Understanding the impacts of coastal deoxygenation in nitrogen dynamics: an observational analysis

Biological production and outgassing of greenhouse gasses (GHG) in Eastern Boundary Upwelling Systems (EBUS) are vital for fishing productivity and climate regulation. This study examines temporal variability of biogeochemical and oceanographic variables, focusing on dissolved oxygen (DO), nitrate, nitrogen deficit (N deficit), nitrous oxide (N₂O) and air-sea N₂O flux. This analysis is based on monthly observations from 2000 to 2023 in a region of intense seasonal coastal upwelling off central Chile (36°S). Strong correlations are estimated among N₂O concentrations and N deficit in the 30-80 m layer, and N₂O air-sea fluxes with the proportion of hypoxic water (4 < DO < 89 µmol L^-1) in the water column, suggesting that N₂O accumulation and its exchange are mainly associated with partial denitrification. Furthermore, we observe interannual variability in concentrations and inventories in the water column of DO, nitrate, N deficit, as well as air-sea N₂O fluxes in both downwelling and upwelling seasons. These variabilities are not associated with El Niño-Southern Oscillation (ENSO) indices but are related to interannual differences in upwelling intensity. The time series reveals significant nitrate removal and N₂O accumulation in both mid and bottom layers, occurring at rates of 1.5 µmol L^-1 and 2.9 nmol L^-1 per decade, respectively. Particularly significant is the increase over the past two decades of air-sea N₂O fluxes at a rate of 2.9 µmol m^-2 d^-1 per decade. These observations suggest that changes in the EBUS, such as intensification of upwelling and the prevalence of hypoxic waters may have implications for N₂O emissions and fixed nitrogen loss, potentially influencing coastal productivity and climate.