Particulate new nitrogen fluxes in the Sargasso Sea

Seasonal variations of up to sevenfold in the flux of new nitrogen as sinking particles are observed at a site near Bermuda between January 1986 and May 1988. Maxima occur in the late winter with minima in the late fall. While this particle flux from the euphotic zone depends on levels of suspended particulate nitrogen (PN), NO3− flux modulated by seasonal variations in the magnitude of vertical transport appears to be the ultimate forcing. However, wintertime increases in euphotic zone NO3− concentration are not required for the high particle fluxes observed for this season. The gradient in NO3− concentration at 100 m is inversely correlated with particle flux resulting in the annual amplitude in new nitrogen flux to be well below the amplitude in vertical transport parameterized as a vertical eddy diffusivity. Yearly flux of sinking PN is 0.2 mol m−2 yr−1, one third the highest estimate reported for new nitrogen flux in this region. The downward mixing of suspended PN may make a significant contribution to the downward flux, particularly in the winter. Its inclusion raises the annual new nitrogen flux to 0.33 mol m−2 yr−1. This estimate is most likely conservative since sporadic events may not have been adequately sampled and the downward flux of nonparticulate forms of nitrogen has not been taken into account.