Mesocosm tracer studies. 2. The fate of primary production and the role of consumers in the pelagic carbon cycle of a mesotrophic lake

Abstract

The fate of primary production in the pelagic food web in mesotrophic Svinsjøen, southeastern Norway, was studied in a 10‐d mesocosm experiment with 14C addition. Specific assimilation rates for all the main compartments in the system were estimated by applying tracer models and further combined to construct a balanced C budget by means of mathematical inverse methods. The specific carbon assimilation rates for the phytoplankton and bacteria were 0.24 and 0.49 d−1, respectively, and 70% of the bacterial production was fueled by phytoplankton exudates. The C budget shows that roughly half of the gross primary production was lost as exudates and 10% was respired. Only 20% of phytoplankton net production was assimilated by consumers. The rest of the net primary production was lost as dissolved and particulate detritus or by sedimentation. Of the net bacterial production, a third accumulated as bacterial biomass, a third was assimilated by bacterivores, and a third was lost as detritus. The coupling between the microbial loop and the classical grazer food web was insignificant, probably due to low Daphnia biomass. The microbial food web thus acted as a sink rather than as a source of C to the higher consumers. Altogether, <2% of gross primary production was incorporated into consumer biomass.