4 8,218.9 8,471.6 108.3 128.3
15 - 24 4 17,835.3 20,375.1 233.9 250.2
East
> 24 3 4,167.5 3,438.8 38.6 29.9
28
Figure 13: A map showing the distribution of Dreissena in Lake Erie 2002; diagram A gives the mean density
while diagram B gives the dry tissue mass distribution (Patterson et al. 2005)
29
Chapter 3: Methodology This chapter of the report goes through the methods used to collect the data for evaluation. The
spectroflourometric method and the data collection process are explained in more detail in this
section. As mentioned previously, the representation of the mussel density distribution estimates
were taken from Patterson et al. (2005), where the data was based on the data gathered during the
summer of 2002. The relevant results and information regarding their findings can be found in
Chapter 4.
3.1. Spectrofluorometric method
The fluoroprobe used in the data collection process is a submersible spectrofluorometer that
measures photosynthetic pigment fluorescence (Beutler et al. 2002). The measurement of
chlorophyll-a concentration provides a reasonable estimate of phytoplankton biomass. Figure 14
shows the different electronic and optical components enclosed within the stainless steel housing.
The pressure sensor at the bottom of the probe measures the actual water depth and the rest of the
data collected from the probe is sent to a connected computer for processing (Beutler et al. 2002).
The flouroprobe, used to assess phytoplankton communities, measures at a rate of approximately
0.1 meters per second at every second for high resolution profiling of the water column (Beutler
et al. 2002). This apparatus allows the measurement of chlorophyll-a fluorescence after each 0.1
meters per second excitation by five light emitting diodes; at wavelengths of 450, 525, 570, 590,
and 610 nanometers (Beutler et al. 2002; Ghadouani & Smith 2005). Each of these five
wavelengths provide a measure of fluorescence associated with differing algal groups such as
chlorophtes, cyanobacteria, diatoms, euglenophytes, etc. (Ghadouani & Smith 2005). However in
this study particularly, the fluorescence captured was transformed through an algorithm to obtain
the total biomass of phytoplankton. The total biomass was expressed in equivalence to
micrograms of chlorophyll-a per liter. This algorithm, which was made available with the probe,
was generated along with a few other algorithms by a research team from Max Planck Institute
(Germany) to produce the signature of differing algal groups (Beutler et al. 2002). In the recent
years, the spectrofluorometric device has demonstrated successful applications in large lakes to
detect deep chlorophyll maxima dominated by toxic cyanobacterial species (Ghadouani & Smith
30
2005). Detailed explanations on the fluoroprobe can be found in the studies conducted by Beutler
et al. (2002; 2003).
Figure 14: A sketch of fluoroprobe and its components (Beutler et al. 2002)
3.2. Data Collection
The present study utilized and considered a unique set of data which was collected back in the
summer of the year 2002. Both nearshore and offshore samples were taken during seven
consecutive cruises abroad the Environment Canada R/V Limnos from April to October. In total,
14 stations were sampled across the eastern region of Lake Erie; samples from site 23, 439, 440,
442, 443, 444, 445, 448, 449, 450, 451, 931, 935, and 938 (Figure 15). The samples were
considered nearshore if their depth ranged between zero to 15 meters. Any depth below 15 meters
would be considered offshore samples. The number of nearshore to offshore samples was an
equal seven to seven ratio. The geographic details and the location for each of the sample sites are
shown in Table 3.
31
In order to take measurements of the temperature along the vertical profile, the probe was out-
fitted with a thermistor manufactured by BBE-Moldaenke, Germany (Ghadouani & Smith 2005).
This thermistor was claimed by its manufacturer to be able to measure up to an accuracy of
0.05°C. The fluoroprobe was deployed three times in each of the stations to obtain measurements
of chlorophyll-a fluorescence and temperature along the vertical depth profile. More information
on the methods used in the laboratory in obtaining the quantified chlorophyll-a data from the
collected samples can be found in the study by Ghadouani & Smith (2005).
Table 3: Geographic detail of each of the 14 sampled sites
Station Latitude Longitude General Location Details Depth Range (m)
23 42.5017 -79.8897 West Central Offshore 50 - 80
439 42.7469 -79.0722 East Offshore 15 - 20
440 42.8531 -79.1664 North East Nearshore 10 - 15
442 42.8417 -79.3931 North Nearshore 10 - 15
443 42.6867 -79.2764 East Offshore 20 - 30
444 42.5433 -79.2517 South Nearshore 10 - 15
445 42.8019 -79.6994 North Nearshore 10 - 15
448 42.2953 -79.7136 South Nearshore 10 - 15
449 42.7672 -79.9708 North West Nearshore 6 - 10
450 42.6969 -79.9500 North West Offshore 20 - 30
451 42.6483 -79.8903 West Central Offshore 30 - 50
931 42.8497 -78.9417 North East Nearshore 6 - 10
935 42.5917 -79.4661 Central Offshore 30 - 50
938 42.6333 -80.0581 West Offshore 30 - 50
32
Figure 15: Map of eastern Lake Erie and indication of some of the sites sampled
33
3.3. Extraction and Use of Data
The dataset collected during the cruises in 2002 was examined, processed, and the sections that
were needed for the present study was extracted and tabulated into Appendix A. The data that
was extracted included, the chlorophyll-a measured in micrograms per liter from the near-bottom
regions of the lake (just above mussel beds), the chlorophyll-a measured two to three meters
above the bed, the temperatures from both the surface and the bottom of the lake.
To investigate the possible depletion of phytoplankton concentration in the near-bottom regions,
the differences in chlorophyll-a concentration between two to three meters (represented as Chl2),
and approximately half to one meter above the bed (just above mussel bed and represented as
Chl1) was examined. Figure 16 below shows with a simplified sketch on how the chlorophyll-a
measurements can provide an indication of depletion. Depletion would be indicated when: [Chl1]
- [Chl2] gives a negative value. If a positive value is to be found, this would indicate the
restoration or addition of phytoplankton biomass.
The temperature data, on the other hand, was used to determine if the specific location in the lake
was stratified or vice versa. As mentioned in the previous chapters, if the surface temperature is
higher than that of the near-bottom temperature, the lake was assumed to be stratified. If the
difference between these two locations were insignificant, then the lake would be assumed to be
well-mixed. Unfortunately, some sections of the extracted data (notably the August period in
most sampled stations) had gaps in them due to the absence of raw data from the cruise
originally. This may, although not to a great extent, affect the final results.
34
Figure 16: A simplified sketch showing the difference between a near-bottom region with and without mussels.
The grazing effects of the mussels were expected to deplete the chlorophyll-a concentrations. Chl2 represents
the chlorophyll-a concentrations at two to three meters above the mussel bed and Chl1 represents the
chlorophyll-a concentrations directly above the mussel bed (roughly one meter from the seabed).
Chlorophyll-a (μg/L) Depth (m) Chlorophyll-a (μg/L) 2 – 3
meters above the
b d
Near-bottom
Depth (m) 2 - 3 meters above the
bed
Near-bottom
Depth
Profile (m)
Depth (m)
No mussels gathered Mussels gathered in high densities
Near-bottom region
Water Surface
Phytoplankton rich water column
No Feeding occurring Feeding occurring
Chlorophyll-a two to three meters above the mussel bed (Chl2)
Chlorophyll-a directly above bed (Chl1)
No Depletion Found Depletion Found
35
Chapter 4: Results
This chapter contains three main elements which present the significant results from the
distribution of the mussel densities, the chlorophyll-a depletion near the bed, and the temperature
differences between the surface and bottom of the lake. Although the initial research regarding
the densities of dreissenids in Lake Erie can be found in Chapter 2, this section aims to detail out
the specifics that were used in the present report as preliminary findings. Each of these elements
is important in achieving the objectives stated in Chapter 1 and also mainly for investigating the
near-bottom depletion of chlorophyll-a, which may be attributed to the filter-feeding of dense
mussel colonies.
4.1. The Dreissenid Distribution of 2002 With regards to the distribution of the Dreissena spp. in the study by Patterson et al. (2005), it
was concluded from their dataset that the densities of zebra mussels were greatest at depths
between four to eight meters while maximum densities of quagga mussels were only found to be
dominant at depths of 15 – 24 meters. In their study, the analysis of the transect-specific northern
shore data indicated that both the species were found to be considerably more abundant at depths
between five to ten meters, than at the depth of two meters.
With regards to the abundance of the mussels on differing substratum, it was found that both
zebra and quagga mussels preferred coarse or hard substrates to sandy or muddy substrates, in the
eastern basin of Lake Erie (Figure 17). Among the two dreissenid species, no significant
differences in density of zebra mussels were found with respect to substrate texture, whereas
quagga mussels were found to occur in significantly greater numbers on hard substrates
(Patterson et al. 2005).
Table 4 represents the essential information extracted on distribution and abundance of the
mussels in eastern Lake Erie. It is worth noting that the density values were based on a
combination of both the zebra and quagga mussels. In essence this should not affect the final
results because it has been found in recent literatures that the Dreissena bugensis (quagga mussel)
have the capability to filter at the same rates or even higher than that of zebra mussels (Baldwin
et al. 2002; Stoeckmann 2003). As a result, the clearance rates should not be affected and may in
fact even increase. The results in Table 4 indicated that total mean densities were found to be
36
highest in the northern nearshore regions of eastern Lake Erie. The deep basins showed signs of
moderate amounts of mussels ranging from 3,000 – 10,000 individuals per squared meter. The
southern region of eastern Lake Erie, regardless of being offshore or nearshore, indicated the
lowest of mussel densities (1-500 individuals per squared meter). These data that were collected
during the summer of 2002 did not study or quantify the abundance of the dreissenid mussels in
the eastern region of east Lake Erie. However, it is a reasonable and logical assumption that any
location nearshore of the northern regions of Lake Erie’s eastern basin, is of higher abundance in
total mussel densities than that of the southern regions.
