A QUALITATIVE STUDY OF ZOOPLANKTON GRAZING IN AN OLIGO-MESOTROPHIC LAKE USING PHYTOPLANKTONIC PIGMENTS AS ORGANIC MARKERS

The impact of grazing by various zooplankton fractions (calanoid copep ods, cyclopoid copepods, cladocerans, and small-sized zooplankton: 80- 315 mu m) on various phytoplankton groups was measured in Lake Pavin ( Central Massif, France) by means of phytoplankton pigments separated b y HPLC as natural organic markers. The impact of grazing was determine d from April to November in the depth-sampling layers (0-7 and 7-20 m) in which most zooplankton organisms were concentrated. Selective graz ing by consumers created spatial (vertical) and temporal variation in mortality exerted on the various phytoplankton groups. For example, th e avoidance of dinoflagellates (characterized by peridinin) by most of the zooplankton organisms studied led to low grazing impacts (<280 ml liter(-1) d(-1)) on this phytoplankton group during all seasons. In c ontrast, preferential consumption of diatoms and chrysophytes (charact erized by fucoxanthin) and chlorophytes and cyanobacteria (characteriz ed by lutein and zeaxanthin) was recorded, depending on the zooplankto n fraction in question. The grazing impact exerted on these phytoplank tonic groups was much higher than that measured on dinoflagellates, re aching values of 2,300 ml liter(-1) d(-1). Grazing mostly affected dia toms and chrysophytes in the 0-7-m layer and chlorophytes and cyanobac teria in the 7-20-m layer; the differences between these two layers we re related to the heterogeneity in the vertical distribution of both p hytoplankton cells and grazing organisms.