Coupled physical/biogeochemical modeling including Osub2/sub-dependent processes in the Eastern Boundary Upwelling Systems: application in the Benguela

摘要

The Eastern Boundary Upwelling Systems (EBUS) contribute to one fifth of theglobal catches in the ocean. Often associated with Oxygen Minimum Zones(OMZs), EBUS represent key regions for the oceanic nitrogen (N) cycle.Important bioavailable N loss due to denitrification and anammox processes aswell as greenhouse gas emissions (e.g, N₂O) occur also in these EBUS.However, their dynamics are currently crudely represented in global models.In the climate change context, improving our capability to properly representthese areas is crucial due to anticipated changes in the winds, productivity,and oxygen content.We developed a biogeochemical model (BioEBUS) taking into account the mainprocesses linked with EBUS and associated OMZs. We implemented this model ina 3-D realistic coupled physical/biogeochemical configuration in the Namibianupwelling system (northern Benguela) using the high-resolution hydrodynamicROMS model. We present here a validation using in situ and satellite dataas well as diagnostic metrics and sensitivity analyses of key parameters andN₂O parameterizations. The impact of parameter values on the OMZ offNamibia, on N loss, and on N₂O concentrations and emissions is detailed.The model realistically reproduces the vertical distribution and seasonalcycle of observed oxygen, nitrate, and chlorophyll a concentrations, andthe rates of microbial processes (e.g, NH₄+ and NO₂−oxidation, NO₃− reduction, and anammox) as well. Based on oursensitivity analyses, biogeochemical parameter values associated with organicmatter decomposition, vertical sinking, and nitrification play a key role forthe low-oxygen water content, N loss, and N₂O concentrations in the OMZ.Moreover, the explicit parameterization of both steps of nitrification,ammonium oxidation to nitrate with nitrite as an explicit intermediate, isnecessary to improve the representation of microbial activity linked with theOMZ. The simulated minimum oxygen concentrations are driven by the polewardmeridional advection of oxygen-depleted waters offshore of a 300 m isobathand by the biogeochemical activity inshore of this isobath, highlighting aspatial shift of dominant processes maintaining the minimum oxygenconcentrations off Namibia.In the OMZ off Namibia, the magnitude of N₂O outgassing and of N loss iscomparable. Anammox contributes to about 20% of total N loss, an estimatelower than currently assumed (up to 50%) for the global ocean.