Bruce Sharfstein

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.

Growth and survival of the Florida apple snail (Pomacea paludosa) fed 3 naturally occurring macrophyte assemblages

The Florida apple snail (Pomacea paludosa) is a critical food source for the endangered snail kite, as well as other regional wildlife. However, very little information is available on the apple snail’s feeding habits. To address this information gap, Florida apple snail juveniles from Lake Okeechobee, Florida, were grown in 12 indoor aquaria for 32 d. Snails were provided with freshly collected, approximately equal volumes of either a Utricularia sp./periphyton complex, an Eleocharis sp./periphyton complex, or metaphyton (loosely attached benthic algae) as food sources. Snail growth and survival were significantly higher in the Utricularia treatment than in the metaphyton treatment, but no significant differences in growth were noted between either the Utricularia vs Eleocharis or Eleocharis vs metaphyton treatments. Overall, Utricularia had the highest nutritive quality based on its relatively low C:N ratio and ash content, and high P and chlorophyll (chl) concentrations. Eleocharis had a high C:N ratio and low P content, but also had moderate chl and low ash concentrations. The metaphyton assemblage had a low C:N ratio, but high ash and low chl concentrations. Changes in snail biomass and length were positively correlated with treatment chl content and P content. Changes in snail survival were positively correlated with treatment chl content and inversely correlated with treatment ash content. Snail excreta, collected at the end of the experiment, showed significant differences in N content among all treatments, with the excreta N content of metaphyton-reared snails being twice that of the other 2 treatments. Changes in snail biomass were inversely correlated with snail excreta N content.

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.

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.

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.

The effects of shading on Chara zeylanica Klein ex Wild. and associated epiphytes

The effect of shading on the growth of Chara zeylanica Klein ex Wild. and associated epiphytes, mainly Rhizoclonium africanum KUTZ., was investigated in a large outdoor tank using water, sediment, and plants from Lake Okeechobee, Florida. Plants were grown in peat sediment and lake water, under ambient temperature (27.1-30.3 °C) and photoperiod (13L:11D). Treatments were established by differentially shading plants with varying numbers of layers of fiberglas window screen. Photon flux density (PFD) ranged from 8 to 153 μmole photons m -2 s -1 , or 1.1 to 21.6% of average incident photosynthetically active solar radiation (PAR), based on percent transmittance in the tank and averaged continuous daytime measurements from a mid-lake PAR sensor. Parameters examined included the ash-free dry mass (AFDM) and growth rate of C. zeylanica and its associated epiphytic community. Both C. zeylanica and epiphytic AFDM, but not their AFDM ratio, decreased linearly with decreasing PAR, were strongly correlated, and had statistically significant treatment effects. The apparent photosynthetic PFD for no net growth (NNG) of heavily epiphytized C. zeylanica, measured approximately a quarter meter above the sediment surface, was estimated to average 16 μmole photons m -2 s -1 , or 2.3 % of mean incident PAR, with an upper 95 % confidence limit of 33 μmole photons m -2 s -1 , or 4.7% of mean incident PAR. The apparent NNG photosynthetic PFD estimate mean of the epiphytic community, however, was negative, with an upper 95 % confidence limit of 10 μmole photons m -2 s -1 , or 1.4% of mean incident PAR. These results suggest that C. zeylanica, and particularly its associated epiphytes, can grow in very low light, which may be an important adaptation given the poor light climate typical of this and many other culturally eutrophic waterbodies.

Phytoplankton photosynthesis-irradiance relationships in a large, managed, eutrophic, subtropical lake: The influence of lake stage on ecological homogeneity

Phytoplankton photosynthesis-irradiance curves are routinely generated as part of the long-term plankton monitoring program of a large, eutrophic, subtropical lake (Lake Okeechobee, Florida). Because previous water chemistry and phytoplankton research found the lake to be more heterogeneous under lower lake stage than under higher lake stage conditions, these photosynthesis data were used to investigate whether phytoplankton from four geographically distinct stations exhibited similar photosynthetic behavior during a period of high lake stage as opposed to spatially heterogeneous photosynthetic behavior after a managed recession and drought reduced water depths. Integrated samples were collected February 1997-October 2001 from the four stations. Photosynthesis-irradiance curves ( 14 C-bicarbonate method) were generated, leading to mean lake-wide values (± SD) of photosynthetic parameters, α B , P m B , and E k , of 0.049 ± 0.046 mg C (mg Chl-a) -1 h -1 (μmol photons m -2 s -1 ) -1 , 7.71 ± 5.81 mg C (mg Chl-a) -1 h -1 , and 189 ± 101μmol photons m -2 s -1 , respectively. The parameters did not differ between stations under high lake stage conditions (with one exception), but α B and P m B differed significantly between stations after the reduction in lake stage, suggesting a shift toward ecological heterogeneity. Variation in photosynthetic parameters was predicted by a different suite of environmental variables for low and high lake stage conditions. These results point to the importance of lake stage to the ecological function of this shallow, subtropical system.