Cyanobacterial mass-occurrences have increased since the end of the 1970s in the Baltic Sea (Suikkanen et al., 2007). The main bloom-formers are Aphanizomenon flos-aquae and Nodularia spumigena. Anabaena sp. is also present, but usually at less than 10% of total community. Anabaena can occur as the dominant cyanobacterium especially in the northern part of the Baltic Sea (Seppa¨la¨ et al., 2007). N. spumigena was considered for a long time as the only toxin-producing cyanobacterium in the Baltic Sea; however, recently Anabaena sp. has also proven to be toxic (Halinen et al., 2007). Since then tens of strains have been isolated, of which several produce microcystin (Vaitomaa et al., 2003; Halinen et al., 2008). It is, thus, worthwhile to study the ecological role of toxin production of Baltic Sea Anabaena more thoroughly.