Phytoplankton growth is frequently limited by the supply of nitrogen (N), phosphorus (P), or sometimes other elements. N₂-fixing cyanobacteria are favoured by N limitation, which they may alleviate by adding new N to the ecosystem. Focussing on the Baltic Sea, this thesis deals with the regulating mechanisms for such cyanobacteria, the magnitude of the N₂ fixation, and the effect of the newNon the pelagic ecosystem. This is done mainly through fieldstudies of the seasonal variation in nutrient elemental composition of the plankton.

The C:N:P stoichiometry of zooplankton was found to differ only slightly from that of seston, implying that zooplankton recycle a relatively balanced N:P ratio compared to general phytoplankton demand. Thus, from this aspect, zooplankton should have a small effect on the development of cyanobacterial blooms in the Baltic Sea.

In the Baltic proper, there was a recurrent summer increase in upper mixed layer total N, which coincided with the biomass build-up of heterocystous cyanobacteria. N₂ fixation in the Baltic proper estimated from this increase ranged between 180-430 thousand tons N yr^-1, several times higher than commonly assumed. The upper end estimate is almost as large as the entire riverine load and twice the atmospheric load of N. The N₂ fixation is sufficient to sustain 30-90% of the summer pelagic net community production, and thus important for the nourishment of pelagic zooplankton and fish.

Seston element ratios were generally close to Redfield values (C₁₀₆N₁₆P₁) in the upper 0-20 m. Consistent withNlimitation of the spring bloom, the C:N ratio increased (to ~9) when nitrate, but not phosphate, was depleted. From June, surface (5 m) seston C:P and N:P ratios increased through summer and peaked at the biomass maximum of N₂-fixing cyanobacteria. This suggests that new nitrogen from N₂ fixation and competition for P resulted in P deficiency, albeit weak, also in non-N₂-fixers.

The P content of was very high in spring and early summer (~3% of dwt, C:P=30-40, N:P=6-7) and decreased as the biomass increased in summer. The storage of P in biomass in early summer was potentially important for the later bloom formation. The culmination of the C:P and N:P ratios at the biomass peak (C:P=380-480, N:P=60-80, P=0.25-0.3% of dwt) strongly indicate biomass P limitation. The iron (Fe) content of was relatively low compared to reported values for other N₂-fixing cyanobacteria but did not decrease in summer, thus not indicating Fe limitation. Also had low P content in summer and higher Fe content, indicating P-deficiency. In , the specific C uptake rate, measured in short-term (3 hrs) incubations, was mainly correlated to temperature and not to P content. This is however compatible with the large total-N-increase, which suggests substantial N-leakage from growing cyanobacteria.

In conclusion, this thesis shows the importance of N₂-fixing cyanobacteria in the Baltic Sea and indicates that P has a critical role as controlling factor. Despite the large annual N₂ fixation, biogeochemical nutrient cycling favour N limitation in the Baltic proper. This thesis demonstrates how the stoichiometric approach can be used for reconciling element cycles with population dynamics of plankton in aquatic ecosystems.