Tidal geomorphology affects phytoplankton at the transition from forested streams to tidal rivers

Tidal geomorphology affects phytoplankton at the transitionfrom forested streams to tidal rivers

SCOTT H. ENSIGN*, MARTIN W. DOYLE †, 1 AND MICHAEL F. PIEHLER ‡

*Curriculum for the Environment and Ecology, University of North Carolina at Chapel Hill, Chapel Hill, NC, U.S.A.†Department of Geography, University of North Carolina at Chapel Hill, Chapel Hill, NC, U.S.A.‡Institute of Marine Sciences, University of North Carolina at Chapel Hill, Morehead City, NC, U.S.A.

SUMMARY

1. Coastal rivers can have long tidally influenced reaches that are affected by tides but do not

contain saline water. These tidal freshwater reaches have steep geomorphic gradients where the

river transitions from narrow, heavily shaded streams to wide, unshaded channels. The influence

of these gradients on river ecosystem production is poorly understood.

2. We characterised gradients in irradiance, geomorphology, water clarity and chlorophyll a along

9- to 16-km tidal freshwater reaches of the Newport and White Oak Rivers in North Carolina,

USA, and examined the effect of nutrient enrichment on phytoplankton growth in the Newport

River. Underwater irradiance was modelled at 2–4 week intervals along both rivers using

measurements of the above-canopy irradiance, canopy cover, water column light attenuation (Kd)

and water depth. Suspended material (TSS), dissolved organic carbon (DOC) and chlorophyll a

were sampled at 2-week interval at five sites on the Newport River and on four dates at four sites

on the White Oak River over the course of one year.

3. Phytoplankton nutrient limitation was assessed at three locations along the tidal gradient. River

water was collected during March, April, June and October, and incubated in 10-L plastic outdoor

containers under ambient water temperature and sunlight. Additions of inorganic nitrogen and

phosphorus served as treatments; growth rate during the 4 days of incubation was calculated from

the change in chlorophyll a concentration over time.

4. Canopy cover decreased from more than 90% to <10% over the length of both tidal freshwater

rivers. Water column irradiance and phytoplankton biomass increased as tree canopy cover

decreased and channel width increased. Channel width exceeded predictions for non-tidal rivers

by threefold because of tidal influence. TSS and DOC decreased significantly along the length of

the Newport River, but no significant gradients were observed in the White Oak River. Kd did not

vary along the tidal gradient of either river.

5. Mesocosm experiments indicated that inorganic nitrogen and phosphorus enhanced the

growth of phytoplankton advected from the non-tidal river into the tidal freshwater river during

spring and summer. Phytoplankton in the tidal freshwater reach were generally not nutrient

limited.

6. Tidal hydrology (in the absence of saltwater) directly affected the morphology of the channel

and indirectly affected biological growth and production. The significant increase in river width,

irradiance and phytoplankton biomass distinguished these tidal freshwater ecosystems from their

upstream (non-tidal fluvial) counterparts, while the strong influence of riparian shading

distinguished them from the saline estuaries downstream. Future development of ecosystem and

biogeochemical models for tidal freshwater rivers will benefit from the linkages between

geomorphology and biological processes identified here.

Correspondence: Scott H. Ensign, USGS National Research Program, 12201 Sunrise Valley Dr, Mail Stop 430, Reston VA 20190, U.S.A. E-mail:

[email protected] address: Martin W. Doyle, Nicholas School of the Environment, Duke University, Box 90328, Durham, NC 27708, U.S.A.

Freshwater Biology (2012) doi:10.1111/j.1365-2427.2012.02856.x

Published 2012. This article is a U.S. Government work and is in the public domain in the USA 1

Keywords: fluvial geomorphology, hydraulic geometry, irradiance, phytoplankton, tidal freshwater river

Introduction

Along low-gradient coastal plains world-wide, there is a

transition zone between rivers and estuaries in which

water flow is affected by tides but not saltwater. This tidal

freshwater zone extends hundreds of kilometres inland in

large rivers such as the Amazon (644 km, Wheeler, 1893),

Mississippi (370 km, Galler & Allison, 2008) and Gambia

(276 km, Amphlett & Brabben, 1991), and encompasses up

to half the length of smaller coastal plain rivers (Ander-

son, 1986). Across the US coastal plain, a vast network of

tidal freshwater rivers sustains 192 873 ha of tidal fresh-

water wetlands (Field et al., 1991). Tidal freshwater rivers

around the globe constitute a unique ecosystem along the

riverine-estuarine gradient (Van Damme et al., 2009).

Primary production within the tidal freshwater zone is

governed by a combination of riverine and estuarine

factors (Schuchardt, Haesloop & Schirmer, 1993; Van

Damme et al., 2009). Tides from downstream and catch-

ment runoff from upstream affect spatial patterns in

channel morphology (Langbein, 1963) and hydrology

(Dalrymple & Choi, 2007), subsequently affecting the

accumulation of phytoplankton biomass within the tidal

freshwater zone (Lucas, Thompson & Brown, 2009).

Gradients in environmental variables such as irradiance,

water clarity and nutrients are also affected by tides and

affect phytoplankton growth (Bukaveckas et al., 2011).

These spatial gradients in geomorphology and environ-

mental factors underlie a gradient in riparian forest

shading that occurs between coastal plain rivers and

estuaries (Vahatalo, Wetzel & Paerl, 2005). However, this

gradient in tree canopy shading and resultant patterns in

irradiance and phytoplankton growth in tidal freshwater

rivers have not been characterised at the transition from

river to estuary. Consequently, knowledge of the factors

affecting primary production in tidal freshwater rivers is

incomplete.

The percentage of above-canopy irradiance that reaches

a river is a function of topographic shading, forest canopy

shading and the width of the channel (Julian, Doyle &

Stanley, 2008a). In coastal plain environments with low-

relief topography, shading from surrounding hill slopes is

minimal but tree canopy shading can block 99% of the

incoming irradiance (Vahatalo et al., 2005). The width of

the channel also affects how much the tree canopy shades

the river. Even when bordered by a dense riparian forest

canopy, a wide channel will receive more irradiance per

unit of channel width than a narrow channel. Therefore,

the tendency for tidal rivers to widen rapidly in the

downstream direction (Leopold, Wolman & Miller, 1964)

may play an important role in affecting irradiance and

phytoplankton growth along a gradient of tidal influence.

