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In a fishless oligotrophic Alpine lake (Lago Paione Superiore: LPS), with a simple trophic food web, the relationships between microorganisms and zooplankton have been studied. During the ice-free period 1999, bacterial grazing by mixotrophic flagellates (MxFl), autotrophic carbon fixation and excretion (EOC), bacterial production, and zooplankton abundance and biomass were measured weekly. After ice-melt, MxFl, which constituted 94% of total phytoplankton, increased in number until the appearance of Daphnia longispina, the dominant crustacean zooplankton species in LPS. Gross primary production and EOC ranged from 0.2 to 2.5 and from 0.1 to 1.8 μg C l-1 h-1, respectively. EOC accounted for 45 to 90% of total autotrophic production. Community grazing rate of MxFl was higher at 9 m in comparison with 1 m and ranged from 3 to 62 × 103 bact ml-1 h-1. MxFl ingested 0.2 to 8% of the natural bacterioplankton per hour, in summer. The community grazing rate of mixotrophic flagellates was significantly correlated with the phytoplanktonic photosynthetic activity (P = 0.006) and with the bacterial production (P = 0.037). In August, Daphnia grazing caused a rapid decrease of both bacteria and MxFl and of their activities. Phytoplankton photosynthetic efficiency increased in late summer after a phosphorus pulse. Although P excretion by zooplankton was not measured, it is speculated that Daphnia play an important role in the availability of this element firstly by grazing bacteria and MxFl in the water column and, after the exhaustion of such food, by browsing algae at the water-sediment interface.
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