Dynamics of dissolved organic car‐bon lability in a eutrophic lake

The concentration of labile dissolved organic C ([DOC L ]) 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 L 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 µ g C liter −1 and accounted for only 5–9% of total DOC. DOC L oscillated around 800 µ g C liter −1 , and the amplitude over 7‐d periods was within ± 16% of the in situ bacterial carbon demand. One period (3 weeks) with consistently increasing [DOC L ] took place concomitant with the increase in macrozooplankton and bacterial biomass. Thus, DOC L was positively related to decreases in chlorophyll and zooplankton grazing. The increase in DOC L was followed by 3 weeks with decreasing concentrations, when the relationship between DOC L 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 L 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 m ).