SEASONAL AND SPATIAL VARIATION IN ZOOPLANKTON COMMUNITY STRUCTURE ANDTHEIR 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. Approximate ly 25% of the lake surface area is occupied by an extensive Littoral z one. From August 1988 to June 1992, approximate to 2000 zooplankton sa mples 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 aver age abundance of adult crustaceans (cladocerans and copepods) (c. 30 l (-1)) varied little regardless of lake stage. 3. Although only minor d ifferences were apparent when lake-wide means in rotifers and adult cr ustaceans for each lake stage were compared, pronounced differences we re 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 conc entrated in the transition area between the central lake and the litto ral fringe. The abundance of all zooplankton groups was inversely corr elated with lake stage, but the relationship was much stronger for rot ifers than crustaceans. Both rotifer and crustacean zooplankton popula tion densities were positively related to increased phytoplankton biom ass (as measured by chlorophyll a) but the relationship was much stron ger for rotifers than crustaceans.