Nitrogen fixation by Trichodesmium and small diazotrophs in the subtropical northeast Atlantic

Abstract

We measured gross and net nitrogen fixation in fractionated samples (organisms > 10 mu m and < 10 mu m), and the density of Trichodesmium, during a cruise along the northeast Atlantic boundary current system and during 2 mesoscale experiments in the upwelling systems of Cape Silleiro (northwest Iberia) and Cape Ghir (northwest Africa). The density of Trichodesmium (< 0.5 trichomes l(-1)) and its associated rates of nitrogen fixation (< 0.1 mu mol N m(-2) d(-1)) were low. Trichomes appeared to accumulate at frontal sites-such as upwelling filaments and the Azores Front. Gross and net rates of nitrogen fixation were always < 0.4 nmol N l(-1)d(-1) except off the northwest African coast where a gross nitrogen fixation peak of 0.98 nmol N l(-1) d(-1) was measured. The < 10 mu m fraction contributed more to both gross and net nitrogen fixation than did the > 10 mu m fraction in most of the areas studied. The < 10 mu m fraction was responsible for 70 to 92% of the total nitrogen fixation in cold nutrient-rich areas. The contribution of small diazotrophs to nitrogen fixation in the upwelling sites suggests that the distribution and activity of these organisms are more widespread than previously thought.