The experiment was repeated at two sites, at Tva¨ rminne in the western Gulf of Finland, and Reposaari in the Bothnian Sea (Fig. 1). The sites differ in salinity, eutrophication level and cyanobacteria bloom frequency. Tva¨ rminne is more saline 5.3– 6.1%, than Reposaari that is almost fresh 1.1–2%, due to the large influence by River Kokema¨kijoki. The sites also differ in level of eutrophication. The Gulf of Finland has a bad status concerning eutrophication, whereas Bothnian Sea has a poor status (HELCOM, 2009). Cyanobacteria blooms have increased significantly in the Gulf of Finland since the end of 1970s (Suikkanen et al., 2007).