Therese L. East

N:P ratios, light limitation, and cyanobacterial dominance in a subtropical lake impacted by non-point source nutrient pollution

A long-term (28-year) data set was used to investigate historical changes in concentrations of phosphorus (P), nitrogen (N), N:P ratios, and Secchi disk transparency in a shallow subtropical lake (Lake Okeechobee, Florida, USA). The aim was to evaluate changes in the risk of N2-fixing cyanobacterial blooms, which have infrequently occurred in the lakes pelagic zone. Predictions regarding bloom risk were based on previously published N:P ratio models. Temporal trends in the biomass of cyanobacteria were evaluated using phytoplankton data collected in 1974, 1989-1992, and 1997-2000. Concentrations of pelagic total P increased from near 50 microg l-1 in the mid-1970s to over 100 microg l-1 in the late 1990s. Coincidentally, the total N:P (mass) ratio decreased from 30:1 to below 15:1, and soluble N:P ratio decreased from 15:1 to near 6:1, in the lake water. Published empirical models predict that current conditions favor cyanobacteria. The observations confirm this prediction: cyanobacteria presently account for 50-80% of total phytoplankton biovolume. The historical decrease in TN:TP ratio in the lake can be attributed to a decreased TN:TP ratio in the inflow water and to a decline in the lakes assimilation of P, relative to N. Coincident with these declines in total and soluble N:P ratios, Secchi disk transparency declined from 0.6 m to near 0.3 m, possibly due to increased mineral turbidity in the lake water. Empirical models predict that under the turbid, low irradiance conditions that prevail in this lake, non-heterocystous cyanobacteria should dominate the phytoplankton. Our observations confirmed this prediction: non-N2-fixing taxa (primarily Oscillatoria and Lyngbya spp.) typically dominated the cyanobacteria community during the last decade. The only exception was a year with very low water levels, when heterocystous N2-fixing Anabaena became dominant. In the near-shore regions of this shallow lake, low N:P ratios potentially favor blooms of N2-fixing cyanobacteria, but their occurrence in the pelagic zone is restricted by low irradiance and lack of stable stratification.

Nutrient ratios and phytoplankton community structure in the large, shallow, eutrophic, subtropical Lakes Okeechobee (Florida, USA) and Taihu (China)

Analysis of ten- and four-year datasets for the large, shallow, subtropical, and eutrophic Lakes Okeechobee (USA) and Taihu (China), respectively, suggest that resource-ratio explanations for cyanobacteria dominance may not apply to these two lakes. Datasets were examined to identify relationships between nutrient ratios [total nitrogen (TN):total phosphorus (TP) and ammonium (NH4+):oxidized N (NOx)] and phytoplankton community structure (as proportions of cyanobacteria and diatoms to total phytoplankton biomass). Datasets were pooled by sampling month, averaged lake-wide, and analyzed with linear regression. In Okeechobee, the cyanobacteria proportion increased and the diatom proportion decreased with increasing TN:TP. In Taihu, cyanobacteria decreased with increasing TN:TP, but the opposite trend observed for diatoms was marginally significant. Okeechobee cyanobacteria increased and diatoms decreased with increasing NH4+:NOx, but no significant relationships between phytoplankton and NH4+:NOx were observed in Taihu. Both lakes had significant relationships between phytoplankton community structure and total nutrients, but these relationships were the opposite of those expected. Relationships between phytoplankton community structure and water quality parameters from the previous month resulted in improved relationships, suggesting a predictive capability. Statistical analysis of the entire datasets (not pooled) supported these and additional relationships with other parameters, including temperature and water clarity.

Hurricane effects on a shallow lake ecosystem, Lake Okeechobee, Florida (USA)

This unique case study of Lake Okeechobee - a large, shallow and culturally eutrophic lake in south Florida - documents the effects of hurricanes on its water quality, sediment, phytoplankton and submersed aquatic vegetation (SAV). Three hurricanes (Frances and Jeanne in 2004 and Wilma in 2005) that swept directly over the lake led to a number of expected changes throughout the system: 1) high winds produced large seiches, strong waves and currents that redistributed bottom sediments and uprooted SAV and emergent macrophytes; 2) sediment disturbance resulted in increased suspended solids and nutrients in the water column, reduced Secchi transparency and affected SAV recovery, phytoplankton biomass and phytoplankton species dominance; and 3) heavy rainfall increased flows, nutrient loads and lake water levels. Changes in suspended solids, most nutrient concentrations, phytoplankton and SAV persisted for two years after the hurricanes. This persistence was attributed to unconsolidated surface sediment that increased in thickness because of the storms and was more easily resuspended during subsequent wind events. Drought conditions and low lake levels in the second year after the hurricanes led to some recovery of SAV, primarily in the form of the non-vascular musk grass (Chara spp.). The absence of high-intensity hurricanes in the near future should aid in SAV recovery and return the nearshore region to a macrophyte-dominated clear-water state. Our results demonstrate the importance of sediment disturbance and water levels in shallow lakes that are vulnerable to extreme weather events.

