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1. The seasonal development of heterotrophic nanoflagellates (HNF), bacteria, rotiferans and crustacean zooplankton was studied in the epilimnion of Lake Pavin, an oligomesotrophic lake in the Massif Central of France. 2. HNF abundance varied from 0.1 to 2.5 × 103 mL–1. Free‐living HNF reached their highest density in spring when the copepod Acanthodiaptomus denticornis dominated the metazooplankton. They were present in low numbers when rotifers and cladocerans were numerous. 3. Attached HNF, consisting of bicoecids and choanoflagellates, were fixed to large diatoms and to the colonial cyanobacterium Anabaena flos‐aquae. The abundance of attached HNF was significantly correlated to bacterial abundance, which fluctuated between 1.1 and 2.7 × 106 mL–1. Highest abundance of these epiphytic protists was recorded when free‐living heterotrophic nanoflagellates declined. 4. The comparison of the dynamics of heterotrophic nanoflagellates, bacteria, and the impact of zooplankton grazing suggested that prey abundance, the presence of suitable attachment sites and limited competition from the free‐living forms were the main factors controlling the development of the epiphytic flagellate protists. In contrast, the low abundance of free‐living forms during the period of rotiferan and cladoceran development suggests the prevalence of a top‐down control by predation of the metazoopankton.

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International audienceThe authors discuss the reliability of a new method used to depict precisely the authigenic calcite precipitation process in the water column of a mesotrophic alpine lake (Lake Bourget, Savoie, France). Complete chemical analysis of dissolved major elements shows that, in the epilimnion, calcium concentration and alkalinity are the only chemical species presenting large variations. The behaviour of these two parameters is only related to calcite precipitation. The method is based on in situ specific conductance measurements (C-25). Experimental values were compared with theoretical calculations. The results demonstrate clearly that dissolved calcium concentration can be directly related to measured specific conductance. Hence, a mathematical relationship was developed to compute calcium concentration profiles in the water column with a centimetre spatial resolution through the whole water column. Finally, authigenic calcite production can be observed and quantified on a time scale covering two years or experimental data. Calcite production is estimated to 4.8 mol m(-2) yr(-1). Extrapolation to the whole lake surface leads to 16.000 +/- 2000 tons yr(-1) as CaCO3. specific conductance is an easy-to-measure in situ parameter which could be used to trace authigenic calcite production in other hard water alpine lakes. (C) 2000 Elsevier Science Ltd. All rights reserved