Rates of carbon (C) specific growth and nitrogen (N2) fixation were monitored in cultures of Baltic Sea Nodularia and Aphanizomenon exposed to gradual limitation by inorganic phosphorus (P). Both cyanobacteria responded by decreased cellular P content followed by lowered rates of growth and N2 fixation. C-specific growth and cellular N content changed faster in Aphanizomenon both when inorganic P was lowered as well as during reintroduction of P. Aphanizomenon also showed a more rapid increase in N-specific N2 fixation associated with increased C-specific growth. When ambient concentrations of inorganic P declined, both cyanobacteria displayed higher rates of alkaline phosphatase (APase) activity. Lower substrate half-saturation constants (KM) and higher Vmax : KM ratio of the APase enzyme associated with Nodularia suggest a higher affinity for dissolved organic P (DOP) substrate than Aphanizomenon. Aphanizomenon, which appears more sensitive to changes in ambient dissolved inorganic P, may be adapted to environments with elevated concentrations of P or repeated intrusions of nutrient-rich water. Nodularia on the other hand, with its higher tolerance to increased P starvation may have an ecological advantage in stratified surface waters of the Baltic Sea during periods of low P availability.

Annual rates of N 2 fixation were measured over 3 yr (1998)(1999)(2000) at an open water station (BY31) and 2 coastal stations (H4 and X1) in the Baltic Sea. This is the first report on depthintegrated rates of N 2 fixation from more than one complete growth season in the Baltic Sea. Annual estimates of N 2 fixation ranged from 56 000 to 125 000 t N in the Baltic Proper, and 18 000 to 162 000 t N at the inshore stations (Himmerfjärden). Rates of N 2 fixation were measured in situ at 4 depths between 0 and 25 m using the 15 N tracer technique for size fractionated organisms larger and smaller than 20 μm. Maximum rates of N 2 fixation were found in surface waters (0 to 4 m depth), and a major part of this activity (80% in coastal and 89% in offshore waters) took place during daylight hours. Integrated rates of N 2 fixation in cells > 20 μm followed the average abundance of filamentous cyanobacteria (primarily Aphanizomenon sp.) in the water column. Molar C:N mass ratios in particles > 20 μm, i.e. filamentous cyanobacteria, suggest that this size fraction was N-sufficient during summer, whereas the molar C:P mass ratios indicated P-limitation during this period. A reduction in sewage discharge to the Himmerfjärden bay area during the study period appears not to have been compensated for by increased rates of N 2 fixation. The patchy distribution of cyanobacteria and the high seasonal variability in N 2 fixation rates emphasize the need for adequate spatial and temporal sampling strategies in studies of N 2 fixation in coastal and open waters of the Baltic Sea.KEY WORDS: Baltic Sea · Aphanizomenon · Nodularia · N 2 fixation · HimmerfjärdenResale or republication not permitted without written consent of the publisher