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

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 Tricho -
desmium (<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.