Cyanobacterial blooms, mass developments of cyanobacteria floating at the surface of waterbodies, have become a recurrent and increasingly important phenomenon in freshwaters worldwide over recent decades. The formation of such blooms in surface waters is closely linked to water eutrophication (Chorus, 2001). These nuisance blooms represent major hazards and potential risks to human and animal health, and interfere in various negative ways with the sustainable use of surface waters, for e.g. drinking water treatment, recreation, irrigation and fisheries. Between 25 and at least 70% of these blooms produce potent cyanotoxins (Sivonen, 1996).