Phosphorus accumulation in the littoral zone of a subtropical lake

In situ mesocosm experiments were performed under summer (1997) and winter (1999) conditions in the littoral zone of a subtropical lake in Florida, USA. The objective was to quantify phosphorus (P) accumulation by various components of the community after adding pulsed doses of dissolved inorganic P. A short-term experiment also was done to quantify the rate of P loss from the water column, with simultaneous use of an inert tracer to confirm that P depletion was not due to leakage of the tanks. In the experiments, added P was rapidly removed from the water; samples collected 3–4 days after adding spikes of near 100 μg l^?1 P contained little or no soluble reactive P. In the short-term experiment, we documented that the half-life of added P was approximately 6–8 h in the water column, and that the tanks were not exchanging water with the surrounding lake. Little of the added P ended up in plankton, rooted vascular plants, or sediments. The main sink for P was periphyton, including surface algal mats, benthic algal mats and detritus, and epiphyton. In the summer 1997 experiment, the periphyton was intimately associated with a non-rooted plant (Utricularia), which also may have sequestered P from the water. Structure of the littoral community varied between summer and winter, and this influenced which periphyton component accounted for most of the P removal. In regard to P mass balances, we accounted for 54% of the added P in 1997, when coarse sampling was done. In 1999, when there was more detailed sampling of the community, 92% of the added P was located in various community components. Subtropical littoral periphyton can be a large sink for P, as long as depth and underwater irradiance conditions favor its growth.     

The influence of large water level fluctuations and hurricanes on periphyton and associated nutrient storage in subtropical Lake Okeechobee,USA

The abundance, community structure and nutrient content of periphyton, and the host plant taxa Chara, Hydrilla, Potamogeton, Vallisneria and Scirpus were studied in Lake Okeechobee, USA. Water levels were generally high during the study period (August 2002–January 2006), but substantial fluctuations occurred. All host plant biomass was seasonally variable but only Vallisneria biomass was spatially variable. All submerged plant beds disappeared after the passage of two hurricanes in September 2004, and a third hurricane passed over the lake in October 2005. Periphyton assemblages were statistically separated most by substrate and then by season. Prior to the hurricanes, annual maxima of periphyton biovolumes and those of summer submerged plant coverage coincided. During all study years, the diatom taxa dominated periphyton total biovolumes. Periphyton biomass was generally highest during the summer or prior to the hurricanes (in the case of epiphytes) and was spatially variable in the case of both Scirpus and Vallisneria. Epiphytic nutrient contents within each host plant group seasonally varied except for nitrogen and carbon in the Vallisneria epiphytes. Epipelic nutrient contents were spatially variable and seasonally variable for carbon. Nutrient contents in epipelon were significantly higher than that in Scirpus epiphytes and were similar but lower among all epiphytic communities. The total annual areal potential epiphytic phosphorus storage extrapolated during this study (2.0 × 10⁻⁴ metric tons ha⁻¹ year⁻¹) was underestimated because storage estimates for epipelon, Chara and Hydrilla-associated epiphytes were omitted. The Chara and Hydrilla-associated epiphytic nutrient storage values were omitted because of limited data, whereas the epipelic data may have not been spatially representative. For periphyton biovolume, host substrate type, water level fluctuation and hurricane impacts on host substrates appear to be more important than seasonal variation in such factors as temperature and nutrients. Epiphytic nutrient storage appears to be influenced most by water level fluctuation and hurricane-related impacts, while the host substrate type appears to be a less important factor than it is for periphyton biovolume. Maximum periphyton biomass and high nutrient storage in shallow subtropical and tropical eutrophic lakes may only occur at consistently lower water levels and during infrequent periods of disturbance, which enhance host substrate colonizable area.     

