A. Fernández, B. Mouriño-Carballido, A. Bode, M. Varela, and E. Marañón

Biogeosciences, 7, 3167–3176, 2010. www.biogeosciences.net/7/3167/2010/. doi:10.5194/bg-7-3167-2010
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 be- tween Trichodesmium abundance and community N2 fixa- tion. The highest Trichodesmium abundances (mean = 220 trichomes L?1) and community N2 fixation rates (mean = 60 ?mol m?2 d?1) occurred in the Equatorial region between 5? S–15? N. In the South Atlantic gyre, Trichodesmium abun- dance was very low (ca. 1 trichome L?1) but N2 fixation was always measurable, averaging 3 and 10 ?mol m2 d?1 in 2007 and 2008, respectively. We suggest that N2 fixation in the South Atlantic was sustained by other, presumably unicel- lular, diazotrophs. Comparing these distributions with the geographical pattern in atmospheric dust deposition points to iron supply as the main factor determining the large scale lat- itudinal 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 control- ling the large scale distribution of N2 fixation. Scaling up from all our measurements (42 stations) results in conser- vative 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).

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