“Hot moments” dominate nitrous oxide emissions in the Elbe estuary

Estuaries convert organic carbon and total nitrogen inputs from land to emissions of greenhouse gases such as carbon dioxide (CO₂) and nitrous oxide (N₂O). Many estuarine air-water flux budgets report CO₂ but omit N₂O, which has a much higher global warming potential. N₂O may have more elusive spatial and temporal emission patterns and exhibit short-lived high-emission events. A tide-resolving biogeochemical model for the tidal Elbe estuary, which flows through the German city of Hamburg, shows that CO₂ fluxes vary strongly through the year and often change sign between net uptake and net release. N₂O is always a net source, is highest in the tidal freshwater reach near Hamburg, and remains relevant in the outer estuary where CO₂ fluxes are small. The annual N₂O budget is driven by short events: within each estuary section, the top 10% of emission days contribute 27–38% of the annual flux. Similar N₂O maxima have been reported for other nutrient-rich and urban estuaries like the Scheldt and Humber.  To observe and manage greenhouse gas emissions better, it is essential to identify the spatial and temporal pattern (the “hotspots” and "hot moments") of episodic nitrous oxide emission events, as has been done in our model study. This enables mitigation measures, such as temporary load reduction or artificial water oxygenation be effective.