Deconstructing Methane Emissions from a Small Northern European River: Hydrodynamics and Temperature as Key Drivers

Methane (CH₄