Once light reaches the river water surface, the average

irradiance within the water column is affected by channel

depth and water clarity. The increase in channel depth

along the tidal freshwater zone reduces average water

column irradiance. Water clarity declines along the tidal

freshwater zone because of increased suspended particu-

late matter entrained by tidally enhanced flow velocity

(Chen et al., 2005; Lionard et al., 2005). Dissolved organic

carbon (DOC) also attenuates irradiance, particularly in

blackwater rivers where DOC concentrations are high

(Meyer, 1990). Spatial gradients in DOC may develop

along the tidal freshwater zone because of photo-oxida-

tion (Wiegner & Seitzinger, 2001) and bacterial metabo-

lism (Edwards & Meyer, 1987; Sabater, Meyer & Edwards,

1993; Moran, Sheldon & Sheldon, 1999). It is unknown

what the combined effect of gradients in depth, particu-

late matter and DOC is on light attenuation in tidal

freshwater rivers.

In addition to irradiance, nutrient concentrations can

limit phytoplankton growth in tidal freshwater rivers.

Longitudinal gradients in nitrogen and phosphorus

related to tidal mixing and biogeochemical processes

were found to influence phytoplankton growth and

nutrient limitation in the St Johns River in Florida,

U.S.A., although irradiance was the primary factor limit-

ing phytoplankton growth (Phlips et al., 2000). Nitrogen

and phosphorus both limited phytoplankton growth in

two blackwater, tidal freshwater rivers in North Carolina

during spring and summer (Mallin et al., 2001). However,

in the turbid Neuse River, North Carolina, immediately

upstream from the tidal freshwater zone nutrients limited

phytoplankton growth only under light-saturated condi-

tions (Whalen & Benson, 2007). These studies indicate that

nutrient limitation of phytoplankton growth in tidal

freshwater rivers is likely to be closely associated with

spatial and temporal trends in light availability along the

tidal gradient.

Riparian shading, geomorphic gradients and water

characteristics affect irradiance in tidal freshwater rivers,

but the combined influence of these factors on reach-scale

patterns in irradiance and phytoplankton growth has not

been examined. This study addressed this knowledge gap

2 S. H. Ensign et al.

Published 2012. This article is a U.S. Government work and is in the public domain in the USA, Freshwater Biology, doi:10.1111/j.1365-2427.2012.02856.x

with observational and manipulative studies of how these

patterns are affected by tidal influence. The first objective

was to determine the spatial and temporal patterns in

irradiance and relate these to patterns in tree canopy

shading, channel morphology and water clarity. The

second objective was to quantify how tide-affected chan-

nel width and depth along the tidal freshwater zone, since

these factors directly and indirectly affect irradiance to the

river and subsequent phytoplankton growth. The third

objective was to examine the spatial and temporal

patterns in inorganic nitrogen and phosphorus concen-

tration and determine their influence on phytoplankton

growth along this gradient. Intensive water sampling was

performed on one river over a year, and the general

patterns found there were corroborated with less-inten-

sive sampling on an adjoining tidal freshwater river. Both

rivers have relatively short fluvial–estuarine transition

zones (9–16 km between the head of tide and oligohaline

estuary), which we expected to create steep gradients in

geomorphic and ecological conditions and facilitate the

detection of spatial trends.

Methods

Study sites and channel morphology

The Newport (34.8�N 76.8�W) and White Oak (34.8�N

77.2�W) Rivers in North Carolina are blackwater rivers

with relatively small drainage areas (150 and 200 km2,

respectively) and tidal freshwater reaches of nine and 23

river km, respectively. General characteristics of these

coastal plain blackwater rivers include low-gradient,

extensive forested riparian zones, and highly coloured

water because of dissolved organic matter. Tidal range is

c. 1 m at the downstream end of each study reach and

diminishes to zero at the upstream end of each study

reach (Ensign et al., 2012). Tertiary-treated waste-water

effluent from the town of Newport enters the Newport

River c. 5 km downstream from the head of tide.

Sampling of physicochemical parameters and chloro-

phyll a was conducted at five sampling sites (A through E)

on the Newport River, located 3 km upstream from the

head of tide, and at 2, 4, 5.5 and 9 km downstream from

the head of tide. Morphology and riparian characteristics

of the Newport River are similar between the non-tidal

site A and head of tide, so we assume that physicochem-

ical parameters change little between site A and the head

of tide 3 km downstream. Therefore, we present data

from site A as distance 0 km from the head of tide. On the

White Oak River, four sites (A through D) were sampled

for physicochemical parameters and chlorophyll a and

were located 4, 5, 9 and 16 km from the head of tide,

respectively. On the Newport River, sites A through D

were bordered by hardwood forests composed of cypress

(Taxodium spp.), red maple (Acer rubrum L) and tupelo

(Nyssa spp.), while the riparian zone at site E was a

transitional cypress swamp-oligohaline marsh. Sites A

through C on the White Oak were bordered by cypress-

maple-tupelo forest, while site D was bordered by a

transitional cypress-oligohaline marsh.

Channel width and depth were measured along these

tidal gradients between the vegetated banks of the river at

500-m interval on the Newport River and 1000-m interval

on the White Oak River. On the Newport River, a line was

strung across the river, and channel depth was measured

at 2-m interval from a boat. This same procedure was

used on the White Oak in the upper half of the tidal

freshwater zone, but was not logistically possible in the

wider portions of the White Oak River. In the lower

portion of the White Oak River channel, therefore, width

was measured from aerial photographs from the US

Geological Survey using Google Earth (Google, 2012), and

channel depth was measured mid-channel using an SM-5

Depthmate portable sounder (Speedtech Instruments,

Great Falls, VA, U.S.A.).

