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2009 SOUTHEASTERN NATURALIST 8(2):255–266
Nutrients and Light Limitation of Phytoplankton
Biomass in a Turbid Southeastern Reservoir:
Implications for Water Quality
Dmitri Sobolev1,*, Kandis Moore2, and Ashley L. Morris2
Abstract - Water column turbidity, chlorophyll-a content, and dissolved inorganic
macronutrients were measured in Ross Barnett Reservoir, a turbid, artificial water
body located in central Mississippi. Measurements indicated that abundant nutrients
were present, whereas levels of chlorophyll-a were substantially below expectation.
Significant inorganic turbidity and the resulting low transparency suggested rapid
extinction of light within the water column. Sediment settling in an experimental microcosm
led to rapid increase in chlorophyll-a, consistent with the notion that algal
biomass abundance was limited by light, rather than nutrients. Based on our results,
an antagonistic relationship between the inorganic turbidity and inorganic nutrients
exists in the reservoir, the former modulating effects of the latter and thus limiting
the potential for algal blooms. Should suspended sediment be removed without
addressing nutrients at the same time, algal biomass is expected to increase dramatically.
Likewise, high levels of dissolved inorganic nutrients, together with limited
competition from the planktonic algae create an ideal set of conditions for fl oating
macrophyte mats to develop.
Algal growth in aquatic environments is often limited by availability of
major nutrients, such as phosphorus (P; Hecky and Kilham 1988) or nitrogen
(N; Ryther and Dunstan 1971), the so-called macronutrients. Increasing
supply of those nutrients leads to increase in primary production, potentially
resulting in overall decline in water quality, sporadic oxygen depletion,
toxic algal blooms, and fish kills (Lindholm et al. 1989, Smayda 1997). The
complex of natural processes associated with increasing primary productivity
of an aquatic system and attendant decline in the water quality is termed
eutrophication. High nutrient loads from municipal wastewater, agricultural
fertilizers, and surface water runoff (Carpenter et al. 1998) are the known to
accelerate eutrophication of many aquatic systems.
It was long known that turbid well-mixed lakes respond less to nutrient
additions than clear, stratified ones. Limiting effects of turbidity upon
algal growth have been well established (Knowlton and Jones 1996), and
a disconnect between nutrient concentrations and chlorophyll-a levels has
been demonstrated for turbid environments (Walker 1982). Light extinction
by inorganic particles can control primary production in turbid waters (e.g.,
Cole and Cloern 1984, Cole et al. 1992, Desmit et al. 2005, Irigoien and
Castel 1997, Pennock 1985, Wofsy 1983).
1Jackson State University, Biology Department, Box 18540, 1400 J.R. Lynch Street,
Jackson, MS 39217. 2Callaway High School, 601 Beasley Road, Jackson, MS 39206.
*Corresponding author - firstname.lastname@example.org.
256 Southeastern Naturalist Vol. 8, No. 2
Light limitation imposed by suspended solids is further modulated by
vertical circulation in the water body. The concept of critical depth (Sverdrup
1953), which is defined as depth where vertically integrated photosynthesis
equals vertically integrated algal respiration, was introduced to account for
such modulation. If the circulation depth of a water body is greater than
its critical depth, integrated gross primary production of the water column
would be less than integrated algal respiration (e.g., Grobbelaar 1985), resulting
in net algal biomass loss. Thus, the water column would be unable to
sustain a community of planktonic algae over significant periods of time; or,
in a more realistic situation, net primary production of the entire system will
be severely limited. This situation is frequently encountered in turbid wellmixed
estuaries and reservoirs, significantly affected by particulate-laden
surface runoff and resuspension (Cole et al. 1992, Irigoien and Castel 1997,
Desmit et al. 2005).
