Stimulation of the microbial food web in an oligotrophic, slightly acidified lake

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Abstract
We investigated the relative importance of inorganic nutrients vs. organic C in controlling bacterial production in a clear‐water, slightly acidified lake. We added dissolved organic C (DOC) as glucose (0.5 mg C liter−1) with and without inorganic nutrients (10 µg P liter−1 and 100 µg nitrate‐N liter−1) to 350‐liter enclosures. The treatment with DOC and nutrients in combination stimulated bacterial production (thymidine incorporation) steadily (from 1 to 9 pmol liter−1 h−1) throughout the 3‐week period. Glucose added alone and N+P added alone (at 2 × lake concn) had little effect on bacterial production. The enhancement of bacterial production by glucose with N+P in turn resulted in an increase in chrysophyte biomass, mainly the mixotroph Dinobryon. The enhancement of bacterial production was transferred up to protozoans (ciliates + heliozoans). These results indicate that bacterioplankton are potentially a link for C in this system and that there is a sensitive balance between bacterial limitation by DOC and inorganic nutrients in this oligotrophic lake.

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