During three years, 1998-2000, the significance of N₂ fixation was studied in coastal and offshore waters of the northern Baltic proper. This is the first seasonal study on N₂ fixation rates in the Baltic Sea using the sensitive ¹⁵N tracer technique. Rates of N₂ fixation in the size fraction >20 µm followed the seasonal and annual fluctuations of cyanobacterial biomass, primarily Aphanizomenon. Light affected the vertical distribution of cyanobacterial biomass and rates of N₂ fixation, with highest rates at the surface. The daily pattern of N₂ fixation activities was also affected by irradiance, with peak activities around noon. Mean N₂ fixation rate at night were 37 % of the average daytime rate.

In 1999, the filamentous cyanobacteria Aphanizomenon and Nodularia were studied along a north-south transect in the Baltic proper. Spatial variations in the phosphate uptake kinetics may be due to adaptations to prevailing conditions. High concentrations of alkaline phosphatases (APases) were detected in association with P-sufficient cyanobacteria, suggesting that Aphanizomenon and Nodularia may be of high importance for the regeneration of phosphorus in the surface waters of the Baltic Sea.
In 1999 and 2000, Aphanizomenon colonies were collected in May, July & September in coastal surface waters of the northern Baltic proper. High P-incorporation rates in spring were followed by low rates in summer and autumn, thereby suggesting P-sufficiency during their biomass peak in summer. Hence, results indicated that the cyanobacteria could provide P for themselves in both open and near-shore waters. However, the size-fractionated seawater samples indicated P-deficiency in the >20 µm size fraction. This may be due to the presence of detrital material in the samples. Also cellular N contents suggested N-sufficiency throughout summer. Positive correlations between cellular Fe:C ratios and specific growth rates of the cyanobacteria, along with seasonal drops in Fe:C coinciding with the biomass peaks of the N₂-fixing cyanobacteria, indicated Fe-limited cyanobacteria. Further, a positive relationship between the Fe:C and P:C ratios suggested some kind of interrelation between dissolved P and Fe in the Baltic Sea.

Additional studies of Aphanizomenon and Nodularia under controlled laboratory conditions indicated a higher sensitivity to low ambient P levels by Aphanizomenon in comparison with Nodularia. However, high growth rates during P-replete conditions indicated that Aphanizomenon may be a stronger competitor in waters characterised by nutrient pulses, such as upwelling areas. Total annual N₂ fixation in the Himmerfjärden bay was estimated to 41, 24 & 68 tons N yr^-1 for 1998, 1999 and 2000, respectively. These results suggest that the N input from N₂ fixation does not compensate for the reduced N discharge in the nearby sewage treatment plant. Total annual N₂ fixation was estimated for the whole Baltic proper. In 1998, 1999 and 2000, total N input from N₂ fixation were 94 000, 78 000 and 27 000 tons N yr^-1, respectively. Hence, biological N₂ fixation may be the third largest source of N input to the Baltic proper, after river and land run-off and atmospheric depositions.