The significance of nitrogen fixation to new production during early summer in the Baltic Sea
2007 (English)In: Biogeosciences, ISSN 1726-4170, Vol. 4, no 1, 63-73 p.Article in journal (Refereed) Published
Rates of dinitrogen (N-2) 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 N-2 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 N-2 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 N-2 fixation was contributed by filamentous cyanobacteria. On average, cyanobacterial cells > 20 mu m were able to supply a major part of their N requirements for growth by N-2 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: NRATE 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 N-2 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 N-2 fixation was found in cells < 5 mu m. Since cells < 5 mu m did not show any detectable rates of N-2 fixation, the N-15-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 N-2 fixation found during the 1999 transect and a higher level of N-15-enrichment in cells < 5 mu m. A variable but significant fraction of total N-2 fixation (1-10%) could be attributed to diazotrophy in cells between 5-20 mu m.
Place, publisher, year, edition, pages
2007. Vol. 4, no 1, 63-73 p.
dissolved organic nitrogen, n-2 fixation, diazotrophic cyanobacteria, vertical-distribution, trichodesmium ims101, pacific-ocean, bloom, buoyancy, dynamics, transformations
IdentifiersURN: urn:nbn:se:su:diva-57117ISI: 000245075200006OAI: oai:DiVA.org:su-57117DiVA: diva2:415973
authorCount :62011-05-102011-05-032011-05-10Bibliographically approved