EXPERIMENTAL STUDIES OF ZOOPLANKTON-PHYTOPLANKTON-NUTRIENT INTERACTIONS IN A LARGE SUBTROPICAL LAKE (LAKE-OKEECHOBEE, FLORIDA, USA)

1. Over a 1-year period, twenty controlled experiments were performed using small mesocosms (20-1 clear plastic carboys) and plankton commun ities collected from four sites in shallow, subtropical Lake Okeechobe e, Florida. In replicated treatments, macrozooplankton grazers were ex cluded by size fractionation (115 mu m), and/or nutrients (N and P) we re added, and impacts on phytoplankton biomass and productivity were m easured after 3-day incubations. 2. In most experiments (fifteen out o f twenty), there was no significant effect of zooplankton exclusion on phytoplankton biomass or productivity, but there were significant inc reases 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 zoopl ankton exclusion were small (5-20%), probably because of the small siz e and relatively low grazing rates of the cladocerans. 5. The results support the hypothesis that phytoplankton biomass in Lake Okeechobee i s little affected by herbivorous macrozooplankton. This may be a commo n feature of lowland tropical and subtropical lakes.