The role of regeneration from the sediments in the supply of nutrients to the euphotic zone in the southern Benguela system

Abstract

Regeneration of nutrients from Benguela sediments taken between Lüderitz and Walker Bay, on the leeward side of upwelling centres, was studied by means of two methods. First, porewater samples were taken from sediment cores and analysed for nutrients. The observed concentration gradient was used to estimate the potential flux by diffusion of nutrients across the sediment/water interface. Second, intact sediment cores were incubated at the temperatures and concentrations of dissolved oxygen found at the interface, and the rate of increase of nutrients in the overlying water was monitored. The systems where samples were taken may be classified as semi-closed, as there is preferential settling of particulate organic carbon and regeneration of nutrients from the sediments. Here, the nutrient concentrations in bottom waters exceeded those in upwelling source water by up to 100 per cent, probably because of the regeneration process. More stable water-column conditions on the leeward side of upwelling centres may inhibit exchange of nutrients between bottom waters and the euphotic zone: implications of this possibility are discussed. A mechanism is proposed as to the manner in which nutrient-rich bottom water from St Helena Bay may be advected southwards during current reversals associated with coastal trapped waves. This mechanism would prime the upwelling centre at Cape Columbine and could result in advection of nutrients back into St Helena Bay euphotic zone waters.