Dynamics of dissolved organic carbon lability in a eutrophic lake

The concentration of labile dissolved organic C ([DOC,]) and the plankton community structure were measured weekly during a diatom spring bloom and the subsequent clear-water phase in a eutrophic, temperate lake. The DOC, concentration was assessed by a bacterial regrowth method based on direct measurements of the increase in bacterial biomass and its respiratory demand. [DOC,] ranged from 560 to 1,130 pg C liter- I and accounted for only 5-9 % of total DOC. DOCL oscillated around 800 pg C liter- I, and the amplitude over 7-d periods was within k 16 % of the in situ bacterial carbon demand. One period (3 weeks) with consistently increasing [DOC,] took place concomitant with the increase in macrozooplankton and bacterial biomass. Thus, DOC, was positively related to decreases in chlorophyll and zooplankton grazing. The increase in DO & was followed by 3 weeks with decreasing concentrations, when the relationship between DOC, and bacterial production was negative. This pattern gives support to the hypothesis that grazing is a,quantitatively important process in the flow of carbon to bacteria. The variations of DOC, were analyzed with a model describing interactions among substrate, bacteria, and grazing. Short-term (days) oscillations probably result from pulses of substrate production that are followed within a few days by bacterial uptake, and long-term (weeks) variations are explained by a variable bacterial substrate affinity (K,,)