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Abstract

In the summer of 1992, Daphnia pulex appeared for the first time in Lake Ciso (NE-Spain). The cladocerans developed an unusually large population (maximum of ca. 2000 ind L-1) which caused drastic changes in the plankton communities found in the epi- and metalimnion. The food web structure in the epilimnion changed to a system that consisted almost exclusively of daphnids and heterotrophic bacteria. The extremely simple composition of the plankton was ideal for examining the controlling factors of the bacterioplankton and the interactions between Daphnia, heterotrophic nanoflagellates (HNF) and bacteria. In situ grazing and growth rates of the organisms were determined by size-fractionation and incubation experiments. Further, the effect of nutrient additions (carbon and phosphorus) on the specific bacterial growth rate was assessed in the presence and absence of Daphnia. The predation impact of D. pulex on protozoans was much higher than on bacteria; feeding rates on nanoflagellates (2.9 mL ind-1 h-1) exceeded those on bacteria (0.4 mL ind-1 h-1) considerably. Despite the low bacterial clearance rate, the high biomass of daphnids was sufficient for keeping bacterioplankton below the carrying capacity. Bacterial growth (about one doubling per day) could be balanced by the grazing activity of D. pulex. Addition of organic carbon strongly enhanced bacterial growth rates whereas phosphorus addition had no effect. After the cladocerans were removed, heterotrophic nanoflagellates developed and became the main bacterivores. They achieved fairly high growth rates (doubling time of 4-8 hours) on the bacteria which were present. The temporary invasion of D. pulex in Lake Ciso created a situation where bacterial abundance was controlled by metazooplankton predation and specific growth rate was determined by the supply of organic carbon. Further, our results suggest that similar predation pressure can be exerted on bacteria by Daphnia and nanoflagellates. However, HNF and other protozoans become unimportant when large Daphnia populations are present.