Extreme weather events influence the phytoplankton community structure in a large lowland subtropical lake (Lake Okeechobee, Florida, USA)

We demonstrate a major ecological change in a large lake ecosystem in response to a series of extreme weather events. Phytoplankton community dynamics in subtropical Lake Okeechobee are described from 2000 through early 2008 with emphasis on inter-relationships among phytoplankton populations and associated environmental variables in this large, shallow eutrophic lake. The lake experienced the physical effects of three hurricanes in 2004–2005, which caused massive sediment resuspension, near total elimination of submerged aquatic vegetation, elevated biologically available nutrients and total suspended solids, and lower water transparency. Patterns of long-term co-dominance by nitrogen (N)-fixing cyanobacteria and meroplanktonic diatoms abruptly changed to dominance by only meroplanktonic diatoms. The planktonic cyanobacteria genera Anabaena and Planktolyngbya both decreased approximately an order of magnitude in the post-hurricane period despite large surpluses of bioavailable nutrients. Meroplanktonic diatoms (mostly Aulacoseira spp.) declined approximately 20%, perhaps because of superior competitive ability for light in a polymictic, turbid water column. Canonical Correspondence Analysis (CCA) suggested that reduction in planktonic cyanobacteria after compression of the photic zone and the persistence of meroplanktonic diatoms were related to light utilization traits for the key algal taxa and indicated that pre-existing light limitation and crustacean grazing pressure were intensified in the post-hurricane period.

Large-Scale Mapping and Predictive Modeling of Submerged Aquatic Vegetation in a Shallow Eutrophic Lake

A spatially intensive sampling program was developed for mapping the submerged aquatic vegetation (SAV) over an area of approximately 20,000 ha in a large, shallow lake in Florida, U.S. The sampling program integrates Geographic Information System (GIS) technology with traditional field sampling of SAV and has the capability of producing robust vegetation maps under a wide range of conditions, including high turbidity, variable depth (0 to 2 m), and variable sediment types. Based on sampling carried out in AugustœSeptember 2000, we measured 1,050 to 4,300 ha of vascular SAV species and approximately 14,000 ha of the macroalga Chara spp. The results were similar to those reported in the early 1990s, when the last large-scale SAV sampling occurred. Occurrence of Chara was strongly associated with peat sediments, and maximal depths of occurrence varied between sediment types (mud, sand, rock, and peat). A simple model of Chara occurrence, based only on water depth, had an accuracy of 55%. It predicted occurrence of Chara over large areas where the plant actually was not found. A model based on sediment type and depth had an accuracy of 75% and produced a spatial map very similar to that based on observations. While this approach needs to be validated with independent data in order to test its general utility, we believe it may have application elsewhere. The simple modeling approach could serve as a coarse-scale tool for evaluating effects of water level management on Chara populations.

Zooplankton response to extreme drought in a large subtropical lake

Plankton data from 1997 to 2005 were used to examine impacts of a managed draw-down, subsequent drought and resulting historic low water levels (during 2000 and 2001) on the zooplankton of Lake Okeechobee, Florida. Prior to the drought the lake supported less than 150 ha of submerged vegetation. Following the drought, over 15,000 ha of submerged vegetation developed around the lake shore and conditions favored greater survival of age 0 fish. The zooplankton changed significantly from the pre- to post-drought period, including: (a) a near-complete loss of all dominant species of cladocerans and rotifers; and (b) an abrupt transition to a community with over 80% of total biomass comprised of Arctodiaptomus dorsalis, a calanoid copepod previously described as being resistant to fish predation. These changes persisted over a 5 year post-drought sampling period. In contrast, there were no systematic changes in biomass of bacteria, phytoplankton, inedible cyanobacteria, algal cell size, suspended solids, or any other physical or chemical attributes known to affect zooplankton in shallow lakes. Evidence points towards increased predation by fish, and perhaps invertebrates, as factors responsible for loss of cladocerans and rotifers following the drought, and indicates a need for future research to link changes in water level to shifts in predation pressure in this and other shallow lakes.

Periphyton nutrient limitation and other potential growth-controlling factors in Lake Okeechobee, U.S.A.

Periphyton nutrient limitation was assessed in Lake Okeechobee, a large, shallow, eutrophic lake in the southeastern U.S.A. Nutrient assays were performed to determine if the same nutrients that limit phytoplankton also limit periphyton growth in the lake. Nutrient diffusing clay substrates containing agar spiked with nitrogen, phosphorus, or both, along with nutrient-free controls, were incubated at four sites in the lake. Three sites were located in a pelagic–littoral interface (ecotone) and one site was located in the interior littoral region. Incubations lasted for 20–26 days, and were repeated on a quarterly basis between 1996 and 1997, to incorporate seasonal variability into the experimental design. The physical and chemical conditions at each site also were measured. Periphyton biomass (chlorophyll a and ash-free dry mass) was highest at the littoral and northern ecotone sites. At the littoral site, nitrogen limited biomass in four of five incubations, although the largest biomass differences between the treatments and controls (≤3 μg cm−2 as chl) were probably not ecologically significant. Periphyton biomass at the western and southern ecotone sites was low compared to the other two sites. Increases in water column depth and associated declines in light penetration strongly correlated with periphyton growth and suggested that they may have limited growth most often at all three ecotone sites. Nitrogen also was found to limit periphyton growth approximately 20% of the time at the ecotone sites and phosphorus was found to limit growth once at the west site.

