Impacts of Agricultural Nutrient Loading on Finnish Watercourses

Abstract

Eutrophication of lakes, rivers and coastal waters is the main environmental problem caused by agriculture in Finland. Water quality of most Finnish watercourses is good or excellent, but in the intensively cultivated 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 association with small particles. According to bioassays the algal availability of this paniculate P is very low. Therefore, the eutrophying effect of agriculture can be estimated on the basis of dissolved nutrients. In shallow lakes, typical in agricultural regions, the release of dissolved 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 introduction of better agricultural practices in the drainage basin has to be coupled with in-lake measures.