Algal responses to experimental nutrient addition in the littoral community of a subtropical lake

1. An in situ experiment was performed in the littoral zone of a large, subtropical lake to quantify effects of phosphorus (P) and nitrogen (N) on algal biomass, productivity, nutrient content and phosphate uptake kinetics. 2. We hypothesized that resident periphyton rapidly sequester added nutrients from the water column, but once a certain threshold is reached, nutrients remain in the water and permit a shift to a phytoplankton-dominated community. 3. Three duplicate sets of 1.2-m diameter mesocosms were treated with 10, 20 or 50 μg P L^??1 in combination with 100, 200 or 500 μg N L^??1, respectively. The nutrients were added thrice weekly for 14 days, after which the treatment doses were doubled for an additional 9 days. The cumulative amounts of P and N added over the course of the study were 700 and 7000 μg L^??1, respectively. Two untreated mesocosms and two open reference sites were used as controls. 4. The total P concentration in the water column of nutrient-treated mesocosms remained low, even after prolonged high dosing. However, there was a two-fold increase in the P content of surface algal mats and epiphyton. This indicates that some of the added P was sequestered by those components of the community. In contrast, metaphyton and epipelon displayed little or no increase in their P content. Large quantities of added P could not be accounted for in the periphyton community, and may reflect unmeasured losses to the sediments or other pools. 5. Nitrogen also was depleted from the water column, but there were no significant increases in periphyton N content. Much of the added N could not be accounted for in mass balances, and may have been lost from the mesocosms through volatilization or other biochemical processes. 6. Chlorophyll-a in epiphyton increased significantly after 14 days in the highest nutrient treatment, where there also was a proliferation of Spirogyra on day 28. 7. On day 28, water column samples from the highest nutrient treatment also displayed a significantly higher rate of carbon uptake, and a significantly higher concentration of midday dissolved oxygen. 8. The hypothesis that phytoplankton become dominant at high nutrient loading rates was not supported. However, there were dramatic changes in community structure (increased dominance by epiphytic Spirogyra) and function (increased productivity and dissolved oxygen) in response to nutrient additions.     

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 g 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.     

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.     

Ecological attributes of a native and exotic emergent subtropical marsh community in lake okeechobee, Florida (USA)

Ecological attributes of native Eleocharis cellulosa and exotic-invasive Panicum repens communities were compared in the western littoral marsh of Lake Okeechobee, USA. Water quality, periphyton and macroinvertebrates were monitored for 2 years. Fish and zooplankton were monitored during the second year. Mean dissolved oxygen, pH and Secchi transparency were significantly higher in Eleocharis while total phosphorus was marginally higher in Panicum. Periphyton biomass was higher in Panicum but biovolumes were higher in Eleocharis. There were clear differences in the within-habitat periphyton assemblages, less difference in the among-habitat assemblages and moderate to clear differences in periphyton over time in both habitats. Both habitats were dominated by small omnivorous fish. Habitat and date were the most influential factors in differences among both macroinvertebrates and zooplankton, although these differences were marginal to moderate in both cases. Macroinvertebrate densities were higher in Eleocharis while zooplankton densities were higher in Panicum. These data suggest that contrary to the paradigm that Panicum is undesirable as habitat, sparse (<1,000 stems m^?2) Panicum in close proximity to Eleocharis may exhibit a similar suite of ecological attributes. Both habitats also appear to contain similar food webs to nearby Everglades marsh communities.     

Daphnia lumholtzi and Daphnia ambigua: population comparisons of an exotica and a native cladoceran in Lake Okeechobee, Florida

The population dynamics of an exotic cladoceran (Daphnia lumholtzi Sars) and a native cladoceran (Daphnia ambigua) were studied over a 12 month period in subtropical Lake Okeechobee, Florida (USA), to quantify the extent of invasion of the exotic species and compare ecological niches. Daphnia lumholtzi accounted for up to 70% of the Daphnia assemblage during the summer months (June-August), while D.ambigua accounted for up to 97% of the Daphnia assemblage from fall to spring (October-April). The densities of the two species were inversely corelated. The exotic species was most concentrated in the shallower, warmer, north and south ends of the lake during the summer. It also was present, but at much lower densities, in the central lake region during the fall. The native species displayed a ubiquitous distribution throughout the lake during spring and winter, but was concentrated in the deeper, cooler, central region during the summer. Relationships of the two species with environmental conditions indicate that water column temperature might affect the seasonal and spatial distribution of the two Daphnia species. The results also indicate that D.lumholtzi may be filling a 'vacant' seasonal or spatial niche when conditions are unfavorable for D.ambigua.