An important part of our analysis was to identify how

channel morphology departed from its upstream, non-

tidal condition along the tidal freshwater reach. To do

this, we predicted channel width and depth along the

tidal freshwater reach using hydraulic geometry equa-

tions developed for non-tidal coastal plain rivers in North

Carolina by Sweet & Geratz (2003). The relationship

between catchment area (Aw, mi2) and bankfull channel

width (Wbkf, ft) was Wbkf = 9.64(Aw)0.38, with an R2 of 0.95;

the relationship between Aw and bank full channel depth

was Dbkf = 0.98(Aw)0.36, with an R2 of 0.92. The 95%

confidence interval of the predicted values on the New-

port and White Oak Rivers was also computed, and all

predictions were converted to metre. Catchment area at

five locations on the Newport River was determined with

elevation-based flow modelling in a GIS (Ensign, Piehler

& Doyle, 2008). Catchment area at three locations on the

White Oak River was determined from the Watershed

Boundary Dataset (http://datagateway.nrcs.usda.gov).

Irradiance modelling

Solar irradiance at the river water surface was modelled

over a 1-year period that encompassed the period of water

sampling on both rivers. Water surface irradiance was

calculated as a function of above-canopy irradiance, tree

canopy shading and orientation of the channel using the

Tides and river ecosystems 3


solar simulator software included with Gap Light Ana-

lyzer (Frazer, Canham & Lertzman, 1999). Average daily

solar irradiances for the study area were obtained from

the North Carolina State Climate Office (http://www.nc-

climate.ncsu.edu/) at three sites near the study area, and

solar irradiance was converted to photosynthetically

active radiation (PAR) using a conversion factor of 2.04

(Meek et al., 1984). Canopy interception of irradiance was

calculated from hemispherical photographs of the tree

canopy taken with a Nikon Coolpix 4500 (Nikon Inc.,

Melville, NY, U.S.A) camera with fisheye lens at 500 m

(Newport River) and 1000 m (White Oak River) intervals

during leaf-on (June in the Newport River, May in the

White Oak River) and leaf-off periods (January–February

in both rivers). The colour photographs were digitised to

black and white using automatic threshold detection

algorithms in SideLook 1.1.01 software (Nobis & Hunz-

iker, 2005), and the per cent canopy cover was determined

using Gap Light Analyzer software (Frazer et al., 1999).

Average daily irradiance to the river’s surface was

modelled with Gap Light Analyzer software. Leaf-on

canopy cover data were used for May–October period,

leaf-off photos were used for December–March, and an

average of leaf-on and leaf-off values were used for April

and November. Topographic shading by the surrounding

coastal plain landscape was assumed to be negligible and

was not parameterised in the model. Average water

column irradiance was calculated using the formula:

Ix = I0 · [1 ) exp()k · z)] ⁄ k · z, where Ix is the average

water column irradiance, I0 is the modelled water surface

irradiance, k is the water column attenuation coefficient

and z is average channel depth at mid-tide.

Water sampling

Water sampling was conducted on the Newport River

every 2 weeks between May 2006 and May 2007. The

White Oak River was sampled on four dates in 2009.

Sampling of the White Oak River was focused around the

period of maximum phytoplankton biomass observed in

the Newport River. On both rivers, we attempted to take

all samples during the ebb tide, although this was not

always possible at all sites during each sampling trip.

During each sampling, water column attenuation of PAR

was measured using a 4-pi irradiance sensor and LI1400

display ⁄ logger (Li-Cor Environmental, Lincoln, NB,

U.S.A.) at 0.25-m interval. A YSI 6800 multiparameter

water quality sonde and 650 display (Yellow Springs

Instruments, Yellow Springs, OH, U.S.A.) were used to

measure salinity and turbidity. Salinity was measured to

provide an indication of the freshwater–saltwater bound-

ary, and turbidity was measured as an indicator of

suspended matter.

Surface water samples (2 L) were collected and

returned to the UNC Chapel Hill Institute of Marine

Science for analysis of chlorophyll a, dissolved organic

carbon (DOC), nitrogen, phosphorus and total suspended

solids (TSS). Fifty to 100 mL aliquots was filtered through

0.7-lm glass fibre filters, and chlorophyll a captured on

the filter was extracted in 90% acetone and analysed on a

Turner Trilogy fluorometer (Turner Designs, Inc., Sunny-

vale, CA, U.S.A.). The filtrate was analysed for DOC on a

Shimadzu 5000TOC analyzer (Shimadzu, Columbia, MD,

U.S.A.). DOC concentration was used as an indicator of

how much attenuation of irradiance was attributed to

dissolved material. Nitrate plus nitrite (referred to here-

after as just NO�3 ), ammonium (NHþ4 ) and dissolved

reactive phosphorus (referred to hereafter as just PO�34 )

were measured using a Lachat Quikchem 8000 autoana-

lyzer (Lachat Instruments, Milwaukee, WI, U.S.A.). TSS

was measured by filtering up to 1 L of water through a

2.7-lm pre-combusted glass fibre filter (Whatman GF ⁄D)

and drying at 105 �C until a constant weight was

achieved. TSS was not measured in the White Oak River,

so we relied on turbidity as an indicator of suspended

material in this river.

We tested for spatial trends in TSS, turbidity, DOC and

Kd which would indicate an effect of tidal influence along

the gradient. For each of these parameters (excluding TSS

in the White Oak River), a mixed-effect linear model was

developed using distance from the head of tide (km) and

river as fixed effects and the date of sampling as a random

effect. This modelling framework accounted for the

correlation between measurements during each sampling

date and tested the null hypothesis of zero slope in each

regression model. All statistical analyses were performed

using R (R Development Core Team, 2012).

Mesocosm experiments

Mesocosm experiments were conducted to determine how

changes in nutrient concentration along the tidal gradient

affect phytoplankton growth. Experiments were per-

formed in April, June and October 2006 and March 2007

using river water from Sites A (non-tidal), C (tidal

freshwater) and E (oligohaline) on the Newport River.

River water (5 L) was incubated in 10-L polyethylene

cubitainers (Hedwin Corp., Baltimore, MD, U.S.A.) in an

outdoor pond at the nearby UNC Institute of Marine

Sciences in Morehead City, North Carolina. A control and

a nutrient treatment were applied in 4 replicates; the

nutrient treatment was comprised of +140 lg N-NO�3 L)1,



+140 lg N-NHþ4 L)1 and +155 lg P-PO�34 L)1 (expressed

as the concentration increase over background). Initial

and final concentrations of NO�3 , NHþ4 and PO�34 are

provided in Table 4. Since the purpose of the experiments

was to measure the effects of nutrient enrichment when

irradiance was not limiting, irradiance was reduced to

66% of ambient conditions using black plastic window

screen. This design allowed phytoplankton much more

irradiance than they receive in the river but was not high

enough to cause photo-inhibition. Irradiance and temper-

ature were equivalent for all mesocosms during each

study. Phytoplankton growth rate was determined from

the slope of mixed-effects linear regression models

developed from log(chlorophyll a) values on days 1, 2

and 4 of each experiment, with fixed effects for site and

nutrient treatment and random effects for each cubitainer

over the course of the experiment. Model simplification

was performed using likelihood ratio tests with ANOVAANOVA

(a = 0.05).

