Uncoupling of bacterial production and flagellate grazing in aquatic sediments: a case study from an intertidal flat

In contrast to planktonic ecosystems, the fate of bacterial production in a quatic sediments is still largely unclear. In this study, we identified the factors regulating the impact of flagellate grazing on benthic bacterial p roduction for a sandy and a silty intertidal sediment. Flagellate grazing r ates were estimated using fluorescently labelled sediment to prevent distur bance of in situ bacterial density and community composition and to account for grazing on attached bacteria. Since flagellate cell size was quite div erse, the grazing rates were determined for 4 size classes. Bacterial produ ction was measured simultaneously with grazing estimates. Bacterial density and production increased with decreasing median grain size of the sediment . Bacterial production was strongly related to the chlorophyll a content of the sediment, indicating resource control of bacterial production. In cont rast to bacteria, flagellate biomass decreased with decreasing median grain size of the sediment. Pairwise comparison of grazing rates between the 2 s ites showed that grazing rates were significantly higher at the sandy site. This suggests that the effect of sediment composition on flagellate biomas s may be mediated by an influence of sediment characteristics on flagellate ingestion rates, The negative relation of bacterial production and the pos itive relation of flagellate biomass and grazing rates with median grain si ze resulted in a significant positive relation between the impact of flagel late grazing on bacterial production and the median grain size of the sedim ent. These results amount to an uncoupling of flagellate grazing and bacter ial production in fine sediments. Our results as well as results from previ ous studies suggest that grazing by flagellates is not an important fate of bacterial production in aquatic sediments, except for sandy sediments duri ng periods of low bacterial production.