Latitudinal distribution of Trichodesmium spp. and N₂ fixation in the Atlantic Ocean

Abstract. We have determined the latitudinal distribution of Trichodesmium spp. abundance and community N₂ fixation in the Atlantic Ocean along a meridional transect from ca. 30° N to 30° S in November–December 2007 and April–May 2008. The observations from both cruises were highly consistent in terms of absolute magnitude and latitudinal distribution, showing a strong association between Trichodesmium abundance and community N₂ fixation. The highest Trichodesmium abundances (mean = 220 trichomes L^−1,) and community N₂ fixation rates (mean = 60 μmol m⁻² d⁻¹) occurred in the Equatorial region between 5° S–15° N. In the South Atlantic gyre, Trichodesmium abundance was very low (ca. 1 trichome L⁻¹) but N₂ fixation was always measurable, averaging 3 and 10 μmol m² d⁻¹ in 2007 and 2008, respectively. We suggest that N₂ fixation in the South Atlantic was sustained by other, presumably unicellular, diazotrophs. Comparing these distributions with the geographical pattern in atmospheric dust deposition points to iron supply as the main factor determining the large scale latitudinal variability of Trichodesmium spp. abundance and N₂ fixation in the Atlantic Ocean. We observed a marked South to North decrease in surface phosphate concentration, which argues against a role for phosphorus availability in controlling the large scale distribution of N₂ fixation. Scaling up from all our measurements (42 stations) results in conservative estimates for total N₂ fixation of ∼6 TgN yr⁻¹ in the North Atlantic (0–40° N) and ~1.2 TgN yr⁻¹ in the South Atlantic (0–40° S).