Reservoirs in the Southeastern United States appear to be similar to
estuaries in some respects. For example, reservoirs are affected by major
non-point nutrient inputs, and these same non-point sources provide suspended
inorganic particulates in the form of clay and silt that is carried into
those systems with runoff (Wendt et al. 1986). Many of the reservoirs are
sufficiently shallow for the bottom sediments to be readily resuspended by
wind and wave action, leading to high levels of inorganic turbidity, especially
during the drawdown periods, when water levels are low and greater
areas of sediments are exposed to circulation (Effl er et al. 1998).
Water quality studies of the Ross Barnett Reservoir have included bacterial
(Kishini et al. 2006, Tchounwou and Warren 2001) and macrophyte
surveys (Wersal et al. 2006). However, little is known about nutrient or
suspended sediment dynamics and their role in phytoplankton abundance
(including algal blooms) in this reservoir. In this work, we used field measurements
of nutrient concentrations and inorganic turbidity in Ross Barnett
Reservoir to identify potential limits on algal biomass, and used experimental
microcosms to indicate the potential response of algae to increased water
clarity that might result from reduction in suspended sediment loading.
Field Site Description
Ross Barnett Reservoir is an impoundment of the Pearl River in central
Mississippi, northeast and upstream of the Jackson metropolitan area
(Fig. 1). The total area of the reservoir is ca. 12,550 ha at full pool (Cooper
and Knight 1985). The reservoir is operated by Pearl River Valley Water
Supply District for the main purpose of supplying water to the city of Jackson.
The reservoir is not substantially impacted by nutrients from municipal
effl uent or urban runoff, but receives runoff from extensive agricultural
lands in the watershed. The same agricultural activities, as well as housing
construction in Rankin and Madison counties (Twumasi and Merem 2005),
contribute visually noticeable suspended particulate matter, which has led to
public concern and press coverage (DiLuizzo 2007).
2009 D. Sobolev, K. Moore, and A.L. Morris 257
Materials and Methods
Samples of surface water were collected five times, bi-weekly, from
27 May through 24 July of 2006 at 5 near-shore locations on Ross Barnett
Reservoir from existing docks/jetties (Fig. 1). GPS coordinates of each
of the sites are listed in Table 1. Measurements of chlorophyll-a (in vivo
fluoresence) and nephelometric turbidity were done in situ using a YSI
multiprobe. The multiprobe unit was maintained and calibrated according
to the manufacturer’s instructions. Sampling from land-connected
structures naturally limited our study locations to near-shore surface sites.
Table 1. Locations of the sampling stations in Ross Barnett Reservoir.
Station Name Latitude Longitude
1 Ross Barnett Reservoir Not sampled Not sampled
2 Brown’s Landing N32o31.136' W89o58.345'
3 Safe Harbor Marina N32o30.305' W89o56.174'
4 Hwy 43 Fishing Pier N32o31.122' W89o56.486'
5 Pelahatchie Bay Park N32o23.553' W90o01.616'
6 Fannin Landing N32o25.079' W90o01.305'
Figure 1. Sampling location in Ross Barnett Reservoir: Station 2, Brown’s Landing;
3, Safe Harbor Marina; 4, Hwy 43 Pier; 5, Pelahatchie Shore Park; 6, Fannin Landing.
258 Southeastern Naturalist Vol. 8, No. 2
Water samples were returned to the lab and preserved by freezing at -20
°C. Although freezing can alter composition of certain nutrient species,
such as ammonia or phosphate, this method was selected due to logistical
limitations. Prior to analysis, samples were thawed overnight at +4 °C, filtered
through 0.22 -μm membrane filter and analyzed for soluble reactive
phosphorus (SRP), ammonia, nitrite, and nitrate. No attempt was made to
measure organic forms of either phosphorus or nitrogen. The analyses were
conducted by use of spectrophotometric kits from Hach, using manufacturer-
supplied equipment and procedures. In short, those kits measure SRP by
ammonium molybdate technique (Hach method number 10210), ammonia
through the Nessler procedure (Hach method number 8038), and nitrate
and nitrite through the Griess reagent procedure (method number 8507);
nitrate being measured as nitrite following reduction by cadmium powder
(method number 10020). Molar measurements of inorganic nitrogen species
were combined to estimate total dissolved inorganic nitrogen content.
