Community N2 fixation and Trichodesmium spp. abundance along longitudinal gradients in the eastern subtropical North Atlantic

We have determined planktonic community N-2 fixation, Trichodesmium abundance, the concentration and vertical diffusive flux of phosphate, and satellite-derived estimates of atmospheric concentration of dust along two longitudinal transects in the eastern subtropical North Atlantic during November 2007 and from April-May 2008. Trichodesmium abundance was particularly low (<3 trichome l(-1)) during the spring 2008 cruise, when low sea surface temperatures were recorded and vertical stratification was less marked. However, community N-2 fixation was always measurable, albeit low compared with other regions of the tropical Atlantic. The average, vertically-integrated N-2 fixation rate was 1.20 +/- 0.48 mu mol N m(-2) d(-1) in autumn 2007 and 8.31 +/- 3.31 mu mol N m(-2) d(-1) in spring 2008. The comparison of these rates of diazotrophy with the observed Trichodesmium abundances suggests that other, presumably unicellular, diazotrophs must have contributed significantly to community N-2 fixation, at least during the spring 2008 cruise. Satellite data of atmospheric dust concentration suggested similar rates of atmospheric deposition during the two surveys. In contrast, vertical diffusive fluxes of phosphate were 5-fold higher in spring than in autumn (14.2 +/- 12.1 mu mol P m(-2) d(-1) and 2.8 +/- 2.6 mu mol P m(-2) d(-1), respectively), which may have stimulated N-2 fixation. These findings agree with the growing view that N-2 fixation is a more widespread process than the distribution of Trichodesmium alone may suggest. Our data also suggest a role for phosphorus supply in controlling the local variability of diazotrophic activity in a region subject to relatively high atmospheric inputs of iron.