Figure 17: Graph shows the mean density of individuals per meter squared by substrate of Dreissena across
Lake Erie, 2002 and the error bars indicate the standard deviation (Patterson et al. 2005)
Table 4: Indication of the Dreissena spp. densities in various locations across eastern Lake Erie. A rating
system was used for the indication of abundance - 5 indicating the highest and 1 being the lowest in density.
General Location Details Depth (m) Densities of Dreissenid
(Individuals m2) Indication of Abundance
Nearshore North West < 15 > 20,000 5
Nearshore North < 15 > 20,000 5
Offshore North 15 - 24 10,000 - 20,000 4
Offshore North West 24 - 40 3,000 - 10,000 3
Offshore Central > 40 3,000 - 10,000 3
Offshore Cental South 24 - 40 500 - 1,500 2
Offshore South 15 - 24 1- 500 1
37
4.2. Near-bottom Chlorophyll-a Depletion and Temperature Differences
According to the extracted results in Appendix A, a large portion of the sampled stations
indicated the existence of near-bottom depletion. From each of these stations three graphs were
produced to show: 1) chlorophyll-a concentrations over time, 2) the temperature difference over
time, and 3) a combined graph indicating the difference in chlorophyll-a concentrations and
temperature over time. The more meaningful graphs were highlighted below and discussed in
more detail while the remaining graphs were included in Appendix B for reference and
comparison purposes. The expected trend to identify here would be that high mussel densities
would cause high chlorophyll-a depletions. For the locations of the stations, refer to Figure 15.
Table 5 gives a detailed summary of each the sampled stations; details regarding the stations that
followed the expected trends, the maximum depletion in percentage for each station, the months
where depletion was found, the near-bottom temperature during the depletion, the state of the
lake (stratified or well-mixed), and the indication of the mussel abundance based on the
preliminary findings (refer to Table 4). The maximum depletion in percentage for each station
was included in the findings (Table 5) to better indicate the depletion of chlorophyll-a as the
percentage demonstrates how much of the chlorophyll-a has been removed from the water
column. Also, the reason behind the missing data in the column “Indicating the Mussel
Abundance” was a result of these stations not being covered in the preliminary dataset. Therefore,
the assumptions made can only apply to those areas. For example, station 440 located on the
North Eastern section of the east basin would be assumed to have high mussel densities (>20,000
individuals per squared meter).
Results from the deep offshore regions showed large differences in temperature with magnitudes
of at least 10°C. This shows that during the July to August period, the lake would commonly
experience thermal stratification. Results have also suggested that depletion would occur during
the periods when the lake stratifies (particularly during the month of June). Whenever signs of
depletion were present, the temperature above the mussel bed was found to be within the range of
7°C to 14°C. There were a total of four noteworthy occurrences (station 439, 440, 443 and 448)
where it appears that biomass have increased compellingly in concentrations. In general, the
results from most sites have suggested that high densities of mussels may have depleted the
chlorophyll-a concentrations. However, a minority of results have also shown that some of the
locations did not follow the expected trend.
38
Table 5: Summary of each of the sampled stations using the extracted data and preliminary findings
Followed Expected Trends General Location StationDepletion Period Depletion Occurred
Temperature above Mussel
Bed (°C)
Highest Percentage
Depleted Stratified
Indication of Mussel
Abundance
West Central 451 June - October 12.24 51.78% 3 North West 450 June & Septmber 9.53 56.54% 3
West Offshore
938 Yes
June 7.48 53.47% Yes
3
North 442 June 12.5 53.80% 5 North West 449*
Yes May - June & September 11.2 41.21% 5
Yes
South Nearshore
448 Yes** May 7.7 34.65% Yes
1
North East Nearshore 440 June 13.4 22.64% N/A (5)*** East 439 May - June & September 11.2 63.26% N/A (4) East
Offshore443
Yes June & September 10.08 52.53%
Yes N/A (3)
Yes
Central Offshore 935 No - - - Yes N/A (2)
North East 931 - - - No N/A (5) North
Nearshore445 - - - 5
West Central Offshore 23 No
- - - Yes
3
No
South Nearshore 444 Yes June 11.6 27.21% Yes**** N/A (1)
N/A(#)* - These indicated that preliminary data was unavailable thus, assumed values were given
Station 449* - An extreme increase in biomass percentage was detected (940%)
Yes** - Other periods showed no significant depletion
Yes**** - Lake region was well-mixed in other months
39
In six of the observed offshore and nearshore stations – 442, 448, 449, 450, 451, and 938, results
have shown that depletion has occurred. These areas also suggested having moderate to high
mussel densities. Station 451 had three incidents where depletion was evident (Figure 18) and
station 450 showed depletion on two occasions (Figure 19). The data from station 449 showed
that while depletion occurred three times during summer, an extreme increase in biomass
percentage was found to have occurred (940% increase) (Figure 20). However, in terms of actual
chlorophyll-a concentrations, the levels were not alarming. Station 448 had an incident of low
depletion which occurred in May. Mussel abundance in this area was suggested to be low as well.
During the other months, no other signs of depletion were noted although in September there was
an increase in biomass (Figure 21). This station was in accordance with the trend suggesting that
low mussel density would cause less depletion in the water column. Station 938 showed a
depletion in a region where mussels have been suggested to be in high abundance (Figure 22).
-1.5
-1
-0.5
0
0.5
1
1.5
2
2.5
3
April May June July August Sept Oct
Time (months)
Chl
-a C
once
ntra
tions
(μg/
L)
Bottom: Chl1Near-bottom: Chl2Difference
Figure 18: The chlorophyll-a differences over time at station 451. Depletion was found during June, July and
September.
40
-2-1.5
-1-0.5
00.5
11.5
22.5
3
April May June July August Sept Oct
Time (months)
Chl
-a C
once
ntra
tions
(μg/
L)
Bottom: Chl1Near-bottom: Chl2Difference
Figure 19: The chlorophyll-a differences over time at station 450. Depletion was found in July and in
September
-2
-1
0
1
2
3
4
5
April May June July August Sept Oct
Time (months)
Chl
-a C
once
ntra
tions
(μg/
L)
Bottom: Chl1Near-bottom: Chl2Difference
Figure 20: The chlorophyll-a differences over time at station 449. Depletion in chlorophyll-a was found
starting May until the end of June
41
-1.5
-1
-0.5
0
0.5
1
1.5
2
2.5
3
April May June July August Sept Oct
Time (months)
Chl
-a C
once
ntra
tions
(μg/
L)
Bottom: Chl1Near-bottom: Chl2Difference
Figure 21: The chlorophyll-a differences over time at station 448. Starting May, there were signs of depletion
in the chlorophyll-a concentrations but the levels decreased to low depletion from July to August. Biomass was
found to increase in September
-1.5
-1
-0.5
0
0.5
1
1.5
2
2.5
April May June July August Sept Oct
Time (months)
Chl
-a C
once
ntra
tions
(μg/
L)
Bottom: Chl1Near-bottom: Chl2Difference
Figure 22: The chlorophyll-a differences over time at station 938. Depletion in chlorophyll-a was evident in June and slight depletions were found in August to September
42
There were a number of stations (stations 439, 440, 443, 444, and 935) which also showed
evidence suggesting chlorophyll-a depletion during the summer months. Unlike the results above,
these findings lacked preliminary data regarding the mussel abundance and thus, assumptions
could only be made about them. Station 440 which showed depletion was assumed to have a
density of 20,000 individuals per squared meter. Station 439 and 443 showed that depletion took
place more than once. In one instance, station 443 had an increase of biomass by 80%. Both
stations 444 (Figure 23) and 935 which were assumed to have very low densities of mussels, had
low depletion of chlorophyll-a as well. Nevertheless, the results from these stations have shown
important results that strongly suggest that mussels may have caused the depletion of the
chlorophyll-a concentrations.
-1
-0.5
0
0.5
1
1.5
2
2.5
3
3.5
April May June July August Sept Oct
Time (months)
Chl
-a C
once
ntra
tions
(μg/
L)
Bottom: Chl1Near-bottom: Chl2Difference
Figure 23: The chlorophyll-a differences over time at station 444
Four out of the 14 stations (stations 931, 445, 23, and 444) sampled, had results which were
unexpected and even contradicting to some extent. The station 931 is one of the two shallowest
stations from the entire dataset (depth of six to ten meters) and is located at the north eastern
region of the eastern basin. There was, however, no compelling evidence of depletion found
(Figure 24). Initially, the region stratified in June and eventually became well-mixed in the
following months (Figure 25). Station 445 which was assumed to have high abundance of
mussels in the near-bottom region did not produce any significant depletion (Figure 26).
43
Similarly the deepest station of the entire dataset, station 23, was also expected to show high
depletion over the near-bottom region but this was not the case (Figure 27).
