Abstract
Changes in the trophic state of Lake Niegocin were characterized based on physical, chemical, and commercial fisheries data. Analysis of nitrogen and phosphorus influx data has shown that the trophic state of the lake can be described as polytrophic for the time period of interest. A significant reduction in the influx of biogenic substances occurred following the construction of a mechanical-biological wastewater treatment plant employing chemical phosphorus precipitation technology. Shortly after the treatment plant went on line, a reduction in the amount of phosphorus and nitrogen was recorded along with an increase in lake water transparency. The changes in physical and chemical parameters presented herein correspond in general with changes in commercial fish catches. Special attention is paid to pikeperch whose yield had increased along with increasing trophic states but then declined once the treatment plant went on line. The paper emphasizes the fact that this species is one of the best indicators of eutrophication in vendace-type lakes.