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

Abstract. 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. 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.