Diazotrophic activity of Trichodesmium and unicellular cyanobacteria in the subtropical NE Atlantic

Abstract

Fractionated (> and <10 μm) diazotrophic activity and Trichodesmium abundance were studied in a macroscale cruise enclosing the boundary current system of the NE Atlantic and during two mesoscale experiments in the upwelling systems of Cape Silleiro (NW Iberia) and Cape Ghir (NW Africa). The abundance and N2 fixation rates of Trichodesmium were low (<0.5 filaments/l and <0.2 μmol N m-2 d-1) and only accumulated at frontal sites, such as the Azores Front or the edge of Cape Ghir’s filament. Mean N2 fixation rates in the macroscale phase were 0.19 nmol N l-1 d-1, comparable to those obtained off Cape Silleiro (0.09 nmol N l-1 d-1) and off Cape Ghir (0.1 nmol N l-1 d-1). Unicellular cyanobacteria provided ∼60% and ∼80% of total N2 fixation in the macroscale and mesoscale cruise phases, respectively. The predominance of <10 μm diazotrophic activity suggests a wider distribution and activity of these organisms than previously thought.