Influence of an episodic flood event on methane and nitrous oxide dynamics in the Elbe estuary

Ongoing anthropogenic-driven climate change patterns prompt for better constraining drivers of coastal methane (CH₄) and nitrous oxide (N₂O) emissions, potent greenhouse gases (GHG) with greater warming potential than carbon dioxide. The temperate Elbe Estuary (Germany) is heavily influenced by anthropogenic activities including high agricultural nitrogen loads and intense dredging in the Hamburg Port, overall contributing to this ecosystem acting as a net source of CH₄ and N₂O. However, interactions between local microbial activity and transport processes translate into pronounced spatial and temporal variability in estuarine CH₄ and N₂O dynamics, thus challenging assessments at time-scales relevant to episodic events. This study presents spatio-temporal CH₄ and N₂O dynamics in the Elbe estuary based on discrete samples collected during three campaigns (in 08/2024, 09/2024 and 08/2025) that captured the effects of an episodic flood event in autumn following dry summer.

During low river flow conditions (08/2024 and 08/2025), CH₄ and N₂O concentrations showed distinct spatial gradients along the estuary, indicative of riverine input and CH₄ production in the mid-estuary (salinities <20), while N₂O production was mainly restricted to the upper and hypoxic estuary. During a river flood event (09/2024), the Elbe discharge rate rapidly increased 4-fold up to 1240 m³ s^-1 within 10 days, which lowered CH₄ and N₂O concentration along the estuary. Consequently, the CH₄ and N₂O dilution during the flood event decreased estuarine diffusive water-air flux rates, while increasing export to the German Bight. While these preliminary findings need to be further evaluated against ancillary environmental data, capturing the effects of episodic flood events across different seasons has the potential to influence estuarine CH₄ and N₂O emission budgets.