-1.000
0.000
1.000
2.000
3.000
4.000
5.000
6.000
April May June July August Sept Oct
Time (months)
Chl
-a C
once
ntra
tions
(μg/
L)
Bottom: Chl1Near-bottom: Chl2Difference
Figure 24: The chlorophyll-a differences over time at station 931. No evidence of depletion were found and
biomass has slightly increased from May to July
-5.000
0.000
5.000
10.000
15.000
20.000
25.000
April May June July August Sept Oct
Time (months)
Tem
pera
ture
(°C
)
BottomSurfaceDifference
Figure 25: The temperature differences over time at station 931. Although June had temperature difference
that may indicate stratification, the lake was considered well-mixed from July to October
44
-1-0.5
00.5
11.5
22.5
33.5
4
April May June July August Sept Oct
Time (months)
Chl
-a C
once
ntra
tions
(μg/
L)
Bottom: Chl1Near-bottom: Chl2Difference
Figure 26: The chlorophyll-a differences over time at station 445. Other than the slight depletion detected in
May, no other significant changes were found
-0.6-0.4-0.2
00.20.40.60.8
11.21.4
April May June July August Sept Oct
Time (months)
Chl
-a C
once
ntra
tions
(μg/
L)
Bottom: Chl1Near-bottom: Chl2Difference
Figure 27: The chlorophyll-a differences over time at station 23
45
Chapter 5: Discussion This section begins by discussing the results from the previous chapter. Explanations were
offered to suggest the implications of the findings. The limitations section describes the
constraints and sources of errors found during the course of the present study. Recommendations
were also prepared to discuss and suggest future works which can assist this project in improving
and continuing into the next phase.
5.1. Chlorophyll-a Depletion
Evidence of zones experiencing chlorophyll-a depletion were found in the near-bottom regions of
most of the sampled stations. Under these circumstances, these findings inclined towards the
trends observed in the study by MacIsaac et al. (1999). The depleted zones have shown hints that
the presence of dreissenids may be one of the main causes. Unfortunately the present study
cannot provide or show this relationship to be statistically significant due to the lack of a
statistical analysis. The results analyzed indicated that depletion of the chlorophyll-a was mostly
occurring during the month of June (Table 6). Other occurrences of depletion were during May,
July and September. Therefore, it was deduced that the stations experience depletion starting
early to mid-summer. The highest depletion detected were in stations 439, 442, and 443 (refer to
Appendix B.2, B.5, and B.6). Sampled stations 444 and 448 were the only stations located in the
southern parts of the basin. The low chlorophyll-a depletion results from these stations also tend
to agree with the abovementioned pattern. Their mussel density according to the preliminary data
was within the range of one to 500 individuals per squared meter (very low abundance).
Three stations in particular, did not show the expected signs of chlorophyll-a depletion. Station
23, an offshore sampled site, indicated minimal depletion although high abundance of mussels
was suggested in the preliminary dataset. This phenomenon was unexpected and there were no
explanations found regarding this result. The other two nearshore stations 445 and 931 were
assumed to have high depletion but showed otherwise. A justification for this could be due to the
temperatures at the near-bottom region. In both sites the temperatures were found to be high (at
least 18°C to 23°C) and were deemed unfavorable for the mussels, with regards to prior research
performed in the literature review (see Chapter 2). Another possible explanation for this was
46
suggested by a study performed by Higgins (2005) where he suggested that mussels have limited
access to the shallow water column (six meters approximately) during much of the summer in the
nearshore areas of the eastern basin. This was due to the dense growths of Cladophora glomerata
that covered most of the hard substrates from May to August.
Table 6: The maximum increase and decrease in chlorophyll-a concentrations found in each of the stations
including the period at which they occurred. The negative value indicates depletion while the positive
highlighted values indicate the increase in biomass
Chlorophyll-a Concentrations Depleted (μg/L)
Stations May June July September
439 -3.78 +0.45 440 -0.77 +0.64
442 -2.55 443 -3.22 +0.95 448 -0.88 +0.50
449 -1.36
450 -1.47
451 -1.02
938 -1.12
In general, the depletion of chlorophyll-a was found to be much higher in offshore stations as
compared to their nearshore counterparts. Also, some of the sampled stations showed signs of
moderate to high depletion even when preliminary data showed low abundance of mussels. An
explanation offered by Yu and Culver (1999) was that, the in situ filtration efficiency of
dreissenids in dense communities was actually compromised when compared to individuals
filtering in less crowded and better circulated water columns. Therefore, considering a stratified
water column, the lower dreissenid abundance may cause an increase in filtering efficiency.
Other previous studies (Ackerman 1999; Burks et al. 2002) have found suggestions that less
dense colonies may have clearance rates equal to that of denser colonies. This was found to be
caused by the size and age difference within the colony. It is undeniable that larger mussels were
found to filter at a higher rate (Ackerman 1999). However, smaller-sized mussels were found to
be able to position themselves to better capitalize on their food intake when compared to larger
mussels. Less dense colonies made out of smaller mussels could result in higher ingestion rate per
unit mass due to their ability to maneuver and adjust according to the flow of food availability.
47
As an assumption, this may have affected some of the results obtained in the dataset. This was
offered as an explanation to some of the results found and not as a fact that determined the
outcome of the results.
5.2. The Increase in Biomass
The increase in biomass was detected in a few stations, as opposed to a zone of cholophyll-a
depletion. Three out of the four stations (stations 439, 440 & 443), which were found to have an
increase in biomass during the month of July (Table 6), were located in the eastern parts of the
basin. In the observed data, slight biomass increase was noticed from station to station to have
surfaced beginning July and subsiding in September. In Culver et al. (1999), they discovered that
blooms of Microcrystis in late August to September may have cause an increase in biomass near
the benthic layer. Despite the capacity of Dreissena polymorpha to consume a wide size range of
phytoplankton, they have been found to preferentially select their food (Birger et al. 1978). Zebra
mussels and quagga mussels have shown in past research to reject Microcrystis spp. as a food
source, thus the enhanced cyanobacteria abundance in the water column (Vanderploeg et al.
2001). This could have been the reason behind the increase in biomass detected in some of the
station results seeing that biomass increased during those similar periods. That aside, these
findings have also shown support to the arguments that the invasion of the mussels could alter
phytoplankton communities and cause important ecosystem consequences.
The incidence of increased biomass could also possibly be due to the presence of the mussels
themselves. Dreissenids have been known for their ability to remove both phytoplankton and
non-phytoplankton particulates from the water column. This in turn provides for an improvement
in water clarity and thus, better light penetration. Finally this allows for the phytoplankton to
compensate for increased removal by grazers (an increased or neutralized biomass). The other
possible rationalization being that the increased rates of nutrient cycling provided by the mussels
has given the phytoplankton enough supply of nutrients to flourish and bloom.
5.3. Temperature Dataset
Initially, through the research of past literature, it was assumed that the state of the lake being
stratified or well-mixed would have an effect on the depletion of phytoplankton biomass at the
48
near-bottom regions. The turbulent transport and wave action in the water column (well-mixed
region), was supposed to aid the replacement of the filtered water. Hence, it would be expected
that there would be no occurrence of depletion. Contrastingly, the results from the present study
did not show any considerable evidence of the state of the lake affecting the depletion. In other
words, signs of depletion still surfaced even when the region was considered to be well-mixed
(refer to Appendix A - stations 439, 448 & 451). The results from the temperature data, however,
did provide for two other noteworthy discoveries. The first finding was that in any region of the
lake where depletion was found, the same location was consistently found to be stratified as well.
This suggests that when the region is stratified, a factor from within the near-bottom region was
depleting chlorophyll-a concentrations.
Back in Chapter 2, the physiological aspects of the mussel was studied and various research
studies indicated that mussel growth and maturity depended on the temperature and physical
environment. The studies concluded that mussels were able to reproduce at temperatures between
12°C to 15°C. Payne (1997) stated that zebra mussels were well-suited in lower and temperate
climate conditions. He also found that when temperature increases above the optimum value, the
clearance rates of the mussels would decline to a point where mussel mortality may even happen
(at approximately 29°C). The second finding in the present study with regards to the temperature
was that, by examining the temperature at the near-bottom regions there may be evidence to
support the trends of dense mussel colonies depleting the chlorophyll-a. For example, if the
temperatures at the near-bottom regions were suitable for the optimum feeding, reproduction, and
growth of the mussels, then it would be expected that there would be a depletion of chlorophyll-a
where dense mussel colonies were to exist at that specific location. These similar conditions were
suggested in the results and findings when maximum depletion was found in areas with
temperatures ranging from 7°C to 14°C and when preliminary data indicated that mussels
accumulated in large numbers (Table 5 in section 4.2 of Chapter 4). Also, whenever the
temperature increased to 20°C and above, there were no signs of chlorophyll-a depletion
(possible explanation to stations 445 & 931). In this case, the results exhibited that the conditions
were appropriate for the mussels to filter-feed at optimum rates, grow, and multiply during
summer. In other words, the high depletion of the chlorophyll-a found during the month of June
may be due partly to the filter-feeding impacts of the mussels.
49
5.4. Limitations and Recommendations
5.4.1. Statistical Analysis
As previously mentioned, more statistical analysis would have been useful in the present research
especially in determining significant differences between factors, and objectively knowing how
significant the difference is. In other words, it would be valuable to prove that the differences
found in the study (such as the difference in mussel densities between high and low temperature
waters) occurred not by chance, but due to some meaningful association. This can be done using
t-test analysis. For example, an analysis could be conducted to determine whether there is a
significant difference in chlorophyll-a depletion between high and low mussel densities in the
near-bottom regions. Another example would be to determine whether the difference in mussel
densities between various temperatures is indeed significant.
Another interesting analysis which could be implemented in the future would be to study the
significant correlation between the density of the mussels, temperature, and chlorophyll-a
concentrations. All these suggested statistical analyses would require the raw dataset of the
mussel density and mussel distribution at the same locations for each of the sampled stations.