Inter-lake comparisons indicate that fish predation, rather than high temperature, is the major driver of summer decline in Daphnia and other changes among cladoceran zooplankton in subtropical Florida lakes

The mid-summer disappearance of Daphnia and the small size of cladocerans in subtropical lakes have been explained as phenomena caused by high water temperature. Recent experimental and observational studies suggest that instead, fish predation could be the cause. In the present study, we evaluated effects of temperature and fish predation by comparing summer occurrence of Daphnia and seasonality of other cladocerans in subtropical lakes where there was (a) vegetation and a high abundance and diversity of planktivorous fish; versus (b) no vegetation, turbid water, and primarily benthivorous and piscivorous fish. Daphnia disappeared from mid-summer plankton samples at all locations with vegetation, yet it persisted year-round at the turbid vegetation-free location despite identical water temperatures (all lakes were shallow and isothermal). Body size of cladocerans, in general, was twofold lower at vegetated locations, and biomass was threefold lower. Smaller species dominated at the locations with vegetation, especially in summer. Our results demonstrate that in subtropical lakes Daphnia is able to tolerate high summer water temperatures, and suggest that fish predation may be a more important regulator than temperature for cladoceran size, biomass and taxonomic structure.

Development of a decision tree model for the prediction of the limitation potential of phytoplankton in Lake Okeechobee, Florida, USA

Conducting long-term algal bioassays in large, complex systems such as Lake Okeechobee is an expensive and time-intensive undertaking, especially in comparison with physical and chemical monitoring. This paper describes a water quality-based decision tree model for predicting whether the phytoplankton in Lake Okeechobee is limited by light or nutrients. The model was developed and validated using the results of algal bioassays coupled with routinely monitored water quality data. Algal bioassays indicated that the factor most commonly limiting phytoplankton production in Lake Okeechobee for the period of October 1997 to November 2000 was light (59 %) followed by nitrogen (41 %). Limitation status of the phytoplankton was positively correlated with irradiance (in terms of Secchi depth/total depth) and phytoplankton biomass (in terms of chlorophyll-a) and negatively related to dissolved inorganic nitrogen and soluble reactive phosphorus concentrations. A cross-tabulation procedure was used to examine how the frequency of occurrence of light limitation and nutrient limitation varied as a function of these variables. The cross-tabulation procedure was also used to derive the empirical threshold values used to construct the model. This result supports both the accuracy of the derived critical threshold values and the validity of using chemical measurements in predicting whether light is limiting or nutrients are limiting in Lake Okeechobee. The model successfully predicted light limitation versus nutrient limitation in three independent validation data sets 70 % to 85 % of the time. When nutrient limiting conditions prevailed, the model did not successfully predict which nutrient (nitrogen, phosphorus, or a combination of nitrogen and phosphorus) was limiting. Our results suggest that the predictive abilities of the model could be enhanced by using time-specific data rather than averaged monthly data.

Thermal Stratification and the Potential for Enhanced Phosphorus Release from the Sediments in Lake Okeechobee, USA

Abstract We inferred the potential importance of enhanced phosphorus (P) release from the sediments during periods of thermal stratification in Lake Okeechobee USA, a large, shallow, eutrophic lake. This was accomplished by determining the frequency and duration of thermal stratification, using monitoring data collected during 1997 and 1999, with multiparameter sondes, which were positioned at a weather platform in the center of the lake. We also monitored dissolved oxygen, pH, specific conductance, turbidity and redox potential, to assess if any relationships existed between these variables and thermal stratification. Thermal stratification was infrequent, documented for 42 and 74 total hours, and occurred for an extrapolated period of at least one hour on approximately 18 days or 5% of both years. Thermal stratification occurred almost exclusively during the summer (May – September), and was brief, typically lasting for one to six hours per event, although there was one event during 1999, where 49 continuous hours of thermal stratification were documented. Dissolved oxygen (DO) concentrations and redox potential (ORP) measurements collected 0.5 m above the sediment surface suggested that the top of the sediment may not have ever become anoxic and was rarely reducing. Chemical characteristics during periods of thermal stratification, compared to those considered favorable for P release based on previous Lake Okeechobee sediment studies, indicate that thermal stratification is usually insufficiently long for chemical conditions (anoxic or low DO and ORP) to develop above the sediment surface that would enable enhanced P release from the sediments.