To further test the degree of turbidity-driven light limitations of primary
production, ca. 120 liters of Ross Barnett Reservoir water collected at Station
6 (Fannin Landing) in August 2006 were placed into triplicate 30-cm deep,
10-gallon aquaria in the ambient sunlight. The aquaria reduced the amount
of suspended sediment in the water column by settling and further increased
light availability because the sides of the aquaria were open to light as well.
The combination of limited circulation, settling sediment, and light input from
the side substantially increased light availability within the water over time.
Decline in suspended sediment and response of the algal community to it was
assessed by turbidity and chlorophyll-a measurements with the YSI probe
as described for natural samples. This experiment was conducted strictly in
a “before-and-after” fashion, with time as a single variable and no negative
controls included, as it would be impossible to maintain the water column turbidity
without introducing additional variables into the system.
Ross Barnett Reservoir exhibited high levels of suspended sediment on
all five sampling dates in this study. Overall turbidity, as measured by in situ
turbidity sensor, remained consistently high, exceeding 40 nephelometric
turbidity units (NTU) at times and averaging 33 NTU. In situ measurements
of chlorophyll-a content using in vivo fl uorescence produced values of 5 to
15 g L-1 chl-a. The chlorophyll content remained fairly uniform among stations
and sampling dates.
Measurements of dissolved inorganic nutrients showed substantial
amounts of both N and P present at all sampling stations and on all sampling
dates. The vast majority (consistently over 80% molar, at times as high as
95%) of N was present as ammonia. Nitrate was the second-most abundant
N species (about 10% molar), and nitrite ranged from below detection limit
to 5% molar of the total dissolved inorganic N. The amounts of total dissolved
inorganic N remained relatively constant throughout the sampling
2009 D. Sobolev, K. Moore, and A.L. Morris 259
period and appeared to be uniform throughout the sampling stations, remaining
between 0.14 and 0.36 mg N L-1 (10–26 μM) for most of the sampling
period (Fig. 2). SRP remained between 0.08 and 0.22 mg P L-1 (2.5–7 μM)
across the reservoir and throughout the sampling season (Fig. 3). The molar
N:P ratio remained below 10 at all times and at all stations. Ample amounts
of both dissolved inorganic nitrogen and phosphorus suggested no nutrient
limitations in our system.
Reduction in water-column depth and wind-wave disturbance in experimental
tanks resulted in a rapid (18 hours) two-fold drop in total turbidity.
Concurrently, chlorophyll-a concentration increased ten-fold over the 72
hours of the experiment (Fig. 4). At higher turbidity levels, there was a linear
relationship between turbidity and chlorophyll-a in the experimental system,
whereas a decrease in turbidity to about 15 NTU resulted in an apparent
uncoupling of the relationship between those two variables. In the natural
environment, such an interaction was not apparent (Fig. 5).
Figure 2. Dissolved inorganic nitrogen (DIN) concentrations in Ross Barnett Reservoir.
Error bars, confidence intervals at α = 0.05, not shown if less than the size of
symbol or for total DIN.
260 Southeastern Naturalist Vol. 8, No. 2
Figure 3. Soluble reactive phosphorus concentrations in Ross Barnett Reservoir. Error
bars, confidence intervals at α = 0.05, not shown if less than the size of symbol.
Figure 4. Turbidity and chlorophyll-a concentrations in shallow (30-cm) microcosms
(without circulation). Time zero measurements were taken immediately after tanks
were established. Error bars, confidence intervals at α = 0.05, not shown if less than
the size of symbol.
2009 D. Sobolev, K. Moore, and A.L. Morris 261
Dissolved inorganic nitrogen and phosphorus concentrations (Figs 2, 3),
as well as the N:P ratio observed in Ross Barnett Reservoir are typical of
those found in highly eutrophic water bodies (Downing and McCauley
1992). While the low N:P ratio observed in our study might suggest a potential
for nitrogen limitation in the Ross Barnett Reservoir, it is highly unlikely
that major nutrients were limiting during the study period.
