Search

Nitrogen (N2) fixation is an important process that fuels export production in the North Pacific Ocean, as evidenced by seasonally low δ15N of sinking organic nitrogen (N) at the Hawaii Ocean Time series station. However, relatively few direct measurements of N2 fixation exist across the North Pacific. On two cruises there in fall 2002 and summer 2003, the abundance and N2 fixation rate of Trichodesmium spp. and Richelia, as well as bulk water samples, were measured. Trichodesmium spp. were only detected in the area near the Hawaiian Islands, in similar densities on both cruises. Despite similar densities, the areal N2 fixation rate of Trichodesmium spp. in fall 2002 was nearly four times greater than in summer 2003 at stations proximal to the Hawaiian Islands. In the central North Pacific Gyre far from the Hawaiian Islands, where Trichodesmium spp. was not present, whole water N2 fixation rates were relatively high (∼100 μmol N m−2 d−1). Presumably unicellular diazotrophs were responsible for activity there. Our studies show a geographical variation in the dominant diazotroph in the North Pacific Subtropical Gyre in the summer with Trichodesmium being dominant around the Hawaiian Islands, Richelia associated with diatoms to be found in high numbers to the south of the islands while unicellular diazotrophs dominated to the west, away from the islands and evidence from the literature suggests iron may play a role.

The broad distribution and often high densities of the cyanobacterium Trichodesmium spp. in oligotrophic waters imply a substantial role for this one taxon in the oceanic N cycle of the marine tropics and subtropics. New results from 154 stations on six research cruises in the North Atlantic Ocean show depth-integrated N2 fixation by Trichodesmium spp. at many stations that equalled or exceeded the estimated vertical flux of NO3− into the euphotic zone by diapycnal mixing. Areal rates are consistent with those derived from several indirect geochemical analyses. Direct measurements of N2 fixation rates by Trichodesmium are also congruent with upper water column N budgets derived from parallel determinations of stable isotope distributions, clearly showing that N2 fixation by Trichodesmium is a major source of new nitrogen in the tropical North Atlantic. We project a conservative estimate of the annual input of new N into the tropical North Atlantic of at least 1.6 × 1012 mol N by Trichodesmium N2 fixation alone. This input can account for a substantial fraction of the N2 fixation in the North Atlantic inferred by several of the geochemical approaches.

We encountered an extensive bloom of the colonial diatom Hemiaulus hauckii along a 2500 km cruise track off the NE coast of South America in autumn 1996. Each diatom cell contained the heterocystous. N₂-fixing cyanobacterial endosymbiont Richeiia intracellularis. Surface Richeiia heterocyst (and filament) densities increased from 10⁶ heterocyst 1⁻¹ in the bloom. Total abundance ranged from 10⁶ eterocyst m⁻² outside the bloom to over 10¹⁰ heterocyst m⁻² within the bloom. Rates of primary production averaged 1.2 g C m⁻² d⁻¹, higher than typical for oligotrophic Open ocean waters. N₂ fixation during the bloom by the Richelia/Hemiaulus association added an average of 45 mg N m⁻² d⁻¹ to the water column. The relative importance of NH₄⁺ uptake over the Course of the bloom increased from 0 to 42% of total N uptake by the Hemiauluslficheiia association. N₂ fixation by Richelia exceeded estimates of 'new' N flux via NO₃ diffusion from deep water and, together with additional N, fixation by the cyanobacterium Trichodesmium, could supply about 25% of the total N demand through the water column during the bloom. Suspended particles and zooplankton collected within the bloom were depleted in ¹⁵N, reflecting the dominant contribution of N₂ fixation to the planktonic N budget. The bloom was spatially extensive, as revealed by satellite imagery, and is calculated to have contributed about 0.5 Tg N to the euphotic zone. Such blooms may represent an important and previously unrecognized source of new N to support primary production in nutrient-poor tropical waters. Furthermore, this bloom demonstrates that heterocystous cyanobacteria can also make quantitatively important contributions of N in oceanic water column environments.