Seasonal variability of δ15N in sinking particles in the Benguela upwelling region

Temporal changes in δ15N values of sinking particles collected with sediment traps in the Benguela upwelling regime off southwest Africa mirrored variations in the input of inorganic nitrogen to the surface water. Reductions in δ15N (to as low as 2.5‰) corresponded to low sea surface temperatures during austral spring and late austral autumn/early winter, indicating increased nitrate availability due to the presence of recently upwelled water. High particulate fluxes accompanied the low δ15N values and sea surface temperatures, reflecting increased productivity, fueled by the upwelled nutrients. High δ15N values (up to 13.1‰) coincided with high sea surface temperatures and low particle fluxes. In this area, the seaward extension of upwelling filaments, which usually occurs twice yearly, brings nutrient-rich water to the euphotic zone and leads to elevated productivity and relatively lower δ15N values of the particulate nitrogen. Satellite images of ocean chlorophyll show that productivity variations coincide with δ15N changes. The observed isotopic pattern does not appear to have been caused by variations in the species composition of the phytoplankton assemblage. Calculations based on δ15N of the sinking particulate nitrogen show that the surface nitrate pool was more depleted during late austral summer/early fall and mid-winter and that supply exceeded demand during the intense spring bloom and in late austral fall. The main uncertainty associated with these estimates is the effect of diagenesis on δ15N and possible variability in preservation of the isotope signal between periods of high and low particle flux.