This was the limitation of the project because such a dataset was not available. In brief, the lack
of the statistical analyses limited the study to just being able to show association in the data and
giving assumptions instead of confirming significance with statistical data.
5.4.2. Sampling the Dataset
Although this feature in the present report could not have been changed, this section was still
included to demonstrate that sampling of the dataset could have been a limitation or may have
provided some sources of error. To illustrate this, the depleted layers in certain sampled sites may
not have extended high enough to reach the lowest sampled point. Therefore, this suggests that
the lower the sample is taken, the better the results would be. Also, since the samples were taken
once a month during the cruise in 2002, the period taken could have affected the dataset.
Ghadouani & Smith (2005) found that the time of sampling could provide a difference in dataset
collected because, the phytoplankton and algae migrate throughout the water column at different
periods to suit their activities.
50
5.4.3. Data on Water Velocity
The reason behind the proposal to study the water velocity is due to the findings where zebra
mussel fouling was found to depend also on velocity. In the study by Claudi & Mackie (1994),
flow velocities exceeding one and a half meters per second minimize mussel settlement in raw
water intakes. This was proven in a mussel-free raw water intake in Ontario which has a flow
velocity exceeding two and a half meters per second. Therefore, this suggests that flow velocity
would be another important factor which can be used to determine and understand the density or
accumulation of zebra mussels in the near-bottom regions of eastern Lake Erie. Another example
would be that the low water velocity at the near-bottom region may cause a localized depletion
zone whenever the filter-feeding mussels remove the food before it can be replenished.
Consequently, this implies that regions with low flow velocity has a higher chance of
demonstrating depletion zones if mussels were to accumulate there. Understanding the water
velocities would definitely bring about an important supplementary understanding to the near-
bottom depletion and having data on the water velocity can be used in various ways discussed in
the following sub-sections below.
5.4.4. Phytoplankton Community Structure
Studying the phytoplankton community dynamics would aid the present study because the results
would have been able to indicate which specific groups were present in the samples taken. Future
studies could also look into the different wavelengths in the spectroflurometer to identify the
different algae types. The present study examined the depletion of total biomass in the dataset and
since most studies have found that mussels tend to selectively feed, studying the different algal
groups would provide a clearer picture as to what types of groups has been depleted in the near-
bottom regions of eastern Lake Erie. If increased biomass were to be detected then, the specific
group causing the increase would have been able to be narrowed down. For more information on
the ‘spectral groups’ of the phytoplankton community please refer to Beutler et al. (2002; 2003).
5.4.5. Simple Modeling
The next step would be to simulate models which can act as tools in determining the removal of
phytoplankton and particulates in the water column by the mussels. The filter-feeding ability of
the mussels to consume particulates in the water column is limited by the rate of which the algae
reaches the near-bottom region. Therefore, a study in modeling the hydrodynamics and physical
forcing would provide for a better understanding of the near-bottom depletions. For example,
51
future research can start by constructing simple dispersion models to simulate the near-bottom
regions and the filter-feeding of the mussels on the suspended food particles. By studying the past
research of various models (see Chapter 2), the present study suggests that this dispersion model
could include physical effects such as wind velocity, water velocity, horizontal-advection and
turbulent mixing effects with regards to the filter-feeding mussels and their food supply (mainly
phytoplankton). The present report has provided, through various literatures, the suggestions that
modeling these clearance rates of the mussel beds may be enhanced by additionally examining
the different size class, age distribution, and positioning of the mussels within a colony. The
models may start off simple at first, but with consequent improvements and alterations, these
models may provide the users with short-term prediction of mussel impacts in the near-bottom
region and as a tool in management of the lakes with regards to the invasive Dreissena spp.
5.4.6. Quagga Mussel Study
Initially the present study was examining the effects of zebra mussels on the near-bottom
depletion of eastern Lake Erie. Through the process, it was found that quagga mussels were also
found in abundance in the eastern regions (Patterson et al. 2005). According to past research the
two species were originally found to colonize in different depths; the zebra mussels on hard
substrates and shallower water while the quagga mussels in the deeper, and colder regions (Mills
et al. 1996; Baldwin et al. 2002). Recent observations even indicated that the expansion of
quagga mussels has progressively extended to shallower regions in Lake Erie (Coakley et al.
2002). Records also confirm that in the past century wherever mussels invade, the quagga
mussels were the dominant of the two Dreissena spp. (Mills et al. 1996; Baldwin et al. 2002).
This suggests that the quagga mussels may ultimately invade the entire deeper regions of Lake
Erie in the future. While quagga mussels (Dreissena bugensis) has shown to remove and retain
phosphorus more efficiently than zebra mussels, the biofouling effects of either species would
still be as detrimental to the ecosystem. Since both types of mussels were causing the same
effects, the current study has taken into account and assumed that the depletion in the results was
caused by both dreissenids. As a result, the depletion found in the deeper regions of eastern Lake
Erie could have been due to the quagga mussels instead of the zebra mussels. Therefore, a
proposal for future works would be to separately study the biofouling effects of the quagga
mussels in depleting the near bed regions. The study may want to encompass the physiology,
filter-feeding habits, life cycle, and similar topics to most of the investigation done in Chapter 2.
This would aid future works in understanding the separate impacts implied by the Dreissena spp.
52
Chapter 6: Conclusion The present study has successfully shown that near-bottom depletions of chlorophyll-a were
evident in various locations across the eastern basin of Lake Erie. While this is true, the present
study has found it to be problematic when pronouncing the zebra mussels to be responsible for
the near-bottom depletions due to the lack of statistical analyses. Significance in results could not
be produced. Therefore, the present study can only suggest that the results imply that near-bottom
depletion may have been caused by the filter-feeding effects of the mussels.
Depletion was found to be of higher order in the offshore regions as compared to the nearshore
areas. This offshore depletion may have been caused by the presence of quagga mussels which
were assumed in this study to be equally detrimental in terms of biofouling an ecosystem. In
addition to that, these findings support that the historic patterns of eastern Lake Erie has reversed
showing patterns unlike that of large lakes. This again implies that the Dreissena spp. may have
contributed in some way.
Depletion was also found to be higher in stations sampled in the northern regions as compared to
the southern regions. This was suggested to be associated with the distinct difference in mussel
densities between the two locations; northern region has the higher density of mussels. The
increase in biomass instead of depletion was also detected in some of the sampled stations in the
eastern regions of the basin and they were suggested to be due to the occasional cyanobacteria
blooms which took place during July to September.
The study has found that eastern basin of Lake Erie has exhibited summer thermal stratification
for short periods ranging from one to two months each time. Peak depletion of chlorophyll-a was
established to occur during the early to mid-summer season, starting the month of June. The
physiology of the mussels was found in past studies to be governed by temperature. These peak
depletions implied that mussels may have caused the drop in chlorophyll-a concentrations
because those areas were predicted to have high mussel densities apart from being temperately
suitable for mussel activities.
53
In conclusion to the findings, the zebra mussels and the quagga mussels have been suggested to
be important agents in determining the changes in the ecosystem and in depleting the near-bottom
regions of Lake. The use of the dataset provided from the spectroflourometric method has shown
its merits in providing accurate and definitive data. The use of the flouroprobe can be extended in
the future works to study the near-bottom depletion of the individual phytoplankton groups
within the community in great detail with regards to effects of the filter-feeding mussels.
During the course of the dissertation, it was found that in order to predict the dynamics of
particles in the water column, an intimate knowledge of the physical, biological, and ecological
implications of the zebra mussels were needed. In order to obtain these criteria’s a more statistical
approach has to be taken to provide for a definitive answer as to whether the depletions in the
near-bottom region were caused by the mussels. The statistical approach can even take a step
further by finding correlations between the densities of the mussels and the severity of depletion
at the near-bottom region. In essence, even if these criteria’s were to be fulfilled, applying the
same concepts to other regions of North America to predict short-term changes due to the effects
of zebra mussels would continue to be a difficult task due to the uniqueness and sensitivity of
each water body. Finally, it is hoped that the results and findings presented in this study will
contribute and serve as a stepping-stone, in the broader aspects of understanding the effects of
zebra mussels in depleting chlorophyll-a concentrations in the near-bottom regions of eastern
Lake Erie.
54
References
Ackerman, J. D. 1995, Zebra Mussel Life History, Environmental Studies Programme, University
of Northern British Columbia.
Ackerman, J. D., Sim, B., Nichols, S. J., & Claudi, R. 1994, ‘A review of the early life history of
the zebra mussel (Dreissena polymorpha): Comparisons with marine bivalves’, Canadian
Journal of Zoology, vol. 72, pp. 1169-1179.
Ackerman, J. D., Loewen, M. R., & Hamblin, P. F. 2001, ‘Benthic-pelagic coupling over a zebra
mussel reef in western Lake Erie’, Limnology and Oceanography, vol. 46, no. 4, pp. 892-
904.
Arnott, D. L., & Vanni, M. J. 1996, ‘Nitrogen and phosphorus recycling by the zebra mussel
(Dreissena polymorpha) in the western basin of Lake Erie’, Canadian Journal of
Fisheries and Aquatic Sciences, vol. 53, pp. 646-659.
Bastviken, D. T. E., Caraco, N. F., & Cole J. J. 1998, ‘Experimental measurements of zebra
mussel (Dreissena polymorpha) impacts on phytoplankton community composition’,
Freshwater Biology, vol. 29, pp. 375–386.
