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Freshwater zooplankton communities typically undergo pronounced seasonal succession and often show dramatic responses to external factors such as changes in zooplanktivore abundance. For this reason it is important to assess how common zooplankton species differ in their grazing impacts on planktonic ecosystems. To accomplish this we used single—species treatments with Diacyclops bicuspidatus thomasi, Daphnia rosea, Diaptomus novamexicanus, and Holopedium gibberum in situ in Castle Lake, California. These taxonomically diverse zooplankters differ markedly in feeding modes and typical seasonal population dynamics. We measured the response of nutrient concentrations, bacterioplankton abundance, phytoplankton species composition and biomass, primary production, a grazing index (phaeophytin/chlorophyll a), and microzooplankton to our single species treatments. The filter—feeding cladocerans Daphnia and Holopedium and the raptorial filter—feeding calanoid copepod Diaptomus showed several effects typical of herbivorous zooplankton. These included increasing dissolved nutrient concentrations, decreasing algal biomass and the abundance of several common algae, increasing a grazing index, increasing the ratio of bacterial to algal biomass, as well as depressing ciliate microzooplankton abundance. The raptorial cyclopoid copepod Diacyclops was apparently exclusively predaceous as it decimated the ciliate and rotifer microzooplankton, but had no notable effect on the other measured parameters relative to zooplankton—free controls. Diacyclops had the greatest effect on the microzooplankton and Daphnia and Diaptomus had the greatest effect on inorganic nutrients and characteristics of the phytoplankton. Holopedium had qualitatively similar but weaker impacts compared to Daphnia and Diaptomus. None of the zooplankton treatments had an effect on bacterioplankton abundance, nor did grazing by any of these zooplankters increase total algal primary production. Our results suggest differences in the grazing effects of common freshwater zooplankton can be pronounced and indicate that both seasonal succession and long—terms shifts in the zooplankton community structure should have marked effects on microzooplankton competitors and prey, the phytoplankton, and nutrient cycling. |
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