The Baltic Sea is one of the largest brackish water areas on Earth and also one of the few brackish water areas where diazotrophic cyanobacteria are an important component of the phytoplankton. Diazotrophs are common in the southern part, the Baltic Sea proper, where winter surface layer inorganic N/P ratios are well below the Redfield ratio, suggesting predominant N-limitation of phytoplankton growth. Nitrogen fixation is here considered one of the largest sources of new nitrogen, after land run-off but similar to atmospheric deposition.

In Baltic Sea proper coastal areas, nutrient discharges from sewage treatment plants with efficient P-removal have high N/P loading ratios that bring about P-limitation of phytoplankton growth in the discharge area. As a result, diazotrophic cyanobacteria are virtually absent in the phytoplankton.

Here we report results from a study in Himmerfjärden Bay, a northern Baltic proper coastal area, where nitrogen discharges from the Himmerfjärden STP were recently reduced by more than 80% by the introduction of a fluidised bed technique. From 20 years of previous results, we predicted that conditions favouring diazotrophs would result if nitrogen discharges were reduced to one third or less. In practice an increased biomass of the heterocystous filamentous cyanobacterium Aphanizomenon sp. resulted. The toxic species Nodularia spumigena known to form extensive surface accumulations in the coastal area outside and in the offshore Baltic Sea proper, has so far been virtually absent.

Our results are being used to tailor the management of nutrient discharges from the Himmerfjärden STP so as to avoid toxic cyanobacterial blooms, and will gradually be used also to adjust nitrogen reduction to avoid inducing nitrogen fixation in summer.