Baldwin, B. S., Mayer, M. S., Dayton, J., Pau, N., Mendilla, J., Sullivan, M., Moore, A., Ma, A.,
& Mills, E. L. 2002, ‘Comparative growth and feeding in zebra and quagga mussels
(Dreissena polymorpha and Dreissena bugensis): implications for North American lakes’,
Canadian Journal of Fisheries and Aquatic Sciences, vol. 59, pp. 680–694.
Beeton, A. M. 2002. ‘Large freshwater lakes: present state, trends, and future’, Environmental
Conservation, vol. 29, pp. 21-38.
Berkmann, P., Garton D., Kennedy G., Lewandoski, A., & Van Bloem, S. 1995, ‘Dreissena patch
mosaics on hard and soft substrates in the western basin of Lake Erie’, in Fifth
International Zebra Mussel Conference, Toronto, Ontario.
Bertram, P. 1993, ‘Total phosphorus and dissolved oxygen trends in the central basin of Lake
Erie,’ Journal of Great Lakes Research, vol. 19, pp. 224-236.
Beutler, M., Wilstshire, K. H., Meyer, B., Moldaenke, C., Lüring, C., Meyerhöfer, M., Hansen,
U.-P., & Dau, H. 2002, ‘A fluorometric method for the differentiation of algal populations
in vivo and in situ’, Photosynthesis Research, vol. 72, no. 1, pp. 39–53.
Beutler, M., Wiltshire, K. H., Arp, M., Kruse, J., Reineke, C., Moldaenke, C., & Hansen, U.-P.
2003, ‘A reduced model of the fluorescence from the cyanobacterial photosynthetic
apparatus designed for the in situ detection of cyanobacteria’, Biochimica et Biophysica
Acta Bioenergetics, vol. 1604, no. 1, pp. 33–46.
55
Birger, T. I., Malarevskaja, A. Y., Arsan, O. M., Solomatina, V. D., & Gupalo, Y. M. 1978,
‘Physiological aspects of adaptations of mollusks to abiotic and biotic factors due to blue-
green algae’, Malacological Review, vol. 11, pp. 100-102.
Bolsenga, S. J., & Herdendorf, C. E. 1993, Lake Erie and Lake St. Clair Handbook, Wayne State
University Press, Michigan.
Bruner, K. A., Fisher, S. W., & Landrum, P. F. 1994, ‘The role of the zebra mussel, Dreissena
polymoprha, in contaminant cycling: II. Zebra mussel contaminant accumulation from
algae and suspended particles and transfer to the benthic invertebrate Gammarus
fasciatus’, Journal of Great Lakes Research, vol. 20, pp. 735-770.
Bunt, C. M., MacIsaac, H. J., & Sprules, G. W. 1993, ‘Pumping rates and projected filtering
impacts of juvenile zebra mussels (Dreissena polymorpha) in western Lake Erie,
Canadian Journal of Fisheries and Aquatic Science, vol. 50, pp. 1017-1022.
Burks, R. L., Tuchman, N. C., & Call, C. A. 2002, ‘Colonial aggregates: effects if spatial position
on zebra mussels responses to vertical gradients in interstitial water quality’, J. N. Am.
Benthol. Soc., vol. 21, pp. 64-75.
Burns, N. M. 1985, Erie, The Lake That Survived, Rowman & Allanheld, New Jersey.
Charlton, M. N. 1980, ‘Oxygen depletion in Lake Erie: has there been any change?’, Canadian
Journal of Fisheries and Aquatic Sciences, vol. 37, pp. 72-81.
Clarke, M., & McMahon, R. F. 1995, Effects of Current Velocity on Byssal Thread Production in
the Zebra Mussel (Dreissena polymorpha), Department of Biology, The University of
Texas, Arlington.
Claudi, R., & Mackie, G. L. 1993, Zebra Mussel Monitoring and Control, Lewis Publishers,
Boca Raton, Florida.
Claudi, R., & Mackie, G. L. 1994, Practical manual for zebra mussel monitoring and control,
CRC Press, Boca Raton, Florida.
Coakley, J. P, Rasul, N., Ioannou, S. E., & Brown, G. R. 2002, ‘Soft sediments as a constraint on
the spread of the zebra mussel in western Lake Erie: Processes and impacts’, Aquatic
Ecosystem Health Management, vol. 5, pp. 329–343.
Commito, J. A., & Rusignuolo, B. R. 2000, ‘Structural complexity in mussel beds: the fractal
geometry of surface topography, Journal of Experimental Marine Biology and Ecology,
vol. 255, pp. 133-152.
Conroy, J. D., Edwards, W. J., Pontius, R. A., Kane, D. D., Zhang, H., Shea, F., Richey, J. N., &
Culver, D. A. 2005, ‘Soluble nitrogen and phosphorus excretion of exotic freshwater
56
mussels (Dreissena spp.): potential impacts for nutrient remineralization in western Lake
Erie’, Freshwater Biology, vol.50, no. 7, pp. 1146-1162.
Conroy, J. D., & Culver, D. A. 2005, ‘Do Dresissenid mussels affect Lake Erie ecosystem
stability processes?’, The American Midland Naturalist, vol. 153, no. 1, pp. 20-32.
Dame, R., Dankers, N., Prinks, T., Jongsma, H., & Smaal, A. 1991, ‘The influence of mussel
beds on nutrients in the western Waden Sea and eastern Scheldt Estuaries’, Estuaries, vol.
14, pp. 130-138.
Dolan, D. M. 1993, ‘Point source loading of phosphorus to Lake Erie’, Journal of Great Lakes
Research, vol. 19, pp. 212-223.
Edwards, W. J., Rehmann, C. R., McDonald, E., & Culver, D. A. 2005, ‘The impact of a benthic
filter feeder: limitations imposed by physical transport of algae to the benthos’, Canadian
Journal of Fisheries and Aquatic Sciences, vol. 62, pp. 205-214.
Environment Canada & U.S. Environmental Protection Agency, (1995), Great Lakes Atlas: An
Environmental Atlas and Resource Book [Online], Government of Canada and United
States Environmental Protection Agency, Available: http://www.epa.gov/glnpo/atlas/
[June 2007].
EPA, (August 2007), Terms of Environment: Glossary, Abbreviations and Acronyms [Online],
U.S. Enivonmental Protection Agency (EPA), Available:
http://www.epa.gov/OCEPAterms/ [July 2007].
Eshenroder, R. E., & Burnham-Curtis, M. K. 1999, Species succession and sustainability of the
Great Lakes fish community, Great Lakes Fisheries Policy and Management, Michigan
State University.
Fahnenstiel, G. L., Bridgeman, T. B., Lang, G. A., McCormik, M. J., & Nalepa, T. F. 1993,
‘Phytoplankton productivity in Saginaw Bay, Lake Huron: effects of zebra mussel
(Dreissena polymorpha) colonization’, Journal of Great Lakes Research, vol. 21, pp.
465-475.
Ghadouani, A., & Smith, R. E. H. 2005, ‘Phytoplankton distribution in Lake Erie as assessed by a
new in situ spectrofluorometric technique’, Journal of Great Lakes Research, vol. 31, no.
2, pp. 154-167.
Gumley, L., (2006), MODIS Image Gallery [Online], Space Science and Engineering Center
(SSEC), Available: http://www.ssec.wisc.edu/~gumley/modis_gallery/ [June 2007].
Hamblin, P. F. 1979, ‘Great Lakes storm surge of April 6’, Journal of Great Lakes Research, vol.
5, pp. 312-315.
57
Higgins, S. N. 2005, PhD Thesis: Modeling the growth dynamics of Cladophora in eastern Lake
Erie, University of Waterloo, Ontario, Canada.
Holland, R. E., 1993, ‘Changes in planktonic diatoms and water transparency in Hatchery Bay,
Bass Island area, Western Lake Erie since the establishment of the zebra mussel,’ Journal
of Great Lakes Research, vol. 19, pp. 617-624.
Horgan, M. J., & Mills, E. L. 1997, ‘Clearance rates and filtering activity of zebra mussel
(Dreissena polymorpha): implications for freshwater lakes’, Canadian Journal of
Fisheries and Aquatic Sciences, vol. 54, pp. 249-255.
Idrisi, N., Mills, E. L., Rudstam, L. G., & Stewart, D. J. 2001, ‘Impact of zebra mussels
(Dreissena polymorpha) on the pelagaic lower trophic levels of Oneida Lake’, Canadian
Journal of Fisheries and Aquatic Sciences, vol. 58, pp. 1430-1441.
Jorgensen, C. B. 1990, Bivalve Filter Feeding: Hydrodynamics, Bioenergetics, Physiology, and
Ecology, Olsen & Olsen, Denmark.
Karatayev, A. Y., Burlakova, L. E., & Padilla, D. K. 2006, ‘Growth rate and longevity of
Dreissena polymorpha (Pallas): a review and recommendations for future study’, Journal
of Shellfish Research, vol. 25, pp. 23–32.
Klerks, P. L., Fraleigh, P. C., & Lawniczak, J. E. 1996, ‘Effects of zebra mussels (Dreissena
polymorpha) on seston levels and sediment deposition in western Lake Erie’, Canadian
Journal of Aquatic Sciences, vol. 53, pp. 2284-2291.
Kornobis, S. 1977, ‘Ecology of Dreissena polymorpha (Pallas) (Dreissenidae, Bivalvia) in lakes
receiving heated water discharges’, Poland Archive for Hydrobiology, vol. 24, no. 4, pp.
531-545.