The fact that the dissolved inorganic nitrogen was represented primarily
by ammonia suggests that conversion of ammonia to nitrite/nitrate is slower
than the rate of ammonia input, with infl ow water or from anoxic sediments
(Quirós 2003). This situation is typical of fairly eutrophic systems; in fact,
the proportion of ammonia can rise with increasing trophic status in lakes
(Quirós 2003). Overall, nutrient abundances and ratios are consistent with
those typical of eutrophic systems.
Although in vivo fl uorescence measurements of biomass could be affected
by dissolved organic compounds (Fuchs et al. 2002), as well as by
composition of the algal community (Heanie 1974) and algae trophic status
(Kruskopf and Flynn 2006), this parameter is expected to provide an estimate
sufficiently accurate for comparison between samples. Chlorophyll-a
values measured in the reservoir were substantially lower than would be expected
from such high inorganic nutrient concentrations. Using the empirical
Figure 5. Relationship between overall turbidity and chlorophyll-a content in experimental
microcosms (open squares). Field turbidity and chlorophyll-a values shown
for comparison (filleded squares).
262 Southeastern Naturalist Vol. 8, No. 2
relationship between total phosphorus and chlorophyll-a cited in Dillon and
Rigler (1974), expected values of chlorophyll-a in Ross Barnett Reservoir
should have ranged between 76 and 375 μg chl-a per liter, almost an order
of magnitude more than observed. In fact, the predicted values were likely
underestimated because we used SRP rather than total dissolved phosphorus
in the predictive equations. Consistently high concentrations of dissolved
nitrogen and phosphorus, even during the prime growing season, together
with chlorophyll concentrations substantially lower than projected from the
amount of nutrients present, suggest that primary productivity and biomass
of phytoplankton in this reservoir is limited by factors other than nutrient
availability. High nephelometric turbidity observed in our work indicates
rapid extinction of light within the water column. The magnitude of observed
chlorophyll-a concentrations suggest that algal biomass did not likely
contribute significantly to the observed turbidity or to self-shading.
High turbidity (and, by extension, high light extinction) and the relative
uniformity of these parameters among stations indicates that the degree to
which light limits algal biomass in differing locations may be controlled
by the water depth at that location. One can speculate that algae in deeper
portions of the reservoir are expected to spend a substantial amount of time
below the compensation depth, resulting in net biomass loss through respiration.
Thus, a portion of the reservoir could act as net biomass sink and could
limit algal primary production and biomass in the entire system despite the
presence of abundant inorganic nutrients (Cole et al. 1992).
A decrease in suspended sediment within the water column (e.g., as a
result of sediment control in the watershed) might decrease light extinction
and increase the critical depth. Therefore, sediment control in the watershed
could reduce the overall amount of suspended material in the water and increase
algal biomass in the reservoir.
The idea that algal biomass in the reservoir is limited by light availability
is consistent with the results of the microcosm study. A ten-fold increase
in algal biomass (as measured by chlorophyll-a concentration) that accompanied
the settling of suspended sediment from the water column over time
occurred without added nutrients (Fig. 4), indicating light, rather than nutrient
A negative relation between turbidity and chlorophyll was observed at
higher (over 15 NTU) turbidity levels early in the experiment (Fig. 5, open
symbols, designated “sloping limb”). Following the initial decline to 15
NTU, chlorophyll-a concentration was effectively uncoupled from turbidity,
rapidly increasing several fold (designated “vertical limb”). Overall
turbidity in the experimental tanks remained unaltered during that increase
(Fig. 5, open symbols). The biramous nature of the turbidity-chlorophyll
relationship suggests that turbidity decrease below a certain point uncouples
primary production from turbidity-driven limitations, resulting in a spike
in algal biomass. We hypothesize that at turbidities above 15 NTU (Fig. 5,
open symbols, sloping limb), algal biomass is sufficiently low as to not
2009 D. Sobolev, K. Moore, and A.L. Morris 263
contribute significantly to the overall turbidity of the system. Increasing algal
biomass during decrease of the inorganic turbity likely does not increase
overall turbidity sufficiently to compensate for the lost inorganic turbidity.
