Homeostatic regulation of dissolved labile organic substrates by consumption and release processes in a freshwater lake

We examined short-term changes and spatial variability of dissolved free amino acids (DFAA) and glucose in mesotrophic Lake Zurich (Switzerland) at 2- to 4-day intervals over 5 weeks during spring. Heterotrophic incorporation, consumption ("gross" uptake), and release of free leucine and glucose were assessed using isotopically labeled tracers. Glucose exhibited significantly higher consumption and release rates but a lower turnover time than leucine. The net uptake, release, and turnover time of both substrates were significantly correlated, suggesting that their microbial processing was closely coupled. Net changes of individual DFAA and glucose during dark short-term incubations with unfiltered samples including phytoplankton were conspicuously similar, indicating that bulk heterotrophic consumption of labile substrates was nonselective and depended primarily on their availability. Changes of DFAA during incubations were significantly related to their initial concentrations. Specifically, DFAA concentrations tended to converge toward their seasonal in situ median concentrations, which likely represented the balanced state of net release vs. consumption. There was a clear difference between relatively stable compounds (glucose, leucine, arginine) and others that showed pronounced concentration changes during incubations (alanine, valine, glycine), pointing to a variable degree of uncoupling between consumption and release. Moreover, short-term changes of DFAA concentrations in incubations were also related to their variability in replicate 5-liter samples taken from the lake, suggesting that biological processes such as consumption and release may influence the mesoscale spatial distribution of DFAA in the epilimnion of a lake. Overall, we document the homeostatic regulation of labile dissolved substrates by varying release and consumption processes.