BOREAL ENVIRONMENT RESEARCH 6: ¶–¶ ISSN 1239-6095 Helsinki ¶¶ © 2001 Evaluating the effects of nutrient load reductions on the biomass of toxic nitrogen- fixing cyanobacteria in the Gulf of Finland, Baltic Sea Mikko Kiirikki 1) , Arto Inkala 2) , Harri Kuosa 3) , Heikki Pitkänen 1) , Minna Kuusisto 1) and Juha Sarkkula 1) 1) Finnish Environment Institute, P.O. Box 140, FIN-00251 Helsinki, Finland 2) Environmental Impact Assessment Centre of Finland Ltd. (EIA Ltd.), Tekniikantie 21 B, FIN-02150 Espoo, Finland 3) Finnish Institute of Marine Research, P.O. Box 33, FIN-00931 Helsinki, Finland Kiirikki, M., Inkala, A., Kuosa, H., Pitkänen, H., Kuusisto, M. & Sarkkula, J. 2001. Evaluating the effects of nutrient load reductions on the biomass of toxic nitrogen-fixing cyanobacteria in the Gulf of Finland, Baltic Sea. Boreal Env. Res. 6: ¶–¶. ISSN 1239-6095 The effects of nutrient load reductions on the biomass of N-fixing cyanobacteria were evaluated in the scale of the Gulf of Finland. The two analysed reduction scenarios were Finnish national agenda and the improvement of phosphorus removal in the present purification plants of St. Petersburg. The effects of load reduction scenarios were tested by using a 3D-ecosystem model with a horizontal resolution of 5 km. According to the results the Finnish national agenda cannot decrease the biomass of N-fixing cyanobacteria, but it seems to be able to reduce the total phytoplankton biomass in the coastal waters. The phosphorus reduction in St. Petersburg decreases the N-fixing cyanobacteria significantly in the central parts of the Gulf of Finland. Improvement of the phosphorus purification efficiency in the present sewage treat- ment plants offers us an opportunity to try to control the intensity of the toxic blooms of nitrogen-fixing cyanobacteria in the scale of the Gulf of Finland. Introduction During the first week of July 1997, the Gulf of Finland (GOF) became covered by floating ac- cumulations of cyanobacteria. Two main genera responsible for the bloom were Nodularia and Aphanizomenon, which both are capable of fixing atmospheric nitrogen. Nodularia-species