The role of fish in the regulation of nutrient cycling in Lake Balaton, Hungary

SUMMARY. 1. In 1984 an enclosure experiment was carried out in order to elucidate the importance of nutrient supply from fish to phytoplankton during the critical summer period in eutrophic Lake Balaton. One enclosure was stocked with fish and the other left empty.
2. Two peaks were recorded in primary production reaching maximum values in the fish enclosure.
3. In the enclosure with fish, inorganic nitrogen concentrations increased gradually till the end of the experiment, with ammonia making up the largest component. Lower dissolved phosphorus concentrations in the fish enclosure, with its higher primary production rate, may indicate an intense cycling of this nutrient.
4. The biological and chemical changes occurring in the enclosure suggest that bream removal can be an effective tool in water quality control for Lake Balaton.