Nitrous oxide from three temperate estuaries discharging in the North Sea: No estuary is like another

Estuaries are potential sources for the important greenhouse gas nitrous oxide (N₂O). Estuaries are among the most complex ecosystems in the world with biogeochemical processes occurring on a range of spatial and temporal scales, depending on geomorphology, tides, and discharge patterns. Due to the high spatiotemporal variability and limited data availability, N₂O emissions from estuaries are associated with significant uncertainty, presenting a big challenge for the global N₂O emission estimates and budgeting of coastal regions.

This study presents N₂O measurements from three temperate German estuaries discharging into the North Sea: Ems, Weser and Elbe, which are all heavily affected by anthropogenic impacts. During a cruise in September 2024, N₂O dry mole fractions were measured continuously using an analyzer based on off-axis integrated cavity output (Picarro G2508) absorption spectroscopy coupled with an equilibrator system. For calibration and quality control, distinct water samples were taken in 30-min intervals and preserved for later GC analysis. Based on these measurements, we calculated N₂O concentrations and fluxes.

Preliminary results showed N₂O oversaturation with distinct peaks observed along the salinity gradient of all three estuaries. The N2O concentration in the Weser estuary was nearly double the concentration recorded in the Ems and Elbe estuaries. The high variability in N2O concentration between the three estuaries indicated potential differences in dominating biological and biogeochemical processes that modulate N₂O production in each estuary. We suspect that turbidity, organic matter quality and degradation, as well as nutrient availability are responsible for the observed differences between the estuaries, which all are heavily impacted by anthropogenic river alterations. Therefore, we aim to elucidate the impact of human alterations on N₂O production and emissions in these temperate estuaries. Overall, our findings highlight the variability of N₂O emissions depending on stream morphology and chemistry, emphasizing the urgent need for comprehensive measurement programs to ensure accurate emission estimates.