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Nutrients and Light Limitation of Phytoplankton Biomass in a Turbid Southeastern Reservoir:
Implications for Water Quality
Dmitri Sobolev, Kandis Moore, and Ashley L. Morris

Southeastern Naturalist, Volume 8, Number 2 (2009): 255–266

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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. Introduction 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 - 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. Results 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 Discussion 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 limitation. 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. Conclusions 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. Acknowledgments 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. 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