Geochemical evidence of denitrification in the Benguela upwelling system

This paper presents analysis of nitrate, phosphate and silicate data from the Benguela upwelling system. Evidence is presented that suggests denitrification occurring close to shore, and also nutrient trapping. Denitrification leaves an imprint on the water properties in terms of a nitrate deficit, that is to say nitrate concentrations that are significantly less than predicted by multiplying the phosphate concentrations by the Redfield ratio. It is probable that denitrification also causes a decoupling of nitrate and carbon compared to Redfield processes, and large-scale losses of nitrate in the Benguela which are not accompanied by losses of carbon. Nitrate-driven CO2 drawdown following upwelling will be less than it might otherwise be, because of denitrification. nt trapping (secondary remineralisation) is apparent as enhanced phosphate concentrations, some of which are several μmol higher than in the offshore source waters for upwelling. Waters also become enriched in silicate and to a lesser extent nitrate as they advect across the shelf. By implication the same process should also “supercharge” waters in dissolved inorganic carbon, leading to stronger outgassing of CO2 immediately after upwelling. The effect is again to increase the size of the estimated Benguela upwelling system CO2 source.