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The impact of grazing by various zooplankton fractions (Calanoid copepods, cyclopoid copepods, cladocerans, and small-sized zooplankton: 80-3 15 pm) on various phytoplankton groups was measured in Lake Pavin (Central Massif, France) by means of phytoplankton pigments separated by HPLC as natural organic markers. The impact of grazing was determined from April to November in the depth-sampling layers (O7 and 7-20 m) in which most zooplankton organisms were concentrated. Selective grazing by consumers created spatial (vertical) and temporal variation in mortality exerted on the various phytoplankton groups. For example, the avoidance of dinoflagellates (characterized by peridinin) by most of the zooplankton organisms studied led to low grazing impacts (~280 ml liter-r d-r) on this phytoplankton group during all seasons. In contrast, preferential consumption of diatoms and chrysophytes (characterized by fucoxanthin) and chlorophytes and cyanobacteria (characterized by lutein and zeaxanthin) was recorded, depending on the zooplankton fraction in question. The grazing impact exerted on these phytoplanktonic groups was much higher than that measured on dinoflagellates, reaching values of 2,300 ml liter-l d-l. Grazing mostly affected diatoms and chrysophytes in the O-7-m layer and chlorophytes and cyanobacteria in the 7-20-m layer; the differences between these two layers were related to the heterogeneity in the vertical distribution of both phytoplankton cells and grazing organisms. |
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