Carbon metabolism in a humic lake: Pool sires and cycling through zooplankton

To characterize the major carbon pathways in a humic lake, we determined carbon pool sizes and main pathways by long-term tracer studies in enclosures. Dissolved organic carbon (DOC) was by far the largest pool and constituted 80–85% of total carbon. In the water column particulate organic carbon was partitioned between detritus, zooplankton, bacteria, and phytoplankton at ratios of 22 : 4 : 3 : 1. Phytoplankton and bacterioplankton production averaged 24 and 32 µg C liter−1 d−1, while crustacean zooplankton production was very low (<5 µg C liter−1 d−1) during the experiment. Total pelagic community respiration was high, giving a net CO2 flux to the atmosphere of 44 µg C liter−1 d−1, while losses by sedimentation were negligible. Most of the particulate carbon available for zooplankton was highly recycled detritus of low nutritional value. The loop of ingestion and defecation of detrital particles was a major carbon pathway, giving detrital particles a turnover rate of 0.39 d−1. Detritus was found to support 46–82% of body carbon in the surveyed species, with Acanthodiaptomus as the upper extreme. Bacterial carbon accounted for 11–42% of body carbon and phytoplankton for 6–19% in the surveyed species.