MacIsaac, H. J. 1996, ‘Potential abiotic and biotic impacts of zebra mussels on the inland waters
of North America’, American Zoologist, vol. 36, pp. 287-299.
MacIsaac, H. J., Sprules, G. W., Johannsson, O. E., & Leach, J. H. 1992, ‘Filtering impacts of
larval and sessile zebra mussel (Dreissena polymorpha) in western Lake Erie’, Oecologia,
vol. 92, pp. 30-39.
MacIsaac, H. J., Sprules, G. W., & Leach, J. H. 1991, ‘Ingestion of small-bodied zooplankton by
zebra mussel (Dreissena polymorpha): can canabalism on larvae influence population
dynamics?’, Canadian Journal of Fisheries Aquatic Sciences, vol. 48, pp. 2051-2060.
MacIsaac, H. J., Johannsson, O. E., Ye, J., Sprules, G. W., Leach, J. H., McCorquodale, J. A., &
Grigorovich, I. A. 1999, ‘Filtering impacts of an introduced bivalve (Dreissena
58
polymorpha) in a shallow lake: application of a hydrodynamic model’, Ecosystems, vol. 2,
pp. 338-350.
Mackie, G. L. 1991, ‘Biology of the exotic zebra mussel, Dreissena polymorpha, in relation to
native bivalves and its potential impact in Lake St. Clair’, Hydrobiologia, vol. 219, pp.
251-268.
Mackie, G. L., Gibbons, W. N., Muncaster, B. W., & Gray, I. M. 1989, The Zebra Mussel,
Dreissena polymorpha: A synthesis of European Experiences and a Preview for North
America, Water Resources Branch, Ontario Ministry of the Environment, Ontario.
Mills, E. L., Leach, J. H., Carlton, J. T., & Secor, C. L. 1993, ‘Exotic species in the Great Lakes:
a history of biotic crisis and anthropogenic introductions’, Journal of Great Lakes
Research, vol. 19, pp.1-54.
Nalepa, T. F., & Schloesser, D. W. 1993, Zebra Mussels: Biology, Impact, and Control, Lewis
Press, Florida USA.
NALM, (April 2007), Water-Words Glossary [Online], North American Lake Management
Society (NALM), Available: http://www.nalms.org/Resources/Glossary.aspx [July 2007].
Nichols, S. J. 1996, ‘Variations in the reproductive cycle of Dreissena polymorpha in Europe,
Russia, and North America’, American Zoologist, vol. 36, pp. 311-325.
Nichols, S. J., Black, M. G., & Garling, D. L. 1996, ‘Food habits of Dreissena polymorpha as
compared to other freshwater bivalves’, in The Sixth International Zebra Mussel and
Other Aquatic Nuisance Species Conference, Dearborn, Michigan.
Nichols, S. J., & Kollar, B. 1991, ‘Reproductive cycle of zebra mussels (Dreissena polymorpha)
in western Lake Erie at Monroe, Michigan’, in Second International Zebra Mussel
Conference, Rochester, New York.
Nicholls, K. H., Hopkins, G. J., & Standke, S. J. 1999, ‘Reduced chlorophyll to phosphorus ratios
in nearshore Great Lakes waters coincide with the establishment of dreissenid mussels’,
Canadian Journal of Fisheries and Aquatic Sciences, vol. 56, pp. 153–161.
Nuttall, C. P. 1990, ‘Review of the caenozoic heterodont bivalve superfamily Dreissenacea’,
Paleontology, vol. 33, pp. 707-737.
NOAA, (2005), Great lakes lab on mission to Lake Erie dead zone [Online], National Oceanic
and Atmospheric Administration (NOAA), Available:
http://www.noaanews.noaa.gov/stories2005/s2427.htm [June 2007].
59
Noonburg, E. G., Shuter, B. J., & Abrams, P. A. 2003, ‘Indirect effects of zebra mussels
(Dreissena polymorpha) on planktonic food web’, Canadian Journal of Fisheries and
Aquatic Sciences, vol. 60, pp. 1353-1368.
O’Riordan, C. A., Monismith, S. G., & Koseff, J. R. 1995, ‘The effect of bivalve excurrent jet
dynamics on mass transfer in the benthic boundary layer’, Limnology and Oceanography,
vol. 40, pp. 330-344.
Patterson, W. R., Ciborowski, J. H., & Barton, D. R. 2005, ‘The distribution and abundance of
Dreissena species (Dreissenidae) in Lake Erie, 2002’, Journal of Great Lakes Research,
vol. 31, no. 2, pp. 223–237.
Payne, B. S. 1997, ‘Thermal biology of the zebra mussel, Dreissena polymorpha, in North
America’, in The Seventh International Zebra Mussel and Other Aquatic Nuisance
Species Conference, New Orleans, Louisiana.
Prins, T. C., Escaravage, V., Smaal, A. C., & Peeters, J. C. H. 1995, ‘Nutrient cycling and
phytoplankton dynamics in relation to mussel grazing in mesocosm experiment’, Ophelia,
vol. 41, pp.289-315.
Raikow, D. F. 2002, How the feeding ecology of native and exotic mussels affects freshwater
ecosystems, Doctoral Dissertation, Michigan State University.
Ramcharan, C. W., Padilla, D. K., & Dodson, S. I. 1992, ‘Models to predict potential occurrence
and density of zebra mussel, Dreissena polymorpha’, Canadian Journal of Fisheries
Aquatic Sciences, vol. 49, pp. 2611-2620.
Reed-Anderson, T., Carpenter, S. R., Padilla, D. K., & Lathrop, R. C. 2000, ‘Predicted impact of
zebra mussel (Dreissena polymorpha) invasion on water clarity in Lake Mendota’,
Canadian Journal of Fisheries Aquatic Sciences, vol. 57, pp. 1617-1626.
Ricciardi, A., Whoriskey, F. G., & Rasmussen, J. B. 1996, ‘Impact of Dreissena polymorpha on
native unionid bivalves in the upper St. Lawrence River’, Canadian Journal of Fisheries
and Aquatic Sciences, vol. 53, pp. 1434–1444.
Richards, R. P., & Baker, D. B. 2002, ‘Trends in water quality in LEASEQ rivers and streams
(Northwestern Ohio)’, Journal of Environmental Quality, vol. 31, pp. 90-96.
Roditi, H. A., Fisher, N. S., & Sanudo-Wilhelmy, S. A. 2001, ‘Uptake of dissolved organic
carbon and trace elements by zebra mussels’, Nature, vol. 407, pp. 78-80.
Sakai, A. K., Allendorf, F. W., Holt, J. S., Lodge, D. M., Molofsky, J., With, K. A., Baughman,
S., Cabin, R. J., Cohen, J. E., Ellstrand, N. C., McCauley. D. E., O’Neil, P., Parker, I. M.,
60
Thompson, J. N., & Weller, S. G. 2001, ‘The population biology of invasive species’,
Annual Review of Ecology and Systematics, vol. 32, pp. 305-332.
Sly, P. G. 1976, ‘Lake Erie and its basin’, Journal of the Fisheries Research Board of Canada,
vol. 33, pp. 355-370.
Sprung, M. 1987, ‘Ecological requirements of developing Dreissena polymorpha eggs’, Archive
for Hydrobiology Supplement, vol. 79, pp. 69-86.
Sprung, M., & Rose, U. 1988, ‘Influence of food size and food quality on the feeding of the
mussel Dreissena polymorpha’, Oecologia, vol. 77, pp. 526-532.
Stanczykowska, A. 1977, ‘Ecology of Dreissena polymorpha (Pallas) (Bivalvia) in lakes’,
Poland Archive for Hydrobiology, vol. 24, pp. 461-530.
Stoeckmann, A. 2003, ‘Physiological energetics of Lake Erie dreissenid mussels: a basis for the
displacement of Dreissena polymorpha by Dreissena bugensis’, Canadian Journal of
Fisheries Aquatic Sciences, vol. 60, pp. 126–134.
Strayer, D. K., Caraco, N. F., Cole, J. J., Findlay, S., & Pace, M. L. 1999, ‘Transformation of
freshwater ecosystems by bivalves: a case study of zebra mussels in the Hudson River’,
Bioscience, vol. 49, pp. 19-27.
Strayer, D. L., & Malcom, H. M. 2006, ‘Long-term demography of a zebra mussel (Dreissena
polymorpha) population’, Freshwater Biology, vol. 51, no. 1, 117–130.
Turgeon, D. D., Quinn, J. F., Bogan, A. E., Coan, E. V., Hochberg, F. G., & Lyons, W. G. 1998,
Common and scientific names of aquatic invertebrates from the United States and
Canada: Mollusks, American Fisheries Society Special Publication, USA.
USGS, (August 2007), Zebra Mussel Page [Online], U.S. Geological Survey (USGS), Available:
http://nas.er.usgs.gov/taxgroup/mollusks/zebramussel/default.asp [June 2007].
Vanderploeg, H. A., Nalepa, T. F., Jude, D. J., Mills, E. L., Holeck, K. T., Liebig, J. R.,
Grigorovich, I. A., & Ojaveer, H. 2002, ‘Dispersal and emerging ecological impacts of
Ponto-Caspian species in the Laurentian Great Lakes’, Canadian Journal of Fisheries and
Aquatic Sciences, vol. 59, no. 7, pp. 1209-1228.
Wilson, A. E., & Sarnelle, O. 2002, ‘Relationship between zebra mussel biomass and total
phosphorus in European and North American lakes’, Archive for Hydrobiology
Supplement, vol. 153, pp. 339–351.
