Nitrogen transfers and air-sea N₂O fluxes in the upwelling off Namibia within the oxygen minimum zone: a 3-D model approach

Abstract

Abstract. As regions of high primary production and being often associated to Oxygen Minimum Zones (OMZs), Eastern Boundary Upwelling Systems (EBUS) represent key regions for the oceanic nitrogen (N) cycle. Indeed, by exporting the Organic Matter (OM) and nutrients produced in the coastal region to the open ocean, EBUS can play an important role in sustaining primary production in subtropical gyres. Losses of fixed inorganic N, through denitrification and anammox processes and through nitrous oxide (N2O) emissions to the atmosphere, take place in oxygen depleted environments such as EBUS, and alleviate the role of these regions as a source of N. In the present study, we developed a 3-D coupled physical/biogeochemical (ROMS/BioBUS) model for investigating the full N budget in the Namibian sub-system of the Benguela Upwelling System (BUS). The different state variables of a climatological experiment have been compared to different data sets (satellite and in situ observations) and show that the model is able to represent this biogeochemical oceanic region. The N transfer is investigated in the Namibian upwelling system using this coupled model, especially in the Walvis Bay area between 22° S and 24° S where the OMZ is well developed (O2 < 0.5 ml O2 l−1). The upwelling process advects 24.2 × 1010 mol N yr−1 of nitrate enriched waters over the first 100 m over the slope and over the continental shelf. The meridional advection by the alongshore Benguela current brings also nutrient-rich waters with 21.1 × 1010 mol N yr−1. 10.5 × 1010 mol N yr−1 of OM are exported outside of the continental shelf (between 0 and 100-m depth). 32.4% and 18.1% of this OM are exported by advection in the form of Dissolved and Particulate Organic Matters (DOM and POM), respectively, however vertical sinking of POM represents the main contributor (49.5%) to OM export outside of the first 100-m depth of the water column on the continental shelf. The continental slope also represents a net N export (11.1 × 1010 mol N yr−1) between 0 and 100-m depth: advection processes export 14.4% of DOM and 1.8% of POM, and vertical sinking of POM contributes to 83.8%. Between 100 and 600-m depth, water column denitrification and anammox constitute a fixed inorganic N loss of 2.2 × 108 mol N yr−1 on the continental shelf and slope, which will not significantly influence the N transfer from the coast to the open ocean. At the bottom, an important quantity of OM is sequestrated in the upper sediments of the Walvis Bay area. 78.8% of POM vertical sinking at 100-m depth is sequestrated on the shelf sediment. Only 14% of POM vertical sinking reaches the sediment on the slope without being remineralized. From our estimation, the Walvis Bay area (0–100 m), can be a substantial N source (28.7 × 1010 mol N yr−1) for the eastern part of the South Atlantic Subtropical Gyre. Assuming the same area for the South Atlantic Subtropical Gyre as the North Atlantic Subtropical Gyre, this estimation is equivalent to 3.7 × 10−2 mol N m−2 yr−1 for the Walvis Bay area, and 0.38 mol N m−2 yr−1 by extrapolating for the entire Benguela upwelling system. This last estimation is of the same order as other possible N sources sustaining primary production in the subtropical gyres. The continental shelf off Walvis Bay area does not represent more than 1.2% of the world's major eastern boundary regions and 0.006% of the global ocean, its estimated N2O emission (2.9 × 108 mol N2O yr−1), using a parameterization based on oxygen consumption, contributes to 4% of the emissions in the eastern boundary regions, and represents 0.2% of global ocean N2O emission. Hence, even if the Walvis Bay area is a small domain, its N2O emissions have to be taken into account in the atmospheric N2O budget.