Identifying the factors that affect phytoplankton com-

munities is a critical step in understanding and managing

aquatic systems. Nutrient bioassays have been used

extensively to try to provide this information (Paerl &

Bowles, 1987). Results from nutrient addition bioassays

reveal the response of the native phytoplankton to

identifiable changes in nutrient conditions, but generali-

sations from the experiments must be drawn with caution

(Hecky & Kilham, 1988). Nutrient addition bioassays are

often conducted on small scales and always include a

containment effect. Some have argued that their results

have little relationship to the overall ecology of the system

in which they are conducted (Carpenter, 1996). However,

our experiments were a direct assessment of the role of

nutrients and provide data to inform future experiments

and modelling. We interpreted our data within the

context of knowledge of the overall system and an

understanding that the experiments were not a perfect

tool.

Results

Channel morphology, canopy cover, irradiance and

phytoplankton biomass

The width of both rivers was within the predicted range

of non-tidal rivers in the upper third of the tidal

freshwater zone, but width exceeded the range of non-

tidal rivers by up to threefold in the lower tidal freshwater

zone (Figs 1 and 2). The depth of the Newport was within

the range of predicted non-tidal rivers in the upper half

but greater than predictions in the lower half of the tidal

freshwater zone (Fig. 1). The White Oak River depth was

within the range of non-tidal predictions throughout the

tidal freshwater zone (Fig. 2).

Canopy cover on both rivers was higher during

summer than winter, ranging from more than 80% in

summer at the upstream end to <10% at the downstream

end during winter (Fig. 3a,b). In the Newport River, a

Distance from head-of-tide (km)

Cha

nnel

wid

th (

m)

0 41 2 3 5 6 7 8 9

0

5

10

15

20

25

30

35

Observation at high tideBankfull prediction for a non−tidal river

0.0

0.5

1.0

1.5

2.0

2.5

3.0

Cha

nnel

dep

th (

m)

Fig. 1 Observed and predicted bankfull channel width along the

tidal freshwater Newport River. Vertical bars on bankfull predictions

indicate the 95% confidence interval of the predicted value.

0 2 4 6 8 10 12 14 16

0102030405060708090

Cha

nnel

wid

th (

m)

Observation at mean tide

Bankfull prediction for anon-tidal river

0.00.51.01.52.02.53.03.54.04.5

Cha

nnel

dep

th (

m)

Distance from head-of-tide (km)

Fig. 2 Observed and predicted channel width and depth on the tidal

freshwater White Oak River. Vertical bars on bankfull predictions

indicate the 95% confidence interval of the predicted value. Only a

single mid-channel depth was measured in the White Oak River, so

the average cross-channel depth was estimated by multiplying mid-

channel depth measurement by the ratio of mean depth ⁄ maximum

depth found in the Newport River. The grey-shaded area shows the

estimated mean cross-channel depth in the White Oak River using

the minimum and maximum of the Newport River ratio.



considerable decrease in canopy cover began to occur

3 km downstream from the head of tide during the

summer period (Fig. 3a), which is approximately the same

location where channel width increased markedly (Fig. 1).

The decrease in canopy cover on the White Oak was most

dramatic c. 4 km downstream (Fig. 3b), although the

(a) (b)

(c) (d)

(e) (f)

(g) (h)

Fig. 3 Per cent tree canopy during summer and winter on the Newport (a) and White Oak (b) Rivers; irradiance at the water surface on the

Newport (c) and White Oak (d) Rivers; average water column irradiance on the Newport (e) and White Oak (f) Rivers; chlorophyll a on

the Newport (g) and White Oak (h) Rivers. The data for January to early April are from 2007, while the data for late April to December are

from 2006; data were plotted by month as opposed to chronological order. Chlorophyll a in the Newport River was linearly interpolated

between the five sampling locations at each sampling date.



channel did not begin to widen significantly until 6 km

downstream (Fig. 2). These locations where canopy cover

decreased closely correspond with increased water sur-

face irradiance (Fig. 3c,d). The highest water surface

irradiance occurred in the lower tidal region of both

rivers during April when above-canopy irradiance was

highest (Fig. 4). Water surface irradiance was always

greater downstream than upstream, but this contrast was

greater during summer due in part to the more foliated

tree canopy during leaf-on conditions. Average water

column irradiance followed the general temporal patterns

in water surface irradiance, with deviations because of

the longitudinal patterns in depth and water clarity

(Fig. 3e,f).

Chlorophyll a peaked during April and May in the

Newport River and June and July in the White Oak River

(Fig. 2g,h). In the Newport River, the peak chlorophyll a

occurred around 5 km downstream from the head of tide

(Fig. 3g). Chlorophyll a was high at both 5 km and 15 km

downstream in the White Oak River (Fig. 3H). These

locations and periods of high chlorophyll a coincided with

high water column irradiance.

Water characteristics and clarity

Sites A to D on the Newport River were fresh throughout

the study period (salinity <0.5), and site E was oligohaline,

with salinity ranging from 0.04 to 2.6 (Table 1). Sites A

0

200

400

600

800

µmol

esP

AR

m–2

s–1

Jan Mar May Jul Sep Nov

Newport RiverWhite Oak River

Fig. 4 Average hourly solar irradiance above the tree canopy

during the annual period of irradiance modelling in the Newport and

White Oak Rivers.

