Climatological satellite observations in the tropical North Atlantic generally show a wintertime surface chlorophyll-a (Chl-a) maximum except over a broad region in the western North Atlantic that has a summer Chl-a maximum. This region also shows decoupling between Chl-a and vertical nutrient flux, based on the positive relationship between sea surface height anomaly (SSH), sea surface temperature, and Chl-a. An analogous summer Chl-a maximum is simulated in a model including a dynamic representation of Trichodesmium and N2-fixation, but not in runs without. These results suggest that the growth is fuelled by N2-fixation. Using the observed summertime increase in Chl-a and the model efficiency for N2-fixation transfer to phytoplankton biomass, we calculate a nitrogen fixation rate of 220 ¿mol N m-2day-1 in this region. This constitutes the first satellite observation of the effect of nitrogen fixation on Chl-a, and may ultimately provide a means of deriving new global N2-fixation estimates.