WOW, (April 2007), Resources: Glossary [Online], Water on The Web (WOW), Available:
http://waterontheweb.org/resources/glossary.html [September 2007].
61
Yu, N., & Culver, D. A. 1999, ‘Estimating the effective clearance rate and refiltration by zebra
mussels, Dreissena polymorpha, in a stratified reservoir’, Freshwater Biology, vol. 41, pp.
481-492.
Appendix A: Processed Data Set
Chlorophyll-a (μg/L) Temperature (°C) Station Months Just above the
mussel bed Two to three meters
above the mussel bed Difference in Chlorophyll-a
Bottom Surface Difference in Temperature
April 0.5 0.39 0.110 3.3 4.84 -1.540
May - - - - - -
June 0.76 0.81 -0.050 7.18 13.97 -6.790
July 0.75 1 -0.250 6.93 21.03 -14.100
August 0.4 0.51 -0.110 7.2 21.155 -13.955
Sept 0.88 0.86 0.020 7.8 21.19 -13.390
23
Oct 0.83 1.24 -0.410 9.08 12.46 -3.380
Chlorophyll-a (μg/L) Temperature (°C)
Station Months Just above the mussel bed
Two to three meters above the mussel bed
Difference in Chlorophyll-a
Bottom Surface Difference in Temperature
April 1.56 1.37 0.190 3.89 4.94 -1.050
May 7.485 8.54 -1.055 7.09 7.225 -0.135
June 2.195 5.975 -3.780 12.185 16.175 -3.990
July 1.8 1.35 0.450 17.79 21.185 -3.395
August - - - - - -
Sept 2 2.86 -0.860 17.22 21.71 -4.490
439
Oct 1.255 1.165 0.090 13.62 13.37 0.250
62
Chlorophyll-a (μg/L) Temperature (°C)
Station Months Just above the mussel bed
Two to three meters above the mussel bed
Difference in Chlorophyll-a
Bottom Surface Difference in Temperature
April 0.600 0.800 -0.200 4.690 6.990 -2.300
May 0.75 0.8 -0.050 7.95 9.27 -1.320
June 2.630 3.400 -0.770 13.360 16.340 -2.980
July 2.65 2.01 0.640 20.41 21.345 -0.935
August - - - - - -
Sept - - - - - -
440
Oct 1.11 1.635 -0.525 12.805 13.54 -0.735
Chlorophyll-a (μg/L) Temperature (°C)
Station Months Just above the mussel bed
Two to three meters above the mussel bed
Difference in Chlorophyll-a
Bottom Surface Difference in Temperature
April 1.230 1.210 0.020 4.610 4.680 -0.070
May 1.540 1.980 -0.440 8.590 8.840 -0.250
June 2.190 4.740 -2.550 12.250 16.450 -4.200
July 1.845 1.650 0.195 19.990 21.535 -1.545
August 2.840 2.635 0.205 19.990 22.675 -2.685
Sept - - - - - -
442
Oct 0.910 0.815 0.095 13.305 13.870 -0.565
63
Chlorophyll-a (μg/L) Temperature (°C)
Station Months Just above the mussel bed
Two to three meters above the mussel bed
Difference in Chlorophyll-a
Bottom Surface Difference in Temperature
April 0.96 1.1 -0.140 3.68 5.01 -1.330
May 6.2 6.7 -0.500 6.34 7.69 -1.350
June 2.91 6.13 -3.220 10.08 16.64 -6.560
July 2.13 1.18 0.950 15.52 21.65 -6.130
August - - - - - -
Sept 1.17 2.53 -1.360 13.96 22.63 -8.670
443
Oct 1.75 1.78 -0.030 13.6 13.69 -0.090
Chlorophyll-a (μg/L) Temperature (°C)
Station Months Just above the mussel bed
Two to three meters above the mussel bed
Difference in Chlorophyll-a
Bottom Surface Difference in Temperature
April - - - - - -
May 1.56 1.85 -0.290 8.7 9.6 -0.900
June 2.14 2.94 -0.800 11.6 19.8 -8.200
July 0.885 1.01 -0.125 22.6 23.01 -0.410
August - - - - - -
Sept 1.54 1.72 -0.180 22.2 22.4 -0.200
444
Oct 1.65 1.765 -0.115 13.3 13.4 -0.100
64
Chlorophyll-a (μg/L) Temperature (°C)
Station Months Just above the mussel bed
Two to three meters above the mussel bed
Difference in Chlorophyll-a
Bottom Surface Difference in Temperature
April 1.26 1.23 0.030 4.92 4.93 -0.010
May 2.58 2.98 -0.400 7.92 8.69 -0.770
June 3.6 3.4 0.200 12.66 15.88 -3.220
July 2.23 2.165 0.065 18.61 21.04 -2.430
August - - - - - -
Sept 2.79 2.62 0.170 21.8 22.6 -0.800
445
Oct 1.07 1.36 -0.290 12.63 12.98 -0.350
Chlorophyll-a (μg/L) Temperature (°C)
Station Months Just above the mussel bed
Two to three meters above the mussel bed
Difference in Chlorophyll-a
Bottom Surface Difference in Temperature
April - - - - - -
May 1.66 2.54 -0.880 7.7 9.64 -1.940
June 2.38 2.39 -0.010 16.74 17.7 -0.960
July 1.01 1.24 -0.230 8.18 12.58 -4.400
August 1.46 1.7 -0.240 22.41 22.35 0.060
Sept 2.75 2.25 0.500 20.79 21.01 -0.220
448
Oct 0.88 0.92 -0.040 11.575 12.52 -0.945
65
Chlorophyll-a (μg/L) Temperature (°C)
Station Months Just above the mussel bed
Two to three meters above the mussel bed
Difference in Chlorophyll-a
Bottom Surface Difference in Temperature
April 0.52 0.05 0.470 3.71 4.12 -0.410
May 3.32 4.08 -0.760 8.04 8.51 -0.470
June 1.94 3.3 -1.360 11.205 14.63 -3.425
July 2.155 1.995 0.160 17.9 19.39 -1.490
August - - - - - -
Sept 1.85 2.56 -0.710 13.4 20.9 -7.500
449
Oct 1.1 1.4 -0.300 12.23 12.405 -0.175
Chlorophyll-a (μg/L) Temperature (°C)
Station Months Just above the mussel bed
Two to three meters above the mussel bed
Difference in Chlorophyll-a
Bottom Surface Difference in Temperature
April 0.71 0.67 0.040 3.61 4.79 -1.180
May 1.31 1.5 -0.190 5.8 6.91 -1.110
June 1.5 2.28 -0.780 8.8 15.65 -6.850
July 1.59 1.965 -0.375 14.9 19.3 -4.400
August - - - - - -
Sept 1.13 2.6 -1.470 9.53 21.16 -11.630
450
Oct 1.46 1.5 -0.040 12.96 12.96 0.000
66
Chlorophyll-a (μg/L) Temperature (°C)
Station Months Just above the mussel bed
Two to three meters above the mussel bed
Difference in Chlorophyll-a
Bottom Surface Difference in Temperature
April 0.4 0.41 -0.010 3.49 5.12 -1.630
May 0.79 0.77 0.020 5.09 5.89 -0.800
June 1.445 2.4 -0.955 7.48 14.57 -7.090
July 1.15 2.14 -0.990 8.56 19.5 -10.940
August - - - - - -
Sept 0.95 1.97 -1.020 7.98 21.22 -13.240
451
Oct 1.31 2.12 -0.810 12.24 13.12 -0.880
Chlorophyll-a (μg/L) Temperature (°C)
Station Months Just above the mussel bed
Two to three meters above the mussel bed
Difference in Chlorophyll-a
Bottom Surface Difference in Temperature
April 1.150 1.210 -0.060 4.370 5.090 -0.720
May 5.330 5.000 0.330 9.070 8.460 0.610
June 3.550 3.230 0.320 13.850 16.250 -2.400
July 0.990 0.930 0.060 22.500 22.500 0.000
August 1.050 0.930 0.120 22.500 22.500 0.000
Sept 1.860 1.800 0.060 21.750 21.840 -0.090
931
Oct 1.635 1.695 -0.060 12.865 12.930 -0.065
67
68
Chlorophyll-a (μg/L) Temperature (°C)
Station Months Just above the mussel bed
Two to three meters above the mussel bed
Difference in Chlorophyll-a
Bottom Surface Difference in Temperature
April 0.86 0.83 0.030 3.58 4.17 -0.590
May 1.09 1.19 -0.100 4.93 6.25 -1.320
June 2.23 1.995 0.235 8.76 18.01 -9.250
July 0.83 0.95 -0.120 8.65 22.03 -13.380
August 1.22 1.27 -0.050 9.65 21.97 -12.320
Sept 1.46 2.03 -0.570 10.19 20.22 -10.030
935
Oct 1.79 1.75 0.040 14.23 14.6 -0.370
Chlorophyll-a (μg/L) Temperature (°C)
Station Months Just above the mussel bed
Two to three meters above the mussel bed
Difference in Chlorophyll-a
Bottom Surface Difference in Temperature
April 0.45 0.64 -0.190 3.47 5.28 -1.810
May 0.92 0.95 -0.030 5.86 6.88 -1.020
June 0.97 2.085 -1.115 7.48 14.85 -7.370
July 1.73 1.69 0.040 8.95 22.26 -13.310
August 0.58 0.78 -0.200 9.6 21.92 -12.320
Sept 0.93 1.22 -0.290 10.47 20.76 -10.290
938
Oct 1.76 1.76 0.000 14.2 14.53 -0.330
69
Appendix B: Chlorophyll-a and Temperature Graphs
B.1 Station 23
Temperature differences over time at station 23
-20
-15
-10
-5
0
5
10
15
20
25
April May June July August Sept Oct
Time (months)
Tem
pera
ture
(°C
)
BottomSurfaceDifference
Difference in Chl-a Concentrations and Temperature over time at station 23