Table 1 Summary statistics of salinity (psu), total suspended solids (TSS mg L)1), turbidity (NTU), dissolved organic carbon (DOC) (mg L)1)

and Kd (m)1) on the Newport and White Oak Rivers; values are means ± standard deviations, minimum-maximum and sample size

River Site Salinity TSS Turbidity DOC Kd

Newport River A 0.04 ± 0.02 2.7 ± 3.3 0.2 ± 1.2 36.4 ± 6.7 6.9 ± 1.1

0.02–0.12 0–11 0–6 22.7–50.0 5.2–10.8

28 23 27 25 25

B 0.05 ± 0.02 2.7 ± 3.3 1.3 ± 1.9 28.6 ± 6.0 6.9 ± 1.6

0.03–0.11 0.0–11 0–7 16.1–39.9 5.0–10.6

23 23 23 21 10

C 0.06 ± 0.03 4.6 ± 5.5 3.6 ± 4.6 26.7 ± 5.4 6.9 ± 0.9

0.03–0.14 0–23 0–18 16.3–36.0 5.4–9.4

28 25 27 24 22

D 0.07 ± 0.04 5.9 ± 5.5 5 ± 5.1 26.5 ± 6.3 6.8 ± 1.1

0.03–0.20 0–20 0–18 15.4–40.2 4.1–8.6

23 23 23 20 21

E 0.25 ± 0.54 13.3 ± 11.8 9.6 ± 8.9 23.7–5.5 7.3 ± 1.4

0.04–2.60 1.0–46.0 0–36 14.8–35.7 4.9–10.6

28 25 27 25 25

White Oak River A 0.14 ± 0.04 no data 5.0 ± 2.0 no data 4.3 ± 1.2

0.10–0.19 3.2–7.7 3.2–5.9

4 4 4

B 0.12 ± 0.03 no data 4.6 ± 2.1 22.3 ± 5.1 4.0 ± 0.5

0.10–0.16 3.4–7.7 16.4–27.4 3.5–4.6

4 4 4 4

C 0.26 ± 0.32 no data 2.4 ± 2.0 22.3 ± 3.3 4.0 ± 0.3

0.1–0.75 0.4–5.1 18.5–26.5 3.5–4.3

4 4 4 4

D 1.62 ± 2.88 no data 5.6 ± 4.2 21.6 ± 3.5 3.6 ± 0.8

0.13–5.95 1.5–10.9 19.5–26.7 2.8–4.5

4 4 4 4



and B on the White Oak River were fresh while sites C and

D were oligohaline, ranging from 0.1 to 0.75 psu and from

0.13 to 5.95 psu. TSS on the Newport River increased from

a mean of 2.7–13.3 mg L)1 along the tidal gradient

(Table 1). There was a significant linear trend in TSS

of 1.3 ± 0.28 mg L)1 km)1 along the Newport River

(Table 2). Turbidity increased along the Newport River

from a mean of 0.2–9.6 NTU, with a significant linear

trend of 1.1 ± 0.2 NTU km)1. Turbidity on the White Oak

River was lowest at the oligohaline Station C with a

mean of 2.4 NTU; the regression between turbidity

and distance on the White Oak was not significantly

different from zero (0.02 ± 0.8 NTU km)1). DOC on the

Newport River decreased from a mean of 36.5 at Station A

to 23.7 at Station E, with a regression coefficient of

)1.3 ± 0.1 mg L)1 km)1. DOC decreased more modestly

in the White Oak River from 22.3 at Station B to 21.6 at

Station D; the regression coefficient was not significantly

different from zero ()0.09 ± 0.4 mg L)1 km)1). The mean

Kd on the Newport River ranged from of 6.9 m)1 at Station

A to 7.3 m)1 at Station E, and 4.3 m)1 at Station A to

3.6 m)1 at Station D on the White Oak River. Kd did not

change significantly along the tidal gradient of either river

(regression slope estimates were not distinguishable from

zero; Table 2).

Nutrient concentrations and their relationship with

chlorophyll a

NO�3 concentration increased by 22 lg N L)1 km)1 in the

Newport River, but did not change with distance down-

stream in the White Oak River (Tables 2 and 3). Highest

mean NO�3 in the Newport River was found at Station C

(218 lg N L)1), while highest NO�3 in the White Oak was

much lower (96 lg N L)1 at Station A) (Table 3). The

change in NHþ4 with distance along the Newport River

was 1.2 lg N L)1 km)1, but was not distinguishable from

zero in the White Oak River. The mean NHþ4 at all sites

was similar between rivers, except that the maximum in

the Newport River (102 lg N L)1 at Station E) was greater

than the maximum in the White Oak River (52 lg N L)1 at

Station C). Mean PO�34 was greater at all stations on the

White Oak River than the Newport River. PO4) increased

by 1.8 lg P L)1 km)1 in the Newport River, but no

statistically significant relationship existed in the White

Oak River.

High chlorophyll a generally corresponded with high

ambient nitrogen and phosphorus concentrations in the

Newport River, although this was not the case in the

White Oak River (Fig. 5). In the Newport River, variation

in chlorophyll a increased dramatically when nitrogen

Table 2 Summary of mixed-effect modelling results

Model River Estimate Standard error d.f. P value

TSS � distance Newport Intercept = 0.65 0.71 95 0.361

Slope = 1.29 0.28 95 0.000

Turbidity � distance · river Newport Intercept = )0.26 0.30 110 0.400

Slope = 1.06 0.18 110 0.000

White Oak Intercept = 4.27 1.11 29 0.000

Slope = 0.02 0.80 110 0.043

Dissolved organic

carbon � distance · river

Newport Intercept = 32.4 1.3 105 0.000

Slope = )1.3 0.1 105 0.000


Slope = )0.09 0.4 105 0.000

Kd � distance · river Newport Intercept = 6.9 0.19 87 0.000

Slope = 0.03 0.03 87 0.381


Slope = )0.04 0.12 87 0.325

NO�3 � distance · river Newport Intercept = 30 12 101 0.015

Slope = 22 3.0 101 0.000

White Oak Intercept = 87 50 29 0.148

Slope = )3.5 11 101 0.003

NHþ4 � distance · river Newport Intercept = 38 2.4 101 0.000

Slope = 1.2 0.36 101 0.001

White Oak Intercept = 44 9.2 29 0.404

Slope = )1.2 1.4 101 0.018

PO�34 � distance + river Newport Intercept = 9.3 1.4 101 0.000

Slope = 1.8 0.26 101 0.000

White Oak Intercept = 28 5.8 29 0.000

Slope = 1.1 1.1 101 0.331



and phosphorus were greater than about 150 lg N L)1

and about 10 lg P L)1, respectively. In contrast, chloro-

phyll a in the White Oak River spanned nearly the entire

range of the Newport River’s chlorophyll a, even when

nitrogen was <200 lg N L)1. In summary, high nitrogen

and phosphorus concentrations were not always associ-

ated with high chlorophyll a, and high chlorophyll a was

not always associated with high nutrient concentrations.

