Notes: 1. Lake Okeechobee is a large (1732 km2), shallow (mean depth 2.7 m), eutrophic, subtropical lake located in southern Florida. Approximately 25% of the lake surface area is occupied by an extensive littoral zone. From August 1988 to June 1992, ≈ 2000 zooplankton samples were collected throughout the lake.2. During the study period, a severe drought lowered lake levels more than 1 m. At low and normal lake stage, the average lake-wide abundance of rotifers (c. 2000 l–1) was greater than during high water periods (c. 1600 l–1). The average abundance of adult crustaceans (cladocerans and copepods) (c. 30 l–1) varied little regardless of lake stage.3. Although only minor differences were apparent when lake-wide means in rotifers and adult crustaceans for each lake stage were compared, pronounced differences were evident in the distribution of zooplankton communities within Lake Okeechobee. During high and normal lake stage, both rotifer and adult crustacean populations were more uniformly distributed throughout the lake. At low lake stage, the densest zooplankton populations were concentrated in the transition area between the central lake and the littoral fringe. The abundance of all zooplankton groups was inversely correlated with lake stage, but the relationship was much stronger for rotifers than crustaceans. Both rotifer and crustacean zooplankton population densities were positively related to increased phytoplankton biomass (as measured by chlorophyll a) but the relationship was much stronger for rotifers than crustaceans.

Notes: 1. Over a 1-year period, twenty controlled experiments were performed using small mesocosms (20-l clear plastic carboys) and plankton communities collected from four sites in shallow, subtropical Lake Okeechobee, Florida. In replicated treatments, macrozooplankton grazers were excluded by size fractionation (115 μm), and/or nutrients (N and P) were added, and impacts on phytoplankton biomass and productivity were measured after 3-day incubations.2. In most experiments (fifteen out of twenty), there was no significant effect of zooplankton exclusion on phytoplankton biomass or productivity, but there were significant increases in those attributes due to nutrient additions. The magnitude of the responses was a function of light availability at the collection sites.3. In three experiments, zooplankton exclusion led to declines in phytoplankton biomass and productivity, suggesting that animals may sometimes have net positive effects on the phytoplankton, perhaps via nutrient recycling.4. In only two experiments was there evidence of net negative impacts of grazers on the phytoplankton. In both instances, cladocerans (Daphnia ambigua and Eubosmina tubicen) were dominant in the zooplankton. However, the increases in chlorophyll a due to zooplankton exclusion were small (5–20%), probably because of the small size and relatively low grazing rates of the cladocerans.5. The results support the hypothesis that phytoplankton biomass in Lake Okeechobee is little affected by herbivorous macrozooplankton. This may be a common feature of lowland tropical and subtropical lakes.

Notes: 1. This synthesis examines 35 long-term (5–35 years, mean: 16 years) lake re-oligotrophication studies. It covers lakes ranging from shallow (mean depth 〈5 m and/or polymictic) to deep (mean depth up to 177 m), oligotrophic to hypertrophic (summer mean total phosphorus concentration from 7.5 to 3500 μg L−1 before loading reduction), subtropical to temperate (latitude: 28–65°), and lowland to upland (altitude: 0–481 m). Shallow north-temperate lakes were most abundant.2. Reduction of external total phosphorus (TP) loading resulted in lower in-lake TP concentration, lower chlorophyll a (chl a) concentration and higher Secchi depth in most lakes. Internal loading delayed the recovery, but in most lakes a new equilibrium for TP was reached after 10–15 years, which was only marginally influenced by the hydraulic retention time of the lakes. With decreasing TP concentration, the concentration of soluble reactive phosphorus (SRP) also declined substantially.3. Decreases (if any) in total nitrogen (TN) loading were lower than for TP in most lakes. As a result, the TN : TP ratio in lake water increased in 80% of the lakes. In lakes where the TN loading was reduced, the annual mean in-lake TN concentration responded rapidly. Concentrations largely followed predictions derived from an empirical model developed earlier for Danish lakes, which includes external TN loading, hydraulic retention time and mean depth as explanatory variables.4. Phytoplankton clearly responded to reduced nutrient loading, mainly reflecting declining TP concentrations. Declines in phytoplankton biomass were accompanied by shifts in community structure. In deep lakes, chrysophytes and dinophytes assumed greater importance at the expense of cyanobacteria. Diatoms, cryptophytes and chrysophytes became more dominant in shallow lakes, while no significant change was seen for cyanobacteria.5. The observed declines in phytoplankton biomass and chl a may have been further augmented by enhanced zooplankton grazing, as indicated by increases in the zooplankton : phytoplankton biomass ratio and declines in the chl a : TP ratio at a summer mean TP concentration of 〈100–150 μg L−1. This effect was strongest in shallow lakes. This implies potentially higher rates of zooplankton grazing and may be ascribed to the observed large changes in fish community structure and biomass with decreasing TP contribution. In 82% of the lakes for which data on fish are available, fish biomass declined with TP. The percentage of piscivores increased in 80% of those lakes and often a shift occurred towards dominance by fish species characteristic of less eutrophic waters.6. Data on macrophytes were available only for a small subsample of lakes. In several of those lakes, abundance, coverage, plant volume inhabited or depth distribution of submerged macrophytes increased during oligotrophication, but in others no changes were observed despite greater water clarity.7. Recovery of lakes after nutrient loading reduction may be confounded by concomitant environmental changes such as global warming. However, effects of global change are likely to run counter to reductions in nutrient loading rather than reinforcing re-oligotrophication.

