IMPACTS OF AGRICULTURAL NUTRIENT LOADING ON FINNISH WATERCOURSES

Eutrophication of lakes, rivers and coastal waters is the main environ mental problem caused by agriculture in Finland. Water quality of most Finnish watercourses is good or excellent, but in the intensively cul tivated region of southern and western Finland the turbidity of water as well as algal blooms are a common problem. Generally, phosphorus is the limiting nutrient for primary production in Finnish lakes, but in eutrophic lakes and coastal waters the role of nitrogen becomes more important. Nitrogen from agriculture enters the watercourses mainly in dissolved form. By contrast, most phosphorus is transported in associ ation with small particles. According to bioassays the algal availabil ity of this particulate P is very low. Therefore, the eutrophying effe ct of agriculture can be estimated on the basis of dissolved nutrients . In shallow lakes, typical in agricultural regions, the release of di ssolved P from the bottom sediment often induces the development of N2 -fixing blue- green algal blooms. In some cases the internal load may exceed the external load. For the recovery of such a lake the introduc tion of better agricultural practices in the drainage basin has to be coupled with in-lake measures.