Mesocosm growth experiments

Nutrients did not limit the growth of phytoplankton in

mesocosms from any location of the river during April

2006 (Fig. 6a,b). While the nutrient-enriched mesocosms

for site C showed a lower growth rate than the controls

(estimated at the 50% confidence interval), this estimated

growth rate was probably an artefact of abnormally low

chlorophyll a concentrations after 1 day of incubation

(Fig. 6a). In June 2006, growth at all sites was greater in

the nutrient-enriched mesocosms, although only at the

50% confidence interval at site E (Fig. 6c,d). June growth

rates in the controls for sites A and C were similar, as

were the controls at sites C and E (Fig. 6d). In October

2006, no differences in growth rate were observed

(Fig. 6e,f), and mixed-effects modelling showed the nutri-

ent term to not be significant (therefore separate slopes

for the nutrient treatment are not shown in Fig. 6f). In

March 2007, phytoplankton growth differed significantly

between the non-tidal and tidal mesocosms, but those

differences were alleviated by the addition of nutrients to

the non-tidal mesocosms (Fig. 6g,h).

Nutrient additions to site A’s mesocosms increased the

combined NO�3 and NHþ4 as well as PO�34 concentrations

above ambient concentrations at sites C and E (Table 4),

thereby allowing the evaluation of how increased nutrient

concentration would affect phytoplankton advected from

the non-tidal river into the tidal freshwater zone. In June

and March, higher nutrient concentrations in the tidal

freshwater zone enhanced the growth of phytoplankton

from the non-tidal river. Thus, the gradients in nutrients

we observed (Table 2) probably contributed to the peak in

phytoplankton biomass (Fig. 3g) during spring and sum-

mer in the upper tidal freshwater zone.

Nitrogen additions raised the concentration well above

the 200 lg L)1 threshold necessary for nutrient stimula-

tion of phytoplankton growth in blackwater rivers (Mallin

et al., 2004). Combined NO�3 and NHþ4 concentrations

<200 lg L)1 at the conclusion of the experiments in April

and June indicate that phytoplankton growth may have

become nutrient limited over the course of the experi-

ments. Ambient water chemistry in the Newport River

Table 3 Summary statistics of NO�3 ; NHþ4 , total dissolved nitrogen

(TDN) and PO�34 in the Newport and the White Oak Rivers

River Site NO�3 NHþ4 PO�34

Newport

River

A 14 ± 10 35 ± 10 8 ± 3

3–59 18–54 4–17

26 26 26

B 53 ± 23 46 ± 20 11 ± 4

5–120 16–81 6–20

22 22 22

C 218 ± 154 43 ± 16 26 ± 14

20–582 9–70 6–54

22 22 22

D 119 ± 107 45 ± 18 17 ± 14

4–452 13–81 3–60

26 26 26

E 216 ± 107 49 ± 22 25 ± 10

26–493 8–102 10–42

26 26 26

White Oak

River

A no data no data no data

B 96 ± 30 49 ± 3 28 ± 9

55–125 45–52 19–39

4 4 4

C 58 ± 12 29 ± 8 34 ± 10

47–72 20–40 21–44

4 4 4

D 63 ± 22 37 ± 10 37 ± 7

33–86 26–50 29–45

4 4 4

Data are means ± standard deviations, minimum–maximum and

sample size. All values are lg N L)1 or lg P L)1.

0

5

10

15

20

25 ABCDE

0

5

10

15

20

25

NO3−1 + NH4

−1 (µg L−1)

Whi

te O

ak c

hlor

ophy

ll a

(µg)

l/por

t Riv

er c

hlor

ophy

ll a

(µg)

0 200 400 600

BCD

PO4−3 (µg L−1)

0 20 40 60

(a) (c)

(b) (d)

Fig. 5 Chlorophyll a and dissolved inorganic nitrogen concentrations

in the Newport (a) and White Oak River (b), and chlorophyll a and

PO�34 concentration in the Newport (c) and White Oak River (d).



during these experiments was not affected by salinity or

exchange of sea water from the estuary ⁄salinity was

<0.1 psu at all sites (except the lower tidal site in April

2006 when salinity was 0.23 psu). The mesocosm results

therefore represent the growth dynamics of a freshwater

riverine, not estuarine, phytoplankton community.

Discussion

Effects of canopy cover and water clarity on irradiance and

phytoplankton

The first objective of this study was to explore the spatial

and temporal patterns in irradiance and phytoplankton at

the transition from a forested coastal plain river to an

oligohaline tidal river. In both rivers we studied, the

spatial and temporal patterns in water surface irradiance

closely corresponded with seasonal and longitudinal

changes in tree canopy shading. A large increase in

irradiance occurred at the same location that tree canopy

shading decreased. Reduction in water surface irradiance

to 1% of the above-canopy irradiance has also been

documented in other coastal plain rivers (Vahatalo et al.,

2005). There are a range of factors that affect tree canopy

shading in addition to channel width and leaf area,

including vegetation type, channel orientation (Julian,

Stanley & Doyle, 2008b) and riparian timber removal

(Ensign & Mallin, 2001), but the significant effect of tidal

morphology on riparian shading found in this study has

not previously been documented.

While water surface irradiance was related to tree

canopy cover, the water column irradiance was greatly

reduced by particulate and dissolved material. In the

Newport River, TSS increased while DOC decreased

along the tidal gradient, with each factor negating the

other’s effect on light attenuation. Because of these

opposing trends in TSS and DOC, no longitudinal trends

in the light attenuation were found in the Newport River.

No longitudinal trends in light attenuation or DOC were

found in the White Oak River, either. Increases in

suspended matter with proximity to the oligohaline

estuary are common in tidal freshwater rivers (Chen

et al., 2005), reducing light availability for phytoplankton

growth (Lionard et al., 2005; Muylaert, Tackx & Vyver-

man, 2005b; Muylaert et al., 2000; Bukaveckas et al., 2011).

