Short-term changes of protozoan control on autotrophic picoplankton in an oligo-mesotrophic lake

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In May 1994, we investigated the short-term development of the planktonic community in the epi- and metalimnion of an oligo-mesotrophic lake (Piburger See, Tyrol), focusing on trophic links between protists and picoplankton, but also including phyto- and zooplankton. Uptake experiments with fluorescently labelled bacteria (FLB) and picocyanobacteria (FIX) were performed in order to compare the importance of both prey types as carbon sources for bacterivorous protists. Heterotrophic nanoflagellates (HNF) were responsible for —90% of total protozoan picoplanktivory (FLB + FIX); ciliates accounted for —10%. A selectivity index related to prey density showed that both HNF and ciliates clearly preferred FIX over FLB. The mean cell size of autotrophic (prokaryotic) picoplankton (APP) was nearly three times larger (0.323 jim3) and much less variable than mean bacterial cell volume (0.122 (inV). Although APP biomass was on average only 8.6% of total picoplankton biomass, pico- cyanobactena accounted for a mean 15.9% of total HNF carbon uptake. We calculated that total HNF grazing could match potential APP maximum growth rates at the beginning of the study period. A strong decrease in HNF individual clearance rate (CR) on APP was clearly related to a fall in the percentage of choanofiagellates (from 75 to —10% of the HNF community). A simultaneous decrease in HNF biomass and CR was followed by a steep increase in APP abundance; APP abundance and HNF biomass were highly negatively correlated both in the epi- and the metalimnion (/-, = -0.879, r, META = -0.907; P < 0.001). Total rotifer abundance increased by a factor of 50 within 2 weeks and was also negatively correlated with HNF biomass (r, EPI = -0.852, P < 0.001; r, MlrTA = -0.659, P < 0.05). HNF grazing was found to exert a strong short-term control on picocyanobacteria and this link was probably broken by an increase in metazooplankton, especially due to rotifer predation on HNF.