STIMULATION OF THE MICROBIAL FOOD-WEB IN AN OLIGOTROPHIC, SLIGHTLY ACIDIFIED LAKE

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 mug P liter-1 and 100 mug 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 x 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.