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To assess the influence of grazing by cladocerans on dissolved organic matter (DOM), glycolytic and proteolytic activities and bacterial growth were measured by in situ incubation of lake water from the epilimnion of an oligotrophic reservoir in three different treatments: in absence of zooplankton, and in presence of zooplankton (natural abundance and concentrated four-fold). These experiments were conducted at two periods in the succession of plankton populations (May and June 1998), that differed in the quality of the prey ingested (Eudorina sp. compared to Cryptomonas sp. and Rhodomonas sp.) and their grazing intensity (31.8 ± 2.2 μg C l -1 day -1 compared to 10.2 ± 0.5 μg C l -1 day -1 ). A systematic increase in bacterial biomass was measured in the treatments containing the highest zooplankton concentrations. The DOM concentrations produced in situ showed few significant differences between the three treatments, but the assimilation of DOM was higher in the presence of zooplankton than in their absence. These results show that the influence of cladocerans on the DOM was more of a qualitative than a quantitative nature. The protein compounds derived from the grazing activities of metazoans seem to be a major nutrient source for growth for bacteria (r = 0.81, P <0.05). In this study, the highest hydrolytic activities were recorded in the presence of high concentrations of metazoan zooplankton. However, the processes that regulated these activities differed between the two experimental dates (repression compared to enzyme stimulation). Grazing activities could lead to an increase in phytoplanktonic excretion during the growth phase, and therefore the production of low molecular weight compounds that are easily assimilated by the bacterial plankton.

Influence of cladoceran grazing activity on dissolved organic matter, enzymatic hydrolysis and bacterial growth