AME 65:43-53 (2011)  -  DOI: https://doi.org/10.3354/ame01534

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.

KEY WORDS: Nitrogen fixation · Trichodesmium · Upwelling · Subtropical northeast Atlantic · Canary Current Large Marine Ecosystem