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 µm and <10 µm), 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 μmol 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−1d−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 µm fraction contributed more to both gross and net nitrogen fixation than did the >10 µm fraction in most of the areas studied. The <10 µm 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.