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

Abstract

We have determined planktonic community N2 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 N2 fixation was always measurable, albeit low compared with other regions of the tropical Atlantic. The average, vertically-integrated N2 fixation rate was 1.20 ± 0.48mol N m-2 d-1 in autumn 2007 and 8.31 ± 3.31 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 N2 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 mol P m-2 d-1 and 2.8 ± 2.6mol 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. © 2012 International Council for the Exploration of the Sea.