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

Abstract

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 phyto- plankton and zooplankton. This determination takes into consideration the degradation of these pig- ments 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 zoo- plankton community (expressed as the percentage of water volume swept clear of chlorophyll-contain- ing organisms each day) varied between 1 and 48% day1 in the 0-7 m layer (mean value 17.5% day1), and between 0.2 and 19% day1 in the 7-20 m layer (mean value 3.8% day1). The calanoid copepod Acanthodiaptomus denticomis was the main grazer from April to mid-July, but its impact was relatively low (<17% day' in both depth layers). From July onwards, cladocerans became abundant and were the crustaceans with the greatest grazing impact (up to 36% day1). The organisms of the 80-315 |im size class, composed mainly of rotifers, made a major, and sometimes dominant, contribution to grazing, especially in the 7-20 m layer where they accounted for 20-95% of the total grazing by the zooplankton community.