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

Temporal changes in delta(15)N 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 delta(15)N (to as low as 2.5parts per thousand) 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 delta(15)N values and sea surface temperatures, reflecting increased productivity, fueled by the upwelled nutrients. High delta(15)N values (up to 13.1parts per thousand) 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 delta(15)N values of the particulate nitrogen. Satellite images of ocean chlorophyll show that productivity variations coincide with delta(15)N 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 delta(15)N 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 delta(15)N and possible variability in preservation of the isotope signal between periods of high and low particle flux. (C) 2002 Elsevier Science Ltd. All rights reserved.