Coastal plain blackwater rivers have low concentrations of

0.10.51.05.0

10.050.0

100.0

Chl

orop

hyll

a (µ

g L–1

)

Log

(bio

mas

s gr

owth

rat

e) (

day–

1 )

−0.5

0.0

0.5

1.0

1.5

0.10.51.05.0

10.050.0

100.0

0.0

0.5

1.0

1.5

0.10.2

0.51.02.0

0.0

0.5

Bioassay incubation time (days)

Biomass accumulation Biomass growth rate

0 1 2 3 40.10.20.51.02.05.0

10.020.0

0.0

0.5

A(non-tidal)

E(lower tidal)

Site

(a) (b)

(c) (d)

(e) (f)

(g) (h)

Fig. 6 Chlorophyll a response in mesocosm experiments in the Newport River during April (a), June (c), October (e), and March (g), and

the corresponding rate of growth determined using linear, mixed-effects regression modelling (b, d, f, h). Colour of lines on the left-hand panels

corresponds with x axis labels in the right-hand panels; solid lines indicate control mesocosms, and dashed lines indicate nutrient additions.

In the right-hand panels, the darker, thicker bars represent the 50% confidence interval of the slope estimate, and the lighter, longer bars

represent the 95% confidence interval of the slope estimate; filled circles indicate controls and open triangles indicate nutrient treatments.



suspended material (Meyer, 1990; Hupp, 2000), although

the tidal freshwater portion can have significantly higher

suspended material (as demonstrated by the fourfold

increase in TSS in the Newport River). The effect of tidal

hydraulics on river flow is a significant factor affecting

suspended matter and consequently irradiance in tidal

freshwater rivers, and further research is needed on how

the fluvial energy regime is altered by tidal flow.

The highly chromophoric, humic material that com-

prises the majority of DOC in blackwater rivers (Sabater

et al., 1993) has a major effect on light attenuation (Phlips

et al., 2000). Concentrations of DOC in blackwater tidal

rivers (>20 mg L)1) are much higher than those draining

piedmont catchments into tidal freshwater zones

(<10 mg L)1 in the Schelde estuary, Muylaert et al.,

2005a,b), making DOC dynamics particularly important

to ecosystem processes. The decrease in DOC observed

along the tidal freshwater Newport River was presumably

attributed to a combination of photooxidation and

bacterial respiration. Photooxidation can have a major

influence on DOC concentration (reviewed by Wiegner &

Seitzinger, 2001), especially given the longer residence

time in the tidal freshwater zone than in the river

upstream (Lucas et al., 2009). Bacterial respiration in

blackwater rivers also reduces DOC concentration, and

this utilisation can be enhanced by photooxidation (Moran

et al., 1999; Wiegner & Seitzinger, 2001).

Phytoplankton biomass peaked in the same region of

the Newport River that water column irradiance peaked,

and at roughly the same time of year. While fewer

observations of phytoplankton biomass were made on the

White Oak River, a similar correspondence was observed

between phytoplankton and irradiance. While the spatial

and temporal relationships between irradiance and phy-

toplankton were not surprising given that irradiance is

often the primary limiting factor for phytoplankton

growth, it was particularly interesting that the increase

in irradiance and phytoplankton occurred in the same

location where channel width deviated from its non-tidal

pattern. The significance of this tidal influence on channel

morphology and subsequent effects on phytoplankton are

discussed below.

Finally, it should be noted that long water residence

times in tidal freshwater rivers can contribute to increased

phytoplankton biomass, especially when river discharge

is low (e.g. Bennett, Woodward & Shultz, 1986; Schuc-

hardt & Schirmer, 1991). This factor probably played a

role in the development of phytoplankton communities in

the Newport and White Oak Rivers: lowest discharge

during the study period occurred from April–July 2006,

Table 4 Nitrogen and phosphorus concentrations in phytoplankton growth mesocosms

Month Site Treatment

Initial (day 0) Final (day 4)

NO�3 NHþ4 PO�34 NO�3 NHþ4 PO�3

4

April 2006 Non-tidal Control 3 49 17 11 38 8

Nutrients 143 189 172 150 128 162

Upper tidal Control 175 81 44 39 9 11

Nutrients 315 221 199 274 11 115

Lower tidal Control 226 92 32 4 7 7

Nutrients 366 232 187 24 18 78

June 2006 Non-tidal Control 15 37 8 12 26 8

Nutrients 209 185 163 78 31 40


Nutrients 209 185 169 91 23 50


Nutrients 328 190 191 9 22 27

October 2006 Non-tidal Control 17 24 6 14 22 4

Nutrients 157 164 161 153 129 181


Nutrients 145 158 159 213 93 121


Nutrients 314 160 167 324 97 127

March 2007 Non-tidal Control 6 34 7 6 26 2

Nutrients 146 174 162 133 108 123


Nutrients 310 210 175 302 105 141


Nutrients 347 187 174 341 99 135

All values are lg N L)1 or lg P L)1.



and March–April 2007 in the Newport River (Ensign et al.,

2007). Water residence time in the tidal freshwater

Newport River ranged from 3.5 to 6.5 days when calcu-

lated with the modified tidal prism method of Dyer (1997)

(Ensign et al., 2007). The approximate travel time based on

a continuous downstream flow velocity similar to that

measured above tidal influence is less than 1 day,

indicating that tidal hydrology allowed an additional

2–5 days for phytoplankton biomass to accumulate in the

tidal freshwater reach.

Tidal influence on channel morphology

The second objective of this study was to quantify how

tide-affected channel width and depth along the tidal

freshwater zone. Channel width increased along the tidal

freshwater zone of both rivers more rapidly than would

be expected in a non-tidal river of similar size. An obvious

deviation from predicted, non-tidal width did not occur

until one-third of the way downstream through the tidal

freshwater zone of both rivers. Depth also increased more

rapidly than would be expected in a non-tidal river,

although this effect of tide was more pronounced in the

Newport than White Oak River. These tidally affected

trends in channel width and depth strongly influenced

irradiance in opposing ways: the trend in width increased

surface water irradiance while the trend in depth

decreased the average water column irradiance.