-16.000-14.000-12.000-10.000-8.000-6.000-4.000-2.0000.0002.000
April May June July August Sept Oct
Time (months)
Chl
-a C
once
ntra
tions
(μg/
L)
and
Tem
pera
ture
(°C)
Di
ffere
nce
Chl-aTemp
70
B.2 Station 439
Chlorophyll-a differences over time at station 439
-6
-4
-2
0
2
4
6
8
10
April May June July August Sept Oct
Time (months)
Chl-a
Con
cent
ratio
ns (μ
g/L)
Bottom: Chl1Near-bottom: Chl2Difference
Temperature differences over time at station 439
-10
-5
0
5
10
15
20
25
April May June July August Sept Oct
Time (months)
Tem
pera
ture
(°C
)
BottomSurfaceDifference
71
Difference in Chl-a Concentrations and Temperature over time at station 439
-5.000
-4.000
-3.000
-2.000
-1.000
0.000
1.000
April May June July August Sept Oct
Time (months)
Chl
-a C
once
ntra
tions
(μg/
L)
and
Tem
pera
ture
(°C)
Di
ffere
nce
Chl-aTemp
B.3 Station 440
Chlorophyll-a differences over time at station 440
-1.000-0.5000.0000.5001.0001.5002.0002.5003.0003.5004.000
April May June July August Sept Oct
Time (months)
Chl-a
Con
cent
ratio
ns (μ
g/L)
Bottom: Chl1Near-bottom: Chl2Difference
72
Temperature differences over time at station 440
-5.000
0.000
5.000
10.000
15.000
20.000
25.000
April May June July August Sept Oct
Time (months)
Tem
pera
ture
(°C
)
BottomSurfaceDifference
Difference in Chl-a Concentrations and Temperature over time at station 440
-3.500-3.000-2.500-2.000-1.500-1.000-0.5000.0000.5001.000
April May June July August Sept Oct
Time (months)
Chl
-a C
once
ntra
tions
(μg/
L)
and
Tem
pera
ture
(°C)
Di
ffere
nce
Chl-aTemp
73
B.4 Station 442
Chlorophyll-a differences over time at station 442
-3.000
-2.000
-1.0000.000
1.000
2.000
3.0004.000
5.000
6.000
April May June July August Sept Oct
Time (months)
Chl-a
Con
cent
ratio
ns (μ
g/L)
Bottom: Chl1Near-bottom: Chl2Difference
Temperature differences over time at station 442
-10.000
-5.000
0.000
5.000
10.000
15.000
20.000
25.000
April May June July August Sept Oct
Time (months)
Tem
pera
ture
(°C
)
BottomSurfaceDifference
74
Difference in Chl-a Concentrations and Temperature over time at station 442
-4.500-4.000-3.500-3.000-2.500-2.000-1.500-1.000-0.5000.0000.500
April May June July August Sept Oct
Time (months)
Chl
-a C
once
ntra
tions
(μg/
L)
and
Tem
pera
ture
(°C)
Di
ffere
nce
Chl-aTemp
B.5 Station 443
Chlorophyll-a differences over time at station 443
-4
-2
0
2
4
6
8
April May June July August Sept Oct
Time (months)
Chl-a
Con
cent
ratio
ns (μ
g/L)
Bottom: Chl1Near-bottom: Chl2Difference
75
Temperature differences over time at station 443
-15
-10
-5
0
5
10
15
20
25
April May June July August Sept Oct
Time (months)
Tem
pera
ture
(°C
)
BottomSurfaceDifference
Difference in Chl-a Concentrations and Temperature over time at station 443
-10.000
-8.000
-6.000
-4.000
-2.000
0.000
2.000
April May June July August Sept Oct
Time (months)
Chl
-a C
once
ntra
tions
(μg/
L)
and
Tem
pera
ture
(°C)
Di
ffere
nce
Chl-aTemp
76
B.6 Station 444
Temperature differences over time at station 444
-10
-5
0
5
10
15
20
25
April May June July August Sept Oct
Time (months)
Tem
pera
ture
(°C
)
BottomSurfaceDifference
Difference in Chl-a Concentrations and Temperature over time at station 444
-9.000-8.000-7.000-6.000-5.000-4.000-3.000-2.000-1.0000.000
April May June July August Sept Oct
Time (months)
Chl
-a C
once
ntra
tions
(μg/
L)
and
Tem
pera
ture
(°C)
Di
ffere
nce
Chl-aTemp
77
B.7 Station 445
Temperature differences over time at station 445
-5
0
5
10
15
20
25
April May June July August Sept Oct
Time (months)
Tem
pera
ture
(°C
)
BottomSurfaceDifference
Difference in Chl-a Concentrations and Temperature over time at station 445
-3.500
-3.000
-2.500
-2.000
-1.500
-1.000
-0.500
0.000
0.500
April May June July August Sept Oct
Time (months)
Chl
-a C
once
ntra
tions
(μg/
L)
and
Tem
pera
ture
(°C)
Di
ffere
nce
Chl-aTemp
78
B.8 Station 448
Temperature differences over time at station 448
-10
-5
0
5
10
15
20
25
April May June July August Sept Oct
Time (months)
Tem
pera
ture
(°C
)
BottomSurfaceDifference
Difference in Chl-a Concentrations and Temperature over time at station 448
-5.000
-4.000
-3.000
-2.000
-1.000
0.000
1.000
April May June July August Sept Oct
Time (months)
Chl
-a C
once
ntra
tions
(μg/
L)
and
Tem
pera
ture
(°C)
Di
ffere
nce
Chl-aTemp
79
B.9 Station 449
Temperature differences over time at station 449
-20
-15
-10
-5
0
5
10
15
20
25
April May June July August Sept Oct
Time (months)
Tem
pera
ture
(°C
)
BottomSurfaceDifference
Difference in Chl-a Concentrations and Temperature over time at station 449
-14.000
-12.000
-10.000
-8.000
-6.000
-4.000
-2.000
0.000
2.000
April May June July August Sept Oct
Time (months)
Chl
-a C
once
ntra
tions
(μg/
L)
and
Tem
pera
ture
(°C)
Di
ffere
nce
Chl-aTemp
80
B.10 Station 450
Temperature differences over time at station 450
-15
-10
-5
0
5
10
15
20
25
April May June July August Sept Oct
Time (months)
Tem
pera
ture
(°C
)
BottomSurfaceDifference
Difference in Chl-a Concentrations and Temperature over time at station 450
-14.000
-12.000
-10.000
-8.000
-6.000
-4.000
-2.000
0.000
2.000
April May June July August Sept Oct
Time (months)
Chl
-a C
once
ntra
tions
(μg/
L)
and
Tem
pera
ture
(°C)
Di
ffere
nce
Chl-aTemp
81
B.11 Station 451
Temperature differences over time at station 451
-15
-10
-5
0
5
10
15
20
25
April May June July August Sept Oct
Time (months)
Tem
pera
ture
(°C
)
BottomSurfaceDifference
Difference in Chl-a Concentrations and Temperature over time at station 451
-14.000
-12.000
-10.000
-8.000
-6.000
-4.000
-2.000
0.000
2.000
April May June July August Sept Oct
Time (months)
Chl
-a C
once
ntra
tions
(μg/
L)
and
Tem
pera
ture
(°C)
Di
ffere
nce
Chl-aTemp
82
B.12 Station 931
Difference in Chl-a Concentrations and Temperature over time at station 931
-3.000
-2.500
-2.000
-1.500
-1.000
-0.500
0.000
0.500
1.000
April May June July August Sept Oct
Time (months)
Chl
-a C
once
ntra
tions
(μg/
L)
and
Tem
pera
ture
(°C)
Di
ffere
nce
Chl-aTemp
B.13 Station 935
Chlorophyll-a differences over time at station 935
-1
-0.5
0
0.5
1
1.5
2
2.5
April May June July August Sept Oct
Time (months)
Chl-a
Con
cent
ratio
ns (μ
g/L)
Bottom: Chl1Near-bottom: Chl2Difference
83
Temperature differences over time at station 935
-20
-15
-10
-5
0
5
10
15
20
25
April May June July August Sept Oct
Time (months)
Tem
pera
ture
(°C
)
BottomSurfaceDifference
Difference in Chl-a Concentrations and Temperature over time at station 935
-16.000-14.000-12.000-10.000-8.000-6.000-4.000-2.0000.0002.000
April May June July August Sept Oct
Time (months)
Chl
-a C
once
ntra
tions
(μg/
L)
and
Tem
pera
ture
(°C)
Di
ffere
nce
Chl-aTemp
84
B.14 Station 938
Temperature differences over time at station 938
-20
-15
-10
-5
0
5
10
15
20
25
April May June July August Sept Oct
Time (months)
Tem
pera
ture
(°C
)
BottomSurfaceDifference
Difference in Chl-a Concentrations and Temperature over time at station 938
-14.000
-12.000
-10.000
-8.000
-6.000
-4.000
-2.000
0.000
2.000
April May June July August Sept Oct
Time (months)
Chl
-a C
once
ntra
tions
(μg/
L)
and
Tem
pera
ture
(°C)
Di
ffere
nce
Chl-aTemp