Andrew J. Rodusky

Ecological Properties of Charophytes in a Large Subtropical Lake

Benthic charophytes were observed during July 1994 in the southern end of Lake Okeechobee, Florida. Surveys of charophyte distribution and abundance were conducted over the subsequent 30 mo. Charophyte biomass declined progressively from 1994 through 1996; regardless of year, charophyte presence was observed only during the mid to late summer and fall months. Ash free dry mass (AFDM) levels ranged from non-existent to >20 g/m2. Abiotic factors influencing charophyte populations were investigated. Regression analysis showed that charophyte biomass was inversely related to water depth and positively related to Secchi depth, suggesting that irradiance strongly influences charophyte distribution in this lake. Data from photosynthetic measurements and photosynthesis-irradiance curves confirmed this hypothesis, as Ik (irradiance at which photosynthesis is initially saturated) was an order of magnitude greater than the ambient light reaching the charophyte populations. In addition, autotrophic respiration exceeded net photosynthesis under ambient light conditions. Inorganic phosphorus concentration was not significantly related to charophyte biomass. Nutrient kinetics favored phytoplankton growth over Chara, assuming phosphorus limited conditions. Although nutrient concentration may influence Chara phenology and abundance, light appears to be a stronger regulator in this subtropical lentic ecosystem.

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.

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.

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.