Pelagic food web processes in an oligotrophic lake |
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Authors: | Morten Søndergaard Bo Riemann Lars Møller Jensen Niels O G Jørgensen Peter K Bjørnsen Michael Olesen Jens B Larsen Ole Geertz-Hensen Jan Hansen Kirsten Christoffersen Anne-Mette Jespersen Flemming Andersen Suzanne Bosselmann |
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Institution: | (1) Botanical Institute, University of Aarhus, 68 Nordlandsvej, DK-8240 Risskov, Denmark;(2) Institute of Biology and Chemistry, University of Roskilde, P. Box 260, DK-4000 Roskilde, Denmark;(3) Freshwater Biological Laboratory, University of Copenhagen, 51 Helsingørsgade, DK-3400 Hillerød, Denmark;(4) International Agency for 14C Determination, 15 Agern Allé, DK-2970 Hørsholm, Denmark;(5) Department of Microbiology, Royal Veterinary and Agricultural University, 21 Rolighedsvej, DK-1958 Frederiksberg C, Denmark |
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Abstract: | 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 m3) subjected to artificial eutrophication. Simultaneous measurements at three days interval of carbon exchange rates and pools allowed the construction of carbon flow scenarios over a nineteen 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. Uptake of dissolved free amino acids was responsible for 52 to 62% of the gross bacterial production. Thus, amino acids constituted a significant proportion of the EOC. Zooplankton (< 50 µm) grazing on algae and bacteria accounted only for a minor proportion of the particulate production in May. Circumstantial evidence is presented that suggests the chrysophycean alga Dinobryon was the most important bacterial remover.The results clearly demonstrated EOC release and bacterial metabolism to be key processes in pelagic carbon cycling in this oligotrophic lake. |
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Keywords: | primary production extracellular release bacterial production pelagic carbon budgets |
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