Bacterial activity and protozoan grazing potential in a stratified lake

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 × 106 HNAN liter−1) coincided with high bacterial activity. Assuming a conversion factor of 2 × 1018 bacteria mol−1 thymidine, we estimated a maximal bacterial production of about 4 × 109 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 × 108 bacteria liter−1 d−1) and low HNAN numbers (3 × 106 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.