Geochemical evidence of denitrification in the Benguela upwelling system

Abstract

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 CO 2 drawdown following upwelling will be less than it might otherwise be, because of denitrification.Nutrient trapping (secondary remineralisation) is apparent as enhanced phosphate concentrations, some of which are several mmol 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 CO 2 immediately after upwelling. The effect is again to increase the size of the estimated Benguela upwelling system CO 2 source. r

Massive nitrogen loss from the Benguela upwelling system through anaerobic ammonium oxidation

“…2). This decrease has been attributed to the conversion of nitrate to N 2 by denitrifying bacteria (23,24). Nitrite is an intermediate in this process, and nitrite maxima were associated with the decrease in nitrate concentrations at both sites (Fig.…”

“…This condition results from the consumption of O 2 during the decomposition of settling algal biomass (23). A strong N deficit (i.e., a decrease in the ratio of fixed inorganic N to P) (24) in the bottom waters (Fig. 1b) has been attributed to denitrification (23,24).…”

“…A strong N deficit (i.e., a decrease in the ratio of fixed inorganic N to P) (24) in the bottom waters (Fig. 1b) has been attributed to denitrification (23,24). Water in the Benguela OMZ is exchanged rapidly, and concentrations of oxygen vary significantly (23).…”

Variability in upwelling intensity and nutrient regime in the coastal upwelling system offshore Namibia: results from sediment archives

“…Arrows show surface (solid) and subsurface (dashed) hydrographic features (Hardman-Mountford et al 2003). SEC south equatorial current, SECC south equatorial counter current, BC benguela oceanic current, BCC benguela coastal current, ABFZ Angola-Benguela front, PU poleward undercurrent Int J Earth Sci (Geol Rundsch) (2009) 98:309-326 311 in the water column (Kuypers et al 2005;Tyrrell and Lucas 2002). Typical nutrient concentrations in the upwelled water offshore Namibia are 15-25 lM nitrate, 1.5-2.5 lM phosphate and 5-20 lM silicate; these concentrations increase over the shelf (to 10-30 lM nitrate; 2-3 lM phosphate and 20-50 lM silicate) and rapidly decline to \5 lM nitrate, \2 lM phosphate and \1 lM silicate in adjacent pelagic waters over the continental slope (Shannon and O'Toole 2003).…”

“…The area between 20 and 25°S on the shelf-where most of our sediment samples originate-coincides with an area of low-oxygen concentrations in the deeper water column. This coincidence strongly suggests that biological uptake is not the only mechanism that causes low-nitrate concentrations, and that denitrification combined with anaerobic ammonia oxidation (anammox) occurs in the water column at significant rates (Eichner 2001;Kuypers et al 2005;Tyrrell and Lucas 2002). The average d 15 N-value of nitrate in the ocean today is about 5% (Brandes and Devol 2002).…”