Seasonal dynamics of free and attached heterotrophic nanoflagellates in an oligomesotrophic lake

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 x 10(3) 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 x 10(6) 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.