Longitudinal changes in protistan bacterivory and bacterial production in two canyon-shaped reservoirs of different trophic status

Abstract

We studied bacterial production and mortality due to heterotrophic nanoflagellate (HNF) and ciliate grazing on longitudinal transects conducted on two canyon-shaped reservoirs of different trophic status, Řimov and Orlik (Czech republic). Special attention was paid to the changes in these processes related to depth, spatial distribution of sampling points, different periods of planktonic succession and to the taxon-specific bacterivory of ciliates. Bacterial production (BP) in both reservoirs was, on average, reaching similar values (ranging from 1.2 × 106 to 4 × 106 bacteria ml−1 d−1). In Řimov (mesoeutrophic, mean retention time 100 days), when seasonally averaged, total protistan grazing accounted for 35% of BP and was consistantly lower in the layer R (layer with the same temperature and conductivity as the river inflow). On the contrary, the seasonal average for the Orlik reservoir (eutrophic, mean retention time of 23 days) showed roughly two times higher proportion of BP (70% of the total) removed by protists. In both reservoirs, there was little or no difference in contribution of ciliates and HNF to total protistan bacterivory. Overall, oligotrichous ciliates were recognized as the major ciliate bacterivores (accounting for 67% and 48% of total ciliate bacterivory in Řimov and Orlik, respectively) followed by peritrichous ciliates that contributed to total ciliate bacterivory from 23% (Řimov) to 28% (Orlik). A small omnivorous oligotrichous ciliate Halteria cf. grandinella was the major bacterivore within the whole ciliate community in both reservoirs, contributing as much as 48 and 34% to the total ciliate bacterivory, in the Řimov and Orlik reservoirs, respectively.