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 (C), nitrogen (N) and phosphorus (P). Redox conditions, a key parameter in coupled biogeochemical cycling, are highly variable in both space and time; a perennial oxygen minimum zone (OMZ; minimum O2 ∼ 50 µmolL-1) impinges on the slope while the shelf is seasonally oxygen-depleted and even euxinic (i.e. free sulfide 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 marine oxygen loss and associated altered nutrient cycling. Here, we aim to provide a hitherto lacking mechanistic framework for coupled benthic-pelagic cycling of the major nutrients N and P in response to (variable) redox conditions in the NBUS, with specific attention to the occurrence and importance of poorly understood, non-traditional P cycling mechanisms. We combine biogeochemical water-column and sediment data collected from the shelf to slope (100–1500 m water depth) during a cruise in austral summer when shelf bottom waters were low in oxygen. We show shifts in P cycling mechanisms from the slope to the shelf, and major shifts in N: P stoichiometry on the shelf from decoupled N-P cycling: nutrient-N (NO3-, NH4+) is lost as N2 by anaerobic microbial metabolism while excess P is supplied from the sediment. This excess P supply derives from labile sediment P pools other than organic matter: visual core inspection, phosphorus sequential extraction and pore-water chemistry together indicated that a large proportion of the highly reactive sedimentary P pool in shelf sediments consists of (i) P supplied in fish debris rather than marine algal biomass and/or (ii) P accumulated intracellularly by sulfide-oxidizing bacteria (SOB). To further improve our understanding of (de)coupled OMZ nutrient dynamics, we quantify the prominent role of excess benthic P supply in the pelagic N deficit in our study area. In this study, we not only provide an integrated framework for how coupled benthic-pelagic processes shape OMZ nutrient dynamics but also highlight the sensitivity of coupled biogeochemical cycles to spatial and temporal variability in depositional conditions.