Variability in nitrogen and phosphorus limitation for Baltic Sea phytoplankton during nitrogen-fixing cyanobacterial blooms

Baltic Sea phytoplankton responses during N-2-fixing cyanobacterial blooms, dominated by Nodularia spp., Aphanizomenon sp., and Anabaena spp., were studied using nutrient enrichment experiments. Nitrogen fixation, phytoplankton growth, primary productivity, and phytoplankton species' responses were investigated in areas representing open Baltic Sea water and Gulf of Finland water. Responses to additions of N, P, Fe, and the organic chelator EDTA were studied. Phytoplankton biomass and primary productivity were N, P, or N + P-limited. Phosphorus limitation was more prominent at the Gulf of Finland sites, where the dissolved inorganic-nitrogen to phosphorus (DIN:DIP) ratio was > 16:1 during the study period. At the open sea sites, with DIN:DIP ratio < 16: 1, N limitation or N + P co-limitation prevailed. Phosphorus consistently stimulated N-2 fixation, and at times N-2 fixation and total N had a linear relationship with the initial PO43- concentration added. At other times, the relationship was not clear, suggesting other limitations or controls for N-2 fixation. Occasionally, EDTA increased N-2 fixation, but Fe addition did not show a significant impact on phytoplankton biomass, productivity, or N-2 fixation. The chlorophyte Monoraphidium sp., diatom Nitzschia spp., and small flagellates (7.5 to 10 mum) were among the species that exhibited the fastest growth responses under replenishment of the limiting nutrient. The results suggest that high N availability, in relation to P, in the coastal areas in the Gulf of Finland acts to reduce the relative abundance of N-2-fixing organisms. Phosphorus inputs into the upper-mixed layer in the open sea during bloom periods stimulate N-2 fixation, and act to maintain the cyanobacterial blooms.