SPECIES-DEPENDENT EFFECTS OF ZOOPLANKTON ON PLANKTONIC ECOSYSTEM PROCESSES IN CASTLE LAKE, CALIFORNIA

Freshwater zooplankton communities typically undergo pronounced season al succession and often show dramatic responses to external factors su ch as changes in zooplanktivore abundance. For this reason it is impor tant to assess how common zooplankton species differ in their grazing impacts on planktonic ecosystems. To accomplish this we used single-sp ecies treatments with Diacyclops bicuspidatus thomasi, Daphnia rosea, Diaptomus novamexicanus, and Holopedium gibberum in situ in Castle Lak e, California. These taxonomically diverse zooplankters differ markedl y in feeding modes and typical seasonal population dynamics. We measur ed the response of nutrient concentrations, bacterioplankton abundance , phytoplankton species composition and biomass, primary production, a grazing index (phaeophytin/chlorophyll a), and microzooplankton to ou r single-species treatments. The filter-feeding cladocerans Daphnia an d Holopedium and the raptorial filter-feeding calanoid copepod Diaptom us showed several effects typical of herbivorous zooplankton. These in cluded increasing dissolved nutrient concentrations, decreasing algal biomass and the abundance of several common algae, increasing a grazin g index, increasing the ratio of bacterial to algal biomass, as well a s depressing ciliate microzooplankton abundance. The raptorial cyclopo id copepod Diacyclops was apparently exclusively predaceous as it deci mated the ciliate and rotifer microzooplankton, but had no notable eff ect on the other measured parameters relative to zooplankton-free cont rols. Diacyclops had the greatest effect on the microzooplankton and D aphnia and Diaptomus had the greatest effect on inorganic nutrients an d characteristics of the phytoplankton, Holopedium had qualitatively s imilar but weaker impacts compared to Daphnia and Diaptomus. None of t he zooplankton treatments had an effect on bacterioplankton abundance, nor did grazing by any of these zooplankters increase total algal pri mary production. Our results suggest differences in the grazing effect s of common freshwater zooplankton can be pronounced and indicate that both seasonal succession and long-term shifts in the zooplankton comm unity structure should have marked effects on microzooplankton competi tors and prey, the phytoplankton, and nutrient cycling.