Biogeosciences www.biogeosciences.net 1726-4170 1726-4189 4 1 2007 10.5194/bg-4-63-2007 http://www.biogeosciences.net/4/63/2007/ http://www.biogeosciences.net/4/63/2007/bg-4-63-2007.html http://www.biogeosciences.net/4/63/2007/bg-4-63-2007.pdf 63 73 2007-01-18 The significance of nitrogen fixation to new production during early summer in the Baltic Sea U. Ohlendieck ute.ohlendieck@t-online.de K. Gundersen M. Meyerhöfer P. Fritsche K. Nachtigall B. Bergmann Leibniz Institut für Meereswissenschaften, Düsternbrooker Weg 20, 24105 Kiel, Germany Department of Botany, Stockholm University, Lilla Frescativägen 5, 10691 Stockholm, Sweden now at: Department of Marine Sience, University of Southern Mississippi, 1020 Balch Blvd., Stennis Space Center, MS 39529, USA Rates of dinitrogen (N₂) fixation and primary production were measured during two 9 day transect cruises in the Baltic proper in June–July of 1998 and 1999. Assuming that the early phase of the bloom of cyanobacteria lasted a month, total rates of N₂ fixation contributed 15 mmol N m^−2 (1998) and 33 mmol N m^−2 (1999) to new production (sensu Dugdale and Goering, 1967). This constitutes 12–26% more new N than other annual estimates (mid July–mid October) from the same region. The between-station variability observed in both total N₂ fixation and primary productivity greatly emphasizes the need for multiple stations and seasonal sampling strategies in biogeochemical studies of the Baltic Sea. The majority of new N from N₂ fixation was contributed by filamentous cyanobacteria. On average, cyanobacterial cells >20 µm were able to supply a major part of their N requirements for growth by N₂ fixation in both 1998 (73%) and 1999 (81%). The between-station variability was high however, and ranged from 28–150% of N needed to meet the rate of C incorporation by primary production. The molar C:N rate incorporation ratio (C:N_RATE) in filamentous cyanobacterial cells was variable (range 7–28) and the average almost twice as high as the Redfield ratio (6.6) in both years. Since the molar C:N mass ratio (C:N_MASS) in filamentous cyanobacterial cells was generally lower than C:N_RATE at a number of stations, we suggest that the diazotrophs incorporated excess C on a short term basis (carbohydrate ballasting and buoyancy regulation), released nitrogen or utilized other regenerated sources of N nutrients. Measured rates of total N₂ fixation contributed only a minor fraction of 13% (range 4–24) in 1998 and 18% (range 2–45) in 1999 to the amount of N needed for the community primary production. An average of 9 and 15% of total N₂ fixation was found in cells <5 µm. 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