Pelagic food web processes in an oligotrophic lake

Major pelagic carbon pathways including primary production, release of extracellular products (EOC), bacterial production and zooplankton grazing were measured in oligotrophic Lake Almind (Denmark) and in enclosures (7 m super(3)) subjected to artificial eutrophication. Simultaneous measurements at 3 days interval of carbon exchange rates and pools allowed the construction of carbon flow scenarios over a 19 day experimental period. The flow of organic carbon was dominated by phytoplankton EOC release, which amounted from 44 to 58% of the net fixation of inorganic carbon. Gross bacterial production accounted for 33 to 75% of the primary production. The lower values of EOC release (44%) and bacterial production (33%) were found in the enclosures with added nutrients. The release of recently fixed photosynthetic products was the most important source of organic carbon to the bacterioplankton.