DYNAMICS OF DISSOLVED ORGANIC-CARBON LABILITY IN A EUTROPHIC LAKE

The concentration of labile dissolved organic C ([DOCL]) and the plank ton community structure were measured weekly during a diatom spring bl oom and the subsequent clear-water phase in a eutrophic, temperate lak e. The DOCL concentration was assessed by a bacterial regrowth method based on direct measurements of the increase in bacterial biomass and its respiratory demand. [DOCL] ranged from 560 to 1,130 mu g C liter(- 1) and accounted for only 5-9% of total DOC. DOCL oscillated around 80 0 mu g C liter(-1), and the amplitude over 7-d periods was within +/-1 6% of the in situ bacterial carbon demand. One period (3 weeks) with c onsistently increasing [DOCL] took place concomitant with the increase in macrozooplankton and bacterial biomass. Thus, DOCL was positively related to decreases in chlorophyll and zooplankton grazing. The incre ase in DOCL was followed by 3 weeks with decreasing concentrations, wh en the relationship between DOCL and bacterial production was negative . This pattern gives support to the hypothesis that grazing is a quant itatively important process in the flow of carbon to bacteria. The var iations of DOCL were analyzed with a model describing interactions amo ng substrate, bacteria, and grazing. Short-term (days) oscillations pr obably result from pulses of substrate production that are followed wi thin a few days by bacterial uptake, and long-term (weeks) variations are explained by a variable bacterial substrate affinity (K-m).