Seasonal variability of nitrous oxide concentrations and emissions along the Elbe estuary

Abstract. Nitrous oxide (N2O) is a greenhouse gas, with a global warming potential 298 times that of carbon dioxide. Estuaries can be sources of N2O, but their emission estimates have significant uncertainties due to limited data availability and high spatiotemporal variability. We investigated the spatial and seasonal variability of dissolved N2O and N2O emissions along the Elbe estuary (Germany). During nine research cruises done between 2017 and 2022, we measured dissolved N2O concentrations, as well as dissolved nutrients and oxygen concentrations along the estuary and calculated N2O saturation, flux densities and emissions. We found intense N2O production along the Elbe estuary that compensated the effect of decreasing dissolved inorganic nitrogen (DIN) loads since the 1990s. Two hot-spots areas of N2O production have been identified in the estuary: the Port of Hamburg and the mesohaline estuary near the estuarine turbidity maximum (MTZ). N2O production was enhanced by warmer temperatures and fueled by riverine organic matter in the Hamburg Port or marine organic matter in the MTZ. Surprisingly, estuarine N2O emissions where equally high in winter and summer. In winter, high riverine N2O concentrations led to high N2O emissions from the estuary, whereas in summer, estuarine biological N2O production led to equally high N2O emissions. Overall, we find that the Elbe estuary is a year-round source of N2O with estimated annual emissions of 0.24 ± 0.06 Gg yr−1.