Impact of natural metazooplankton assemblage on planktonic microbial communities in a newly flooded reservoir

The grazing impact of a natural assemblage of metazoan zooplankton onpigmented flagellates (PF), heterotrophic nanoflagellates (HNF), ciliates,and non-flagellate algae and microcyanobacteria (NFAM) was measuredin situ during the period of thermal stratification ina newly flooded reservoir (Reservoir de la Sep, France). Experiments wereconducted with diffusion chambers in the meta- and epilimnion over a periodof 7 h. The mean mortalities of PF in the epi- and metalimnion (0.08 0.02 and 0.06 ± 0.03h^-1, respectively), of HNF (0.04 ± 0.02and 0.05 ± 0.02 h^-1) and ciliates (0.09± 0.04 and 0.11 ± 0.04 h^-1)demonstrate the impact of the metazoan zooplankton, and particularly of therotifer Asplanchna priodonta, on the components of themicrobial loop. The mortality of NFAM, accounting for 14 and 18% of totalmortality, remained low throughout the study. The taxa with the highestmortality were pigmented flagellates of 4-19 m, HNF, and small-sized ciliates such asHalteria sp. (0.10 ± 0.02h^-1 at 1 m and 0.30 ± 0.38h^-1 at 7 m) and Urotrichafurcata (0.11 ± 0.05 h^-1 at 1m and 0.12 ± 0.06 h^-1 at 7 m).Large-sized ciliates (Paradileptus elephantinus) andsessile ciliates (Suctorida, Vorticella sp.) had avery low mortality (>0.04 h^-1). Afterreservoir flooding, the organisms in the microbial trophic loop, favouredby the high quantities of allochthonous organic matter, are subject to ahigher mortality than the phytoplankton.