Impact of grazing on phytoplankton in Lake Pavin (France): Contribution of different zooplankton groups

The impact of zooplankton grazing on the phytoplankton community was measured in an oligomesotrophic lake, Lake Pavin (France), during a seasonal study. The feeding activity of various zooplankton groups was determined in the 0–7 and 7–20 m depth layers, by identifying and quantifying chlorophyll a and its derivatives by high-performance liquid chromatography in samples of phytoplankton and zooplankton. This determination takes into consideration the degradation of these pigments into colourless products during their ingestion by zooplankton. A high mean percentage loss, linked with this degradation, has been observed (81.9 ± 11.5%). The grazing impact by the whole zooplankton community (expressed as the percentage of water volume swept clear of chlorophyll-containing organisms each day) varied between 1 and 48% day−1 in the 0–7 m layer (mean value 17.5% day−1), and between 0.2 and 19% day−1 in the 7–20 m layer (mean value 3.8% day−1). The calanoid copepod Acanthodiaptomus denticornis was the main grazer from April to mid-July, but its impact was relatively low (