According to the Nitrate Directive it is necessary to established a protective belt (ecotones) around lakes. Inside these belts, it is forbidden to use fertilize for agricultural purposes. It is believed that it is the most imported measure to protect water quality in the lake. The analysis were conducted to estimate the sources of nitrogen entering the waters of the lake. Some analysis were conducted to estimate the sources of nitrogen entering waters of the lake. It was proved that the biggest load (more than 80%) of contamination is entering the lake with water flowing in streams and ditches. Only 10% of the chemicals are entering the lake with the groundwater filtrating to the lake. It is very important to use a proper methods of agriculture with proper methods of fertilization in the whole area of river basin flowing to the lakes.