The significance of nitrogen fixation to new production during early summer in the Baltic Sea

U. Ohlendieck (1) , K. Gundersen (2) , M. Meyerhöfer (1) , P. Fritsche (1) , K. Nachtigall (1) , B. Bergmann (2)

1 IFM-GEOMAR - Leibniz-Institut für Meereswissenschaften
2 Department of Botany

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 emphasize the need for multiple stations and seasonal sampling strategies in biochemical 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. Since the molar C:N rate incorporation ratio (C:N_RATE) in filamentous cyanobacterial cells was almost twice as high as the molar C:N mass ratio (C:N_MASS) in both years, we suggest that the diazotrophs incorporated excess C on a short term basis (for 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. Since cells <5 µm did not show any detectable rates of N₂ fixation, the ¹⁵N-enrichment could be attributed to regenerated incorporation of dissolved organic N (DON) and ammonium generated from larger diazotroph cyanobacteria. Therefore, N excretion from filamentous cyanobacteria may significantly contribute to the pool of regenerated nutrients used by the non-diazotroph community in summer. Higher average concentrations of regenerated N (ammonium) coincided with higher rates of N₂ fixation found during the 1999 transect and a higher level of ¹⁵N-enrichment in cells <5 µm. A variable but significant fraction of total N₂ fixation (1?10%) could be attributed to diazotrophy in cells between 5?20 µm.

Domaines: Cosmologie et astrophysique extra-galactique [astro-ph.CO]; Astrophysique [astro-ph]; Interfaces continentales, environnement; Océan, Atmosphère; Sciences de la Terre

U. Ohlendieck, K. Gundersen, M. Meyerhöfer, P. Fritsche, K. Nachtigall, et al.. The significance of nitrogen fixation to new production during early summer in the Baltic Sea. Biogeosciences Discussions, 2006, 3 (4), pp.1279-1311. ⟨hal-00297841⟩