Pelagic carbon metabolism in a eutrophic lake during a clear-water phase

Dissolved and paniculate organic carbon (DOC and POC, respectively),primary production, bacterial production, bacterial carbon demandand community grazing were measured for 9 weeks in eutrophicFrederiksborg Slotssø. The period covered the declineof the spring bloom, a clear-water phase and a summer phasewith increasing phytoplankton biomass. The process rates andchanges in pools of organic carbon were combined in a carbonbudget for the epilimnion. The POC budget showed a close balancefor both the post-spring bloom and the clear-water phase, whilea surplus was found in the summer phase. Production of POC wasdominated by phytoplankton (2/3) compared to bacteria (1/3)during all phases, and there was a significant correlation betweenphytoplankton and bacterial production rates (r² = 0.48, P <0.039). Bacterial demand for DOC was balanced by productionand changes in the pool of DOC during the decline of the springbloom, but the calculated demand exceeded the supply by 81 and167%, respectively, during the other two periods. The discrepancywas most probably due to an underestimation of bacterial growthefficiency and an overestimation of in situ bacterial productionin carbon units. Production of bacterial substrate by zooplanktonactivity was estimated to be higher than the direct excretionof organic carbon from phytoplankton. The biological successionwas regulated by the balance between area primary productionand community grazing. The clear-water phase was initiated bya combination of low primary production due to low surface irradianceand high community grazing (100 mmol C m^–2 day^–1),which caused a decrease in phytoplankton biomass. However, dueto the high initial phytoplankton biomass, community grazingwas not high enough to cause a significant decrease in areaprimary production. The summer phase was initiated by a decreasein community grazing followed by an increase in phytoplanktonbiomass. Based on these observations and calculations of areaprimary production as a function of chlorophyll concentrations,we suggest that the possibility for zooplankton to regulatephytoplankton biomass in temperate lakes decreases with increasingnutrient level.