Abstract
Polish rivers, which make up 20% of the Baltic Sea’s catchment area, transport a load of nitrogen and phosphorus compounds into the sea. Theoretically, this load should be strongly correlated with the amount of water flowing in those rivers. However, this is not the case, especially in terms of the phosphorus compounds, since most of them come from point source pollution. The outflow load is also significantly influenced by nonpoint source pollution from farmland, which is very difficult to quantify. About 50% of nitrogen and 30% of phosphorus appear to come from nonpoint source pollution. It is important to realize that the load from nonpoint source pollution also includes the load of phosphorus and nitrogen transformed by lakes. Only recently, however, has this issue been mentioned in the specialist literature. In Polish studies on lake balance this remains a marginal issue. A limited number of observations indicate that lakes are capable of both limiting and increasing the load (mainly from the bottom sediments). This article presents some data on this issue which suggest that the roles of lakes in the transformation of the load of biogenic substances may be significant, but diverse.