PELAGIC CARBON METABOLISM IN A EUTROPHIC LAKE DURING A CLEAR-WATER PHASE

Dissolved and particulate organic carbon (DOC and POC, respectively), primary production, bacterial production, bacterial carbon demand and community grazing were measured for 9 weeks in eutrophic Frederiksborg Slotsso. The period covered the decline of the spring bloom, a clear- water phase and a summer phase with increasing phytoplankton biomass. The process rates and changes in pools of organic carbon were combined in a carbon budget for the epilimnion. The POC budget showed a close balance for both the post-spring bloom and the clear-water phase, whil e a surplus was found in the summer phase. Production of POC was domin ated by phytoplankton (2/3) compared to bacteria (1/3) during all phas es, and there was a significant correlation between phytoplankton and bacterial production rates (r(2) = 0.48, P < 0.039). Bacterial demand for DOC was balanced by production and changes in the pool of DOC duri ng the decline of the spring bloom, but the calculated demand exceeded the supply by 81 and 167%, respectively, during the other two periods . The discrepancy was most probably due to an underestimation of bacte rial growth efficiency and an overestimation of in situ bacterial prod uction in carbon units. Production of bacterial substrate by zooplankt on activity was estimated to be higher than the direct excretion of or ganic carbon from phytoplankton. The biological succession was regulat ed by the balance between area primary production and community grazin g. The clear-water phase was initiated by a combination of low primary production due to low surface irradiance and high community grazing ( 100 mmol C m(-2) day(-1)), which caused a decrease in phytoplankton bi omass. However, due to the high initial phytoplankton biomass, communi ty grazing was not high enough to cause a significant decrease in area primary production. The summer phase was initiated by a decrease in c ommunity grazing followed by an increase in phytoplankton biomass. Bas ed on these observations and calculations of area primary production a s a function of chlorophyll concentrations, we suggest that the possib ility for zooplankton to regulate phytoplankton biomass in temperate l akes decreases with increasing nutrient level.