On the other hand, it appears that around 15 NTU, algal biomass becomes
sufficient to make significant contribution to overall turbidity. Consequently,
any decrease in an inorganic turbidity reducing light limitations in the water
column would result in increase in algal biomass and algal turbidity, bringing
overall turbidity values to the point where light is limiting algal growth
again. A combination of declining inorganic turbidity and increasing algal
biomass appears to be responsible for overall turbidity remaining steady as
chlorophyll-a values increase dramatically.
Use of in vivo fl uorescence for chlorophyll-a measurements is confounded
by turbidity absorbing both excitation and emitted light. Even though
reduction in inorganic turbidity alone would increase apparent measured
chlorophyll-a, turbidity introduced error would only represent 0.03 μgL-1 per
NTU (Lambert 2001); thus, decline of turbidity from 40 to 15 NTU would
not be sufficient to account for measured chlorophyll increase from under 20
to over 160 μgL-1 .
Interestingly, much lower chlorophyll-a content at all levels of turbidity
was observed in the field at all stations on all sampling dates (Fig. 5, closed
symbols). This difference could possibly be explained by differences in algal
community composition, or by limitations imposed upon the algal community
by other factors (e.g., nutrients) in addition to light. Additionally, as the
maximum circulation depth in the microcosm was ca. 30 cm (microcosm
depth), substantially less than that expected in the field, one can speculate
that the same level of turbidity would impose a greater light limitation in the
field, compared to the experimental tank (Grobelaar 1985, Sverdrup 1953).
Due to the high levels of turbidity and high light extinction, most of
Ross Barnett Reservoir is expected to have algal respiration levels exceeding
gross primary production levels, consequently not being able to sustain
its phytoplankton population (Cole et al., 1992). Nevertheless, detectable
amounts of chlorophyll-a were observed in the field. Furthermore, while observed
light extinction values were high and relatively uniform throughout
all sampling stations, there are a number of shallow, wind- and current-protected,
vegetated areas not included in this study. Little suspended sediment
is expected in those areas, resulting in less severe light limitation. Those
shallow, vegetated reservoir margins might act as net algal biomass sources,
where algal primary productivity exceeds algal respiration, while the central
deeper pools act as the biomass sink, where algal respiration exceeds
algal primary production, although at this point, this scenario is still only a
speculation. Development of a geo-referenced model, accounting for local
variations in depth, turbidity, and mixing, as well as the relative sizes of various
pools would be necessary to quantitatively understand the relative roles
of the margins and central part of the reservoir in production, transport, and
decomposition of planktonic biomass. Such a model could act as a tool to
264 Southeastern Naturalist Vol. 8, No. 2
assist in ecosystem and water quality management decision-making, as well
as to further understand fundamental interaction between light, nutrients,
and phytoplankton. Additional data collection for nutrients, light extinction,
and algal biomass and productivity, as well as system geometry and circulation
patterns would be required to develop such a model.
This study of the Ross Barnett Reservoir suggests a potential for planktonic
algae to be limited by inorganic turbidity rather than the abundant
inorganic nutrients. This finding has obvious implications for water quality
management, as sediment-mitigation projects may not have the desired outcome
(i.e., increased water clarity may promote algal blooms). Furthermore,
high levels of suspended sediments can favor fl oating macrophytes, by suppressing
phytoplankton and thus reducing competition for nutrients.
This work was supported by the National Oceanic and Atmospheric Administration
grant number NA050AR4811023 and would have been impossible without
assistance of Mrs. Barbara Burns of Callaway High School. Special thanks go to Dr.
Edwin Cruz-Rivera for suggesting the final experiment and for reviewing manuscript
drafts, as well as to the anonymous reviewers. Dataset processing, figure generation,
and manuscript production was accomplished with free open source software; authors
are indebted to Ubuntu, Open Office, and GIMP development communities.
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