IMPACT OF GRAZING ON PHYTOPLANKTON IN LAKE PAVIN (FRANCE) - CONTRIBUTION OF DIFFERENT ZOOPLANKTON GROUPS

The impact of zooplankton grazing on the phytoplankton community was m easured in an oligomesotrophic lake, Lake Pavin (France), during a sea sonal study. The feeding activity of various zooplankton groups was de termined in the 0-7 and 7-20 m depth layers, by identifying and quanti fying chlorophyll a and its derivatives by high-performance liquid chr omatography in samples of phytoplankton and zooplankton. This determin ation 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 (e xpressed 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-Ju ly, but its impact was relatively low (<17% day(-1) in both depth laye rs). From July onwards, cladocerans became abundant and were the crust aceans with the greatest grazing impact (up to 36% day(-1)). The organ isms of the 80-315 mu m size class, composed mainly of rotifers, made a major, and sometimes dominant, contribution to grazing, especially i n the 7-20 m layer where they accounted for 20-95% of the total grazin g by the zooplankton community.