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PERSISTENT EFFECTS OF A SEVERE DROUGHT ON ZOOPLANKTON IN LAKE OKEECHOBEE, FLORIDA

Karl E. Havens University of Florida Gainesville, FL Therese L. East South Florida Water Management District West Palm Beach, FL John R. Beaver BSA Environmental Services Beachwood, OH

Data from 1997 to 2005 were used to examine impacts of a managed draw-down, subsequent drought and resulting historic low water levels (during 2000 and 2001) on the zooplankton of Lake Okeechobee. Prior to the drought the lake supported less than 150 ha of submerged vegetation. Following the drought, over 20,000 ha of submerged vegetation developed around the lake shore and there was substantially increased recruitment of age 0 fish. The zooplankton changed significantly from the pre- to post-drought period, including: (a) a near-complete loss of all dominant species of cladocerans and rotifers after the drought; and (b) an abrupt transition to a community with over 80% of total biomass comprised of the predation- resistant copepod Diaptomus dorsalis. These changes persisted for the entire five years of post- drought sampling. In contrast, there were no systematic changes in biomass of bacteria, phytoplankton, inedible cyanobacteria, algal cell size, suspended solids, or any other physical or chemical attributes known to affect zooplankton in shallow lakes. Evidence points towards an increase in predation by fish and/or invertebrates as a cause for the loss of all but the most predation resistant species from the lake’s zooplankton community, and indicates a need for future research to link climate changes to shifts in predation pressure in this and other shallow lakes in Florida.

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Session 3 – Page 6

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