CARBON BUDGET FOR THE PELAGIC FOOD-WEB OF THE EUPHOTIC ZONE IN A BOREAL LAKE (LAKE-PAAJARVI)

A carbon budget for the pelagic euphotic zone of a mesohumic, boreal l ake was constructed, on the basis of enclosure experiments, in which e ffects of pulses of humic matter and phosphorus were studied. Although production of phytoplankton and bacteria was enchanced by additions o f phosphorus, and of phosphorus and humic matter together, the effects on zooplankton were less clear. The modest response of metazooplankto n 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 mic robial food chain Protozoans had key roles as grazers in the system; h eterotrophic nanoflagellates were the most important grazers of bacter ia, whereas ciliates were more important algal grazers than metazoopla nkton. Indirect evidence suggested that ciliates formed a considerable proportion of the food of metazooplankton. The amount of carbon relea sed through respiration exceeded the sum of that fixed by primary prod ucers and lost through sedimentation. If the climate change in the bor eal zone increases loading of nutrients and humic matter into lakes, t his imbalance between respiration and fixation of carbon can be expect ed to increase.