Spatial characteristics of sediment trace metals in an eastern boundary upwelling retention area (St. Helena Bay, South Africa): A hydrodynamic–biological pump hypothesis

St. Helena Bay, a retention zone located in the southern Benguela upwelling system, is an important fish nursery. However, it suffers from seasonal bottom water hypoxia causing major economic losses. Anoxic conditions are linked to sulfide fluxes from bottom sediments defined by a high sedimentation rate of organic matter. It is proposed that trace metals may play an important role in alleviating part of the ecological stress by forming sulfide complexes in such systems. A spatially intensive data set of sediment biogeochemical characteristics showed that POC and trace metals (Cr, Cu, Zn, Ni, etc.) accumulated in the central zone of the Bay. Furthermore, trace metal concentrations were strongly correlated with both POC and Al. To explain the observed biogeochemical relationships in St. Helena Bay, we propose a hypothesis that links the upwelling retention hydrodynamics, primary productivity and sediment trace metal distribution. Trace metals are incorporated into phytoplankton cells in the euphotic zone but rapidly sediment along with particulate organics, on their senescence. Both, the biological pump and the dispersion of particulates are primarily controlled by the hydrodynamics prevalent within St. Helena Bay, which also govern the retention zone in the shadow of one of the major upwelling cells. The dynamics of entrainmentestratification drives the productivity, while a residual cyclonic gyre concentrates the surface productivity within the bay. Bed-shear stresses spatially constrain the accumulation of biogenic organic matter, which governs the trace metal biogeochemistry of the sediments, along a narrow terrigenous mud belt.