Bacterivory by heterotrophic nanoflagellates and bacterial production in sediments of a freshwater littoral system

We made simultaneous measurements of benthic bacterial production, using [3H]thymidine assays, and bacterivory by benthic heterotrophic nanoflagellates (HNAN), using fluorescently labeled bacteria. Sediment samples were collected at two stations in a eutrophic freshwater littoral in different seasons. Bacterial production ranged from 19.7 to 263.0~ lo9 bacteria dm-3 h-l, with the highest values measured in summer. HNAN grazing rates (3.8-64.2 bacteria flagellate-l h-l) varied significantly with season and station, and rates were correlated with flagellate biovolume. At both stations, there were large discrepancies between bacterial production and HNAN grazing. The estimated bacterivory ranged from 0.4 to 5.2% of bacterial production. The minor impact of HNAN as grazers on bacteria during summer months was caused by high bacterial production and low HNAN abundances. During winter, bacterial production decreased, whereas HNAN abundance increased. However, due to the low grazing rate of these relatively small flagellates, the total HNAN community still consumed only an insignificant amount of the bacterial production. The results of our measurements show that bacterivory by HNAN cannot account for the fate of bacterial production in the littoral studied.