Latitudinal distribution of <i>Trichodesmium</i> spp. and N₂ fixation in the Atlantic Ocean

• Main Authors: E. Marañón, M. Varela, A. Bode, B. Mouriño-Carballido, A. Fernández
• Format: Article
• Language: English
• Published: Copernicus Publications 2010-10-01
• Series: Biogeosciences
• Online Access: http://www.biogeosciences.net/7/3167/2010/bg-7-3167-2010.pdf

We have determined the latitudinal distribution of <i>Trichodesmium</i> 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 <i>Trichodesmium</i> abundance and community N₂ fixation. The highest <i>Trichodesmium</i> abundances (mean = 220 trichomes L^&minus;1,) and community N₂ fixation rates (mean = 60 μmol m^&minus;2 d^&minus;1) occurred in the Equatorial region between 5° S–15° N. In the South Atlantic gyre, <i>Trichodesmium</i> abundance was very low (ca. 1 trichome L^&minus;1) but N₂ fixation was always measurable, averaging 3 and 10 μmol m² d^&minus;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 <i>Trichodesmium</i> 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 &sim;6 TgN yr^&minus;1 in the North Atlantic (0–40° N) and ~1.2 TgN yr^&minus;1 in the South Atlantic (0–40° S).