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.