The grazing impact of daphnia parvula on phytoplankton in a southeastern, eutrophic reservoir

Abstract

Zooplanktonic grazing is increasingly recognized as an important component of pliytoplankton dynamics and composition in natural, north-temperate lakes. The most efficient grazers arc species of the tladoceran genus Daphnia. but in southeastern reservoirs this genus is often sparse in abundance, particularly during summer. Further, the dominant daphnid is Daphnia panwla which is a small-bodied species possibly not capable of exerting similar controls on food resources as larger Daphnia species. Experimental exclosure of fishes from zooplankton in Jordan Lake, a eutrophic, 55 km* reservoir in Piedmont North Carolina, was undertaken to determine if zooplankton would respond sufficiently to infiucncc phytoplankton. Every major crustacean zooplankton taxon doubled its biomass within 2 weeks after protection from predators in at least 1 of six experiments from May to September. The usual numerical dominant, ShistodiapUmus palhdus, increased in every test by at least two-fold. D. pannda increased in four ot the tests and rapidly reduced chlorophyll concentrations in each case. Abundance and associated species changes in phytoplankton were negligible without a Daphnia response, but when Daphnia increased cell densities of all taxa including most blue-green algae decreased. Only Anabaena increased in proportion to total phytoplankton units. When these results were examined in a D. parvula grazing model, output suggested phyto­plankton declines could be caused solely by increased grazing pressure. These results strongly suggest that fish predation is severely limiting the grazing power of zooplankton in Jordan l.ake. © 1994 Taylor & Francis Group, LLC.