Nitrogen fixation by Trichodesmium spp. and unicellular diazotrophs in the North Pacific Subtropical Gyre

Nitrogen (N-2) fixation is an important process that fuels export production in the North Pacific Ocean, as evidenced by seasonally low delta N-15 of sinking organic nitrogen (N) at the Hawaii Ocean Time series station. However, relatively few direct measurements of N-2 fixation exist across the North Pacific. On two cruises there in fall 2002 and summer 2003, the abundance and N-2 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 N-2 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 N-2 fixation rates were relatively high (similar to 100 mu mol 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.