MARY C. GRIFFIN AND WILLIAM E. KELSO, School of Forestry, Wildlife and Fisheries, Louisiana Agricultural Experiment Station, Louisiana State University Agricultural Center, Baton Route, Louisiana 70803, USA

Many factors may influence the abundance and species richness of littoral zooplankton, including plant type, plant density, and water quality. The Atchafalaya River Basin (ARB) is a forested floodplain swamp that supports large expanses of two exotic macrophytes, hydrilla Hydrilla verticillata and water hyacinth Eichornia crassipes. These macrophytes provide substantially different habitats, and we wanted to determine whether these differences in growth habitat were reflected in the abundance and species composition of resident cladocerans. In summer 1996, we collected zooplankton in hydrilla and water hyacinth beds. Preliminary results indicate that: 1) total cladoceran abundance was high in hyacinth beds, but cladocerans were a higher percentage of total zooplankton abundance in hydrilla beds; 2) plant-associated cladocerans were more abundant in hydrilla than in water hyacinth; 3) cladoceran abundance was positively related to macrophyte density in plant types, and 4) total chlorophyll a (phytoplankton and periphyton) differed among sites but not between plant types. The ARB is also characterized by substantial changes in physicochemistry during flooding, and analyses reveal that although cladoceran abundances were higher after the flood pulse, diversity was higher during the flood pulse.