Bacterial activity and protozoan grazing potential in a stratified lake

Abstract

During summer stratification in Lake Vechten, heterotrophic nanoflagellates (HNAN) showed peak densities at very low oxygen concentrations in the lower metalimnion at a depth of 6–7 m. In this stratum, high HNAN numbers (>30 × 10 6 HNAN liter −1 ) coincided with high bacterial activity. Assuming a conversion factor of 2 × 10 18 bacteria mol −1 thymidine, we estimated a maximal bacterial production of about 4 × 10 9 bacteria liter −1 d −1 from thymidine incorporation. In the upper metalimnion (5–6‐m depth) we found a low bacterial activity (estimated production 1 × 10 8 bacteria liter −1 d −1 ) and low HNAN numbers (3 × 10 6 HNAN liter −1 ). HNAN seem to be a link in the food web between bacteria and cyanobacteria on the one hand and ciliates on the other. Grazing experiments with selectively filtered lake water indicated ingestion rates of about 7 bacteria HNAN −1 h −1 . We calculated that the metalimnetic HNAN are able to consume the entire bacterial production even at high bacterial growth rates.