In the studies of larger tidal freshwater rivers, where

riparian shading is not significant, changes in channel

depth significantly affect irradiance and subsequent

phytoplankton growth. In the St. Johns River tidal

freshwater zone, increases in channel depth decreased

the light availability downstream (Phlips et al., 2000). In

contrast, channel depth in the James River, Virginia,

U.S.A., decreased as width increased in the tidal fresh-

water zone and resulted in increased average water

column irradiances that enhanced phytoplankton growth

in this region (Bukaveckas et al., 2011). The James River

enters the tidal freshwater zone as a large piedmont river,

and therefore, the gradient in channel depth may differ

from coastal plain rivers such as the Newport and White

Oak Rivers.

Tidal channels exhibit a more rapid increase in width

and depth with distance downstream than do non-tidal

rivers (Langbein, 1963; Leopold, Wolman & Miller, 1964).

Since the tidal freshwater zone spans a disciplinary divide

between fluvial and estuarine geomorphology, the models

of channel morphology are fundamentally different on

either side of this zone. In rivers, channel morphology is

modelled as a function of catchment size and river

discharge (e.g. the hydraulic geometry models of Sweet

& Geratz, 2003 used in this study, which are based on

earlier research by Leopold & Maddock, 1953). In contrast,

estuarine morphology models are based on tidal prism

(D’Alpaos et al., 2010) and distance from the river mouth

(Davies & Woodroffe, 2010). In the relatively small coastal

plain rivers we studied, channel morphology diverged

from predictions made using a fluvial model at roughly

one-third of the way through the tidal freshwater zone.

We have also computed the relationship between tidal

prism and channel area and found that it differs signif-

icantly from the estuarine channels investigated by

D’Alpaos et al. (2010) (S. H. Ensign, unpublished data).

It is not unexpected that both fluvial and estuarine models

would fail to accurately predict channel morphology at

the transition between these two geomorphic regimes.

Further research is needed to develop empirical models of

channel morphology in the tidal freshwater transition

zone.

Nutrients gradients and their influence on phytoplankton

growth

The third objective of this study was to examine the

spatial and temporal patterns in inorganic nitrogen and

phosphorus concentrations and determine their influ-

ence on phytoplankton growth in this transition zone.

Inorganic nitrogen and phosphorus increased signifi-

cantly through the Newport River tidal freshwater zone,

but not in the White Oak River. In the Newport River,

this pattern was partly a function of treated waste-water

effluent that entered the river between sites C and D

(Ensign et al., 2007). Interestingly, the average NO�3 and

PO�34 concentrations were lower at site D (nearest the

effluent) than site C upstream and site E downstream.

Spatial gradients in nutrient concentration could also be

related to photomineralisation of dissolved and sus-

pended matter that produces inorganic nitrogen and

phosphorus (Southwell et al., 2009), particularly

given the increase in particulate matter, residence time

and surface irradiance in the lower tidal freshwater

river.

Mesocosm experiments showed that in March and

June the growth rate of the phytoplankton community

advected from upstream into the tidal freshwater reach

would increase when exposed to the higher nutrient

concentrations found in the upper tidal zone. Under

these nutrient-enriched conditions, during spring and

summer, the growth rate of the phytoplankton commu-

nity from the non-tidal river was equal to or greater

than the resident community in the tidal zone. The



mesocosm results indicate that the high phytoplankton

growth observed in the middle portion of the Newport

River tidal freshwater zone is partly a result of

gradients in nutrient concentration. However, a lack of

significant response in the bioassay experiment in April

and October, combined with the relatively high phyto-

plankton biomass (i.e. >10 lg L)1) observed at relatively

low nitrogen (<200 lg N L)1) and phosphorus

(<20 lg P L)1) concentrations in both rivers, indicate

that phytoplankton growth is not always limited by

nutrients in the tidal freshwater zone. Previous studies

of nutrient limitation in coastal plain tidal freshwater

rivers have also reported nutrient limitation of phyto-

plankton growth (Mallin et al., 2004). In summary,

phytoplankton growth in the tidal freshwater zone

was sometimes enhanced by nutrient enrichment, but

ambient nutrient concentrations are often adequate to

support considerable phytoplankton growth when other

factors (e.g. irradiance and grazing) are not limiting.

In conclusion, small coastal plain rivers undergo dra-

matic changes in morphology as they become influenced

by tides. Increased channel width drives a significant

increase in irradiance reaching the river surface, even

while hardwood forests border the channel. Increased

primary production within this zone is attributed to

increased irradiance, but the mechanism responsible for

increased irradiance, namely a tidally driven increase in

width and decrease in canopy shading, is unique among

both fluvial and estuarine ecosystems. In the rivers we

studied, there was a zone between one-third and one-half

of the way through the tidal freshwater reach where

channel morphology, irradiance and nutrients were opti-

mal for phytoplankton growth. While additional studies

are needed in a wider variety of coastal plain tidal

freshwater rivers, our data suggest that it may be possible

to develop a geomorphic framework for predicting where

this growth-optimising zone occurs in coastal plain tidal

freshwater rivers. Prediction of where and when phyto-

plankton growth occurs in tidal freshwater rivers based

on geomorphic attributes would help integrate tidal

freshwater river ecosystems with geomorphically based

conceptual models of river (e.g. Vannote et al., 1980;

Thorp, Thoms & Delong, 2006) and estuarine ecosystems.

Acknowledgments

We thank J. Muehlbauer, K. Siporin, A. Smyth, L.

Stevenson, S. Thompson, B. VonKorff and Dr. J. Weiss

for field and laboratory help, statistical support and

reviews. H. Godwin and D. Wilke provided access to field

sites on their property. This project was funded by NSF

REU # 0441504 (M. W. D.), EPA STAR Graduate Fellow-

ship #FP-91686901-0 (S. H. E.), NSF EAR-0815627 (M. F.

P.) and the NOAA Ecological Effects of Sea Level Rise

Program (M.F.P.). The research described in this paper

has been funded in part by the US EPA under the STAR

Graduate Fellowship Program. EPA has not officially

endorsed this publication, and the views expressed herein

may not reflect the views of the EPA.

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Documents

Tidal geomorphology affects phytoplankton at the transition from forested streams to tidal rivers