ABSTRACT: Two strains of the filamentous N₂-fixing cyanobacterium Nodularia spumigena were inoculated separately in Baltic Sea water, and their growth, carbon (C)-fixation, and N₂-fixation rates were monitored during a 21 d laboratory experiment. Low amounts of P (1 µM final concentration) were added to otherwise un-amended Baltic Sea water (<0.5 µM P). Exponential growth was stimulated under P-enriched conditions, indicating that the natural Baltic Sea water contained all residual nutrients essential for growth, and that N. spumigena was limited by P. The molar ratio of C to N₂ fixation was >15 when ammonium was present at concentrations >5 µM on Day 0. This ratio was significantly different between the 2 strains, which indicated variable affinity for ammonium. It decreased in both strains as the ammonium concentration and C assimilation decreased and N₂ fixation increased during the experiment. After 7 d, C- and N₂-fixation rates co-varied with a relatively stable C:N fixation ratio close to or below the C:N ratio of cells (range: 4.7 to 8.6). The C:N cellular ratio and the specific N₂ fixation varied significantly between strains, emphasizing the variability of eco-physiology between strains within the same species. The average growth rate during the experiment, the accumulated biomass, and the total N₂-fixation rate were significantly higher under P-enriched compared to P-limited conditions in both strains. Hence, summers with high influx of P may stimulate growth, CO₂ sequestration, and N₂ fixation by N. spumigena in the Baltic Sea.