Spatial distribution of δ15N of chlorophyll-a in surface sediment of the northern Benguela Upwelling System

We analyzed the δ¹⁵N of chlorophyll-a (Chl-a) in surface sediments of the northern Benguela Upwelling System (BUS), a coastal upwelling system with high primary productivity in surface waters and intensive denitrification in the subsurface oxygen minimum zone (OMZ). Upwelled nitrate is the major N-input to the northern BUS, and is consumed by denitrification and phytoplankton assimilation. Both processes gradually enrich the residual nitrate pool in ¹⁵N that is assimilated to synthesize chlorophyll-a (Chl-a). The δ¹⁵N_Chl-a distribution in surficial sediment should thus characterize the combined effects of denitrification and biological assimilation on the δ¹⁵N of nitrate in the overlying shelf waters, and should image the spatial patterns of denitrification and nitrate assimilation, respectively, in the northern BUS.

Paired δ¹⁵N_Chl-a and bulk sediment δ¹⁵N (δ¹⁵N_sed) data of 27 surface sediment samples lack a significant correlation (R² 0.19, P=0.012). This insignificant correlation and the spatial discrepancies between δ¹⁵N_sed and δ¹⁵N_Chl-a suggest a varying post-depositional bias of diagenetic processes on the δ¹⁵N_sed. The onshore-offshore difference (∆) of δ¹⁵N_Chl-a in the northern sector of the studied area (17°-19°S,11°–14°E) is created mainly by fractionation effects of progressive plankton assimilation, because here the OMZ is weak and water-column denitrification is insignificant. In the southern sector (19°-23°S, 11°–14°E) where the OMZ is prominent, additional nitrate loss through denitrification causes most of the ∆ of δ¹⁵N_Chl-a. Along the inner shelf (17°-23°S), the southward increase of δ¹⁵N_Chl-a along the inner shelf mirrors an increase of δ¹⁵N_NO3 from 17°S to 23°S, from which an annual denitrification rate of ~0.6 Tg yr^-1 is calculated based on the south-north difference in δ¹⁵N_Chl-a of 11.3‰. In coastal upwelling zones where OMZ intensity is coupled to the upwelling dynamics over the shelf, the δ¹⁵N_Chl-a in surface sediment captures the spatial patterns of integrated N-loss intensity.