Community N2 fixation and Trichodesmium spp. abundance along longitudinal gradients in the eastern subtropical North Atlantic
ICES Journal of Marine Science
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 21 ) during the spring 2008 cruise, when low sea surface temperatures were recorded and vertical
... rtical 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 mmol N m 22 d 21 in autumn 2007 and 8.31 + 3.31 mmol N m 22 d 21 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 mmol P m 22 d 21 and 2.8 + 2.6 mmol P m 22 d 21 , 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.