Carbon budget for the pelagic food web of the euphotic zone in a boreal lake (Lake Pääjärvi)

A carbon budget for the pelagic euphotic zone of a mesohumic, boreal lake was constructed, on the basis of enclosure experiments, in which effects of pulses of humic matter and phosphorus were studied. Although production of phytoplankton and bacteria was enchanced by additions of phosphorus, and of phosphorus and humic matter together, the effects on zooplankton were less clear. The modest response of metazooplankton was partly due to the short time of the experiments, but it was also due to loss of carbon through respiration in several steps of the microbial food chain. Protozoans had key roles as grazers in the system; heterotrophic nanoflagellates were the most important grazers of bacteria, whereas ciliates were more important algal grazers than metazooplankton. Indirect evidence suggested that ciliates formed a considerable proportion of the food of metazooplankton. The amount of carbon released through respiration exceeded the sum of that fixed by primary producers and lost through sedimentation. If the climate change in the boreal zone increases loading of nutrients and humic matter into lakes, this imbalance between respiration and fixation of carbon can be expected to increase. Resume : Nous avons Otabli le bilan du carbone pour la zone euphotique pOlagique diun lac borOal mOsohumique, ‡ liaide diexpOriences en enclos dans lesquelles nous avons OtudiO les effets de poussOes de matire humique et de phosphore. Bien que la production phytoplanctonique et bactOrienne ait OtO accrue par liajout de phosphore, et de phosphore et de matire humique ensemble, les effets sur le zooplancton ont OtO moins Ovidents. La rOponse modeste du mOtazooplancton Otait due en partie ‡ la courte durOe des expOriences, mais aussi ‡ la perte de carbone par la respiration ‡ plusieurs Otapes de la chaOne trophique microbienne. Les protozoaires jouaient un rUle clO en tant que brouteurs dans le systme; les nanoflagellOs hOtOrotrophes Otaient les plus importants brouteurs de bactOries, tandis que les ciliOs Otaient des brouteurs dialgues plus importants que le mOtazooplancton. Des donnOes indirectes permettent de penser que les ciliOs formaient une proportion considOrable de la nourriture du mOtazooplancton. La quantitO de carbone libOrO par la respiration dOpassait la somme de celui qui Otait fixO par les producteurs primaires et perdu par la sOdimentation. (Traduit par la ROdaction)

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