MEPS 360:73-84 (2008)  -  DOI:

Seasonal significance of N2 fixation in coastal and offshore waters of the northwestern Baltic Sea

Jenny Degerholm1,2,*, Kjell Gundersen1,3, Birgitta Bergman1, Erik Söderbäck1,4

1Department of Botany, Stockholm University, 106 91 Stockholm, Sweden
2Present address: Faculty of Natural Sciences, Södertörn University College, 141 89 Huddinge, Sweden
3Present address: Department of Marine Science, University of Southern Mississippi, 1020 Balch Boulevard, Stennis Space Center, Mississippi 39529, USA
4Present address: QIAGEN, Frösundaviks Allé 15, 169 70 Solna, Sweden

ABSTRACT: Annual rates of N2 fixation were measured over 3 yr (1998–2000) at an open water station (BY31) and 2 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 56000 to 125000 t N in the Baltic Proper, and 18000 to 162000 t N at the inshore stations (Himmerfjärden). Rates of N2 fixation were measured in situ at 4 depths between 0 and 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 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 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, 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 N2 fixation. The patchy distribution of cyanobacteria and the high seasonal variability in N2 fixation rates emphasize the need for adequate spatial and temporal sampling strategies in studies of N2 fixation in coastal and open waters of the Baltic Sea.

KEY WORDS: Baltic Sea · Aphanizomenon · Nodularia · N2 fixation · Himmerfjärden

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Cite this article as: Degerholm J, Gundersen K, Bergman B, Söderbäck E (2008) Seasonal significance of N2 fixation in coastal and offshore waters of the northwestern Baltic Sea. Mar Ecol Prog Ser 360:73-84.

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