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Bacterial Activity and Protozoan Grazing Potential in a Stratified Lake

Jaap Bloem and Marie-Jose B. Bar-Gilissen
Limnology and Oceanography
Vol. 34, No. 2 (Mar., 1989), pp. 297-309
Stable URL: http://www.jstor.org/stable/2837107
Page Count: 13
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Bacterial Activity and Protozoan Grazing Potential in a Stratified Lake
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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 $(> 30 \times 10^6 HNAN liter^-1)$ coincided with high bacterial activity. Assuming a conversion factor of $2 \times 10^18$ bacteria $mol^-1$ thymidine, we estimated a maximal bacterial production of about $4 \times 10^9$ bacteria $liter^-1$ $d^-1$ from thymidine incorporation. In the upper metalimnion (5-6-m depth) we found a low bacterial activity (estimated production $1 \times 10^8$ bacteria liter^-1 d^-1)$ and low HNAN numbers $(3 \times 10^6 HNAN liter^-1)$. 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.

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