Methane emissions from three European temperate estuaries

Methane (CH₄) is a potent greenhouse gas with a global warming potential by mass 25 times greater compared to carbon dioxide. Its atmospheric concentration has tripled since the industrial revolution, and half of the global CH₄ emissions can be attributed to aquatic ecosystems. However, there is a large spatiotemporal heterogeneity in river and estuary CH₄ emissions leading to challenges in precise quantification. 

This study presents CH₄ diffusive fluxes from three temperate estuaries discharging into the German Bight of the southern North Sea: the Ems, Weser and Elbe, which are all subject to anthropogenic perturbations. During a campaign in autumn 2024 on the RV Heincke, continuous measurements of CH₄ were obtained using cavity ring-down spectroscopy (Picarro G2508 coupled with an equilibrator system). For quality control purposes, discrete water samples were collected, preserved and later measured with gas chromatography analysis. Ancillary biogeochemical variables were measured continuously using a FerryBox system installed on board.

Preliminary results show varied CH₄ diffusive fluxes across all three estuaries ranging between 6 µmol d^-1 m² and 763 µmol d^-1 m². In the Weser and Elbe, the CH₄ fluxes were elevated (701 µmol d^-1 m² and 499 µmol d^-1 m², respectively) in the lower estuaries with salinities of > 17. Concentrations decreased in the mid-regions and then increased in the upper freshwater region to 763 µmol d^-1 m² and 360 µmol d^-1 m² with salinities 0.2 – 0.5. In the Ems, the highest CH₄ flux up to 627 µmol d^-1 m² was observed in the lower estuary with salinity of 28. We postulate that site specific characteristics, such as organic matter degradation and CH₄ production in the actively dredged Hamburg Harbour (upper Elbe Estuary), as well as stronger winds at 16 m s^-1 in the lower Elbe Estuary promoted elevated CH₄ fluxes. We aim to further disentangle the impacts of human alterations to coastal environments on CH₄ production and emissions, by incorporating and assessing the accompanying FerryBox biogeochemical variables along with discrete nutrient samples in these temperate estuaries.