Notes: 1. Seasonal dynamics of the exotic Daphnia lumholtzii and native macro-zooplankton species were studied for 2 years in six inter-connected lakes in Florida, U.S.A. The lakes ranged widely in pH, colour and trophic status, and were dominated by copepods. 
2. All six lakes contained both D. lumholtzii and the native D. ambigua, but the two species did not overlap in time. D. ambigua was dominant in autumn–spring, coinciding with lower water temperature, higher transparency and lower nutrient and chlorophyll a (Chl a) concentrations than in summer, when D. lumholtzii was dominant. 
3. Based on the field observations, temperature optima were 24 °C for D. ambigua and 29 °C for D. lumholtzii, suggesting that temperature plays a role in determining dominance among the daphnids of subtropical Florida lakes. 
4. D. lumholtzii has not displaced native cladocerans but occupies a ‘vacant’ seasonal niche, unexploited due to the inability of native taxa to tolerate high temperature. Furthermore, D. lumholtzii did not significantly alter algal–zooplankton interactions. There was evidence of top–down control by grazing, but it was primarily attributable to the native taxa.

Notes: 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.

Notes: 1. Phosphate uptake kinetics and uptake rates were calculated for planktonic (phytoplankton and bacterioplankton) and benthic (epiphyton and epipelon) assemblages in a large, shallow, subtropical lake. Samples were taken bimonthly over the period of 1 year at three different sites to examine spatial and temporal variability in these processes.2. Two of the sites, located at the edge of the littoral zone next to the open water (ecotone sites), had low irradiance at the sediment surface and high total phosphorus (TP) concentration (annual mean TP = 112 μg L–1). The third site, located in the littoral marsh zone, had high irradiance at the sediment surface and low TP concentration (annual mean TP = 7 μg L–1).3. Based on 32P-PO4 turnover time, P availability varied temporally and spatially. At the two high TP ecotone sites, P concentration was lowest in July and August. At the low TP marsh site, P limited algal production throughout the year.4. The quotient of maximum uptake rate to half saturation constant (Vm/Ks) in the plankton increased by over two orders of magnitude during the P-limited (summer) period at the two ecotone sites, suggesting that plankton used the scarce phosphorus more efficiently. The specific uptake rate of plankton was significantly greater than that of periphyton at all sites, suggesting that the plankton were more efficient than periphyton at taking up phosphate.5. Periphyton biomass, as well as absolute and percentage P uptake rate, was greater at the marsh site than at the ecotone sites, despite the lower P concentrations in the marsh. This was probably a result of rapid nutrient cycling, combined with high light availability in the marsh.

Notes: 1. Variations in the relative biovolumes of dominant cyanobacterial taxa were evaluated in the context of environmental conditions using canonical correlation analysis (CCorrA) and Redundancy Analysis (RDA). The objective was to test a conceptual model in which underwater irradiance determines dominance by bloom-forming (high light adapted) or non-blooming (low light adapted) taxa.2. The data set consisted of 404 contiguous observations, collected over a 3-year period at eight pelagic sites, in shallow Lake Okeechobee, Florida, U.S.A. Data included species biovolumes, total phosphorus (TP), total nitrogen (TN), dissolved oxygen (DO) and chlorophyll a concentrations, as well as two indices: underwater irradiance (Secchi depth) and the ratio of Secchi:total depth.3. The first environmental canonical variable was strongly correlated with the two light-related indices, and negatively correlated with TP. This reflects the predominant role of resuspended P-rich lake sediments in controlling underwater irradiance in the shallow lake. The first species canonical variable displayed a strong negative correlation with Lyngbya limnetica and L. contorta, and positive correlations with Anabaena circinalis, Aphanizomenon flos aquae and Microcystis spp. The results support the conceptual model; the first pair of canonical variables explained 55% of the variation in the species–environmental data set. RDA results provided further support for the hypothesis that irradiance was the major force controlling community structure.4. One unexpected result was a positive association between Oscillatoria spp. dominance and indicators of high irradiance. This conflicts with past research indicating that Oscillatoria is a low light adapted taxon, and the finding that it is the most abundant taxon in Lake Okeechobee. This may reflect the fact that the two Lyngbya taxa were more strongly associated with low light conditions than Oscillatoria. CCorrA results indicated that Oscillatoria densities are strongly controlled by water temperature. There is a need for more detailed studies of cyanobacteria ecophysiology in order to explain fully the seasonality of phytoplankton in this and other shallow subtropical lakes.

Notes: : Algal blooms, defined as chlorophyll α concentrations greater than 40 μg l−1, are common in Lake Okeechobee, Florida. Using logistic regression techniques, we have developed equations that relate limnological variables to algal bloom occurrence in four distinct open-water regions of this large shallow lake: central pelagic, northwest, southwest, and a transition region between the western and pelagic regions. Wind velocity and total phosphorus, which are closely related to resuspended material in the central region, are negatively related to algal bloom occurrence there. In the transition region, algal bloom occurrence is positively related to total nitrogen and wind velocity. Algal bloom occurrence is strongly and positively related to total nitrogen and total phosphorus concentrations in the western regions. The logistic regression model predicts an algal bloom probability greater than 95 percent in the northwest region when total phosphorus exceeds 0.10 mg l−1 and total nitrogen exceeds 2.5 mg l−1. In the southwest region the model predicts algal bloom probability of 100 percent when total phosphorus exceeds 0.10 mg l−1 and total nitrogen exceeds 2.8 mg l−1. Given 1994 mean total phosphorus concentrations of 0.05 and 0.04 mg l−1 in the northwest and southwest regions, respectively, total nitrogen would have to remain below 1.32 and 1.43 mg l−1, respectively, to keep the algal bloom probability below 10 percent. Because the lake is heterogenous, such nutrient standards should be considered on an in-lake regional basis for Lake Okeechobee.