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Heterotrophic plankton grazing was studied in the Gulf of Trieste (Northern Adriatic Sea) from November 1998 to August 1999 using the dilution method. Four sets of experiments were carried out quarterly in order to assess the impact on communities of both phototrophic and heterotrophic prey. Four different trophic models were observed: during the autumn microzooplankton fed on small dinoflagellates, and phototrophic (PNAN) and heterotrophic nanoflagellates (HNAN), but not on the abundant bulk of diatoms. The entire initial HNAN standing stock was removed; we therefore did not observe any mortality of bacteria, whose biomass was the highest in the whole period. In late winter, the intense diatom bloom of Lauderia annulata remained almost untouched as the microzooplankton fed only on a less abundant small sized diatom (Chaetoceros). Microzooplankton also fed on HNAN, halving the mortality of bacteria induced by HNAN grazing only. In late spring, microzooplankton grazed effectively on a large array of prey (small diatoms, PNAN and HNAN). Reduction of bacterial mortality, exerted by microzooplankton through grazing on HNAN, was less evident, possibly due to direct microzooplankton grazing on bacteria. During the summer, we observed an intense grazing on bacteria by microzooplankton, which shifted from the usual nano-sized prey organisms, due to their extreme paucity, to bacteria. In conclusion, microzooplankton grazing was highly selective and vari- able due to the prey composition and to the predator community structure, which were investigated at the species to genus level. Microzooplankton was unable to control a bloom of large-sized diatoms, but showed a high level of control on most of the PNAN fractions. The result of this selection contributed significantly to the shaping of the phytoplankton community structure. Microzooplankton controlled HNAN biomass even more efficiently with relevant indirect effects on bacterial mortality.

Seasonal variations in the dynamics of microbial plankton communities: first estimates from experiments in the Gulf of Trieste, Northern Adriatic Sea