KRAFTIGA BLOMNINGAR AV CYANOBAKTERIER OCH SAMBAND MED NÄRINGSÄMNEN I NÅGRA AV VÄNERNS GRUNDA VIKAR / Heavy blooms of cyanobacteria in Lake Vänern

Lake Vänern is an oligotrophic lake today, but in the northeast part of the lake there are three shallow bays (Ölmeviken, Varnumsviken and Kolstrandsviken) where some heavy blooms with cyanobacteria, occurred during the last ten years. In this article, the blooms and relations to the water quality and the phosphorous loads are reported and discussed. I also give a suggestion to a more flexible strategy for legal decisions regarding phosphorus and nitrogen limitation of outlets to Lake Vänern, which take the role of the trophic state of the local water body into consideration. For eutrophic waters, as the shallow bays above, a nitrogen reduction following a phosphorus reduction must be the main role. General high nitrogen reduction of outlets is a strategy more suitable for oligotrophic water bodies in Lake Vänern