Benthic phosphorus cycling in the northern Benguela upwelling system: excess P supply and altered pelagic nutrient stoichiometry

Abstract. The northern Benguela upwelling system (NBUS) off Namibia is one of the most productive marine regions globally, with intense biogeochemical cycling and burial of essential elements such as carbon, nitrogen and phosphorus. Element cycling is strongly impacted by temporally and spatially variable degrees of oxygen depletion, both in the water column and the sediment; a perennial oxygen minimum zone (OMZ; minimum O₂ ~ 50 µmol L^-1) impinges on the slope while the shelf is seasonally oxygen-depleted and even euxinic (i.e. free sulphide in the water column). Areas such as the NBUS are not only significant in regional and global marine element cycles and budgets but can also help predict the impact of globally increasing eutrophication and deoxygenation. Here, we combine biogeochemical water-column and sediment data to explore the (de)coupling of pelagic and benthic nitrogen and phosphorus cycling as function of depositional conditions, particularly deep-water redox. We show major shifts in N:P stoichiometry in deep waters as a result of pelagic nutrient-N (NO₃^-, NH₄⁺) loss as N₂ and supply of excess P from the sediment into the bottom water. Notably, benthic P supply contributes strongly to the pelagic N deficit (PO₄^3-*16 − ∑(NO₃^-, NO₂^-, NH₄⁺)), which is commonly considered to reflect only the strength of anaerobic N loss. Our results further reflect strong differences in benthic nutrient cycling between depositional environments. The seasonally anoxic shelf hosts organic-rich, strongly reducing sediments with large pools of labile P and high effluxes of DIC and total alkalinity, NH₄⁺, PO₄^3- and HS^-. Chemical analysis indicates that a large proportion of the highly reactive sedimentary P pool consists of (i) P supplied in fish debris rather than marine algal biomass and/or (ii) P accumulated intracellularly by sulphide-oxidizing bacteria (SOB). These SOB modulate benthic phosphate and sulphide fluxes, while iron (Fe) redox cycling plays a minor role as the shelf sediments are depleted in reactive ferric iron phases. In contrast to the shelf, slope sediments including those underlying the OMZ are much less organic-rich and show patterns of coupled N and P cycling, the latter coupled to Fe redox cycling. Overall, we show how the distinct environmental conditions on the highly productive and seasonally anoxic shelf drive the decoupling of pelagic and benthic N and P cycles. This results in strongly altered nutrient dynamics and stoichiometry compared to oxygenated marine systems. Such perturbations are then likely to occur at wider (global) scale in the ocean under conditions of intense oxygen depletion either in the geological past or in the anthropogenic future.