Seasonal and spatial variation in zooplankton community structure and their relation to possible controlling variables in Lake Okeechobee

1. Lake Okeechobee is a large (1732 km(2)), 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, approximate to 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.