Remote sensing of new production fuelled by nitrogen fixation

Abstract

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 μmol 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.