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

The grazing impact of a natural assemblage of metazoan zooplankton on pigmented flagellates (PF), heterotrophic nanoflagellates (HNF), ciliates, and non-flagellate algae and microcyanobacteria (NFAM) was measured in situ during the period of thermal stratification in a newly flooded reservoir (Reservoir de la Sep, France). Experiments were conducted with diffusion chambers in the metaand epilimnion over a period of 7 h. The mean mortalities of PF in the epiand metalimnion (0.08 ± 0.02 and 0.06 ± 0.03 h–1, respectively), of HNF (0.04 ± 0.02 and 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 the rotifer Asplanchna priodonta, on the components of the microbial loop. The mortality of NFAM, accounting for 14 and 18% of total mortality, remained low throughout the study. The taxa with the highest mortality were pigmented flagellates of 4–19 μm, HNF, and small-sized ciliates such as Halteria sp. (0.10 ± 0.02 h–1 at 1 m and 0.30 ± 0.38 h–1 at 7 m) and Urotricha furcata (0.11 ± 0.05 h–1 at 1 m and 0.12 ± 0.06 h–1 at 7 m). Large-sized ciliates (Paradileptus elephantinus) and sessile ciliates (Suctorida, Vorticella sp.) had a very low mortality (<0.04 h–1). After reservoir flooding, the organisms in the microbial trophic loop, favoured by the high quantities of allochthonous organic matter, are subject to a higher mortality than the phytoplankton.