ABSTRACT: The in situ growth rates of the dominant planktonic protists were estimated in Lake Pavin during early spring, from unfiltered (UF, i.e. raw) and fractionated (<50 and <10 µm) water samples incubated in diffusion chambers. The early spring phytoplankton bloom was largely dominated by the plastidic dinoflagellate Gymnodinium uberrimum. Because they were not efficiently removed by filtration through the 10 µm pore-size mesh, small ciliates (10 to 25 µm in length: Urotricha spp., Balanion planctonicum and Strobilidium/Halteria) affected the growth rates of nanoflagellates in the <10 µm samples. Low or negative growth rates were also found for oligotrichs and tintinnids, which appeared to be very sensitive to manipulation and incubation. Except for the latter 2 taxa, species-specific theoretical growth rates (derived from cell volume and temperature) were close to those calculated from changes in natural abundances. Our results suggest that the small ciliate Urotricha may act as an essential trophic link between autotrophic nanoflagellates (ANF) and metazooplankton, and that large ciliates may be able to efficiently transfer energy between heterotrophic nanoflagellate (HNF) prey and metazooplankton grazers during early spring in Lake Pavin. Our experiments highlight the importance of taking into account each individual protist species, and of not considering flagellates and ciliates as being a uniform group in pelagic systems. Because of the great diversity of shape, size and flexibility of these cells, it is essential during fractionation experiments to confirm the effectiveness of the experimental protocols so as to identify possible artefacts and to obtain a more reliable estimate of the flows of material and energy in aquatic ecosystems.

KEY WORDS: Growth rates · Flagellates · Ciliates · Protists · Freshwaters · Lakes

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Published in AME Vol. 24, No. 2 (2001) on May 30
ISSN: 0948-3055. Copyright © Inter-Research, Oldendorf/Luhe, 2001