Nitrous oxide dynamics in coastal marine surface sediments

Coastal seas are especially vulnerable for eutrophication and deoxygenation. These processes can lead to major changes in oxygen concentrations within both coastal bottom waters and surface sediments. The dynamics of the greenhouse gas nitrous oxide (N₂O) are strongly coupled to oxygen dynamics, and changes in oxygenation can thus lead to changes in N₂O production and consumption patterns. Deoxygenation therefore has the potential to affect the exchange of N₂O between surface sediments and bottom waters, which in turn might affect the water column budget of N₂O and eventually its exchange across the sea-air interface. Here, we present depth profiles of oxygen and N₂O as determined with microsensors within coastal marine surface sediments from Lake Grevelingen (The Netherlands) in March 2023. Lake Grevelingen is a eutrophic coastal system suffering from seasonal bottom water anoxia and euxinia. We present results for several field locations and results of laboratory experiments in which oxygen concentrations were manipulated. On-board measurements revealed high concentrations of N₂O in the top sediments (~1 mm), where oxygen was also present. We find that increased depths of oxygen penetration in the sediment lead to higher penetration depths of N₂O. We discuss the potential drivers of N₂O fluxes between the surface sediments and bottom waters under different oxygenation conditions.