MEPS prepress abstract  -  doi: 10.3354/meps07379

ABSTRACT: Annual rates of N₂ fixation were measured over three years (1998-2000) at an open water station (BY31) and two coastal stations (H4 and X1) in the Baltic Sea. This is the first report on depth integrated rates of N₂ fixation from more than one complete growth season in the Baltic Sea. Annual estimates of N₂ fixation ranged from 56 000 to 125 000 tonnes N in the Baltic Proper, while the inshore stations (Himmerfjärden) ranged from 18 000 to 162 000 tonnes N. Rates of N₂ fixation were measured in situ at four depths between 0-25 m using the ¹⁵N tracer technique for size fractionated organisms larger (>) and smaller (<) than 20 µm. Maximum rates of N₂ fixation were found in surface waters (0-4 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 N₂ 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, suggested 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 by increased rates of N₂ fixation. The patchy appearance of cyanobacteria and the high seasonal variability in N₂ fixation rates emphasize the need for implementing spatial as well as temporal sampling strategies in studies of N₂ fixation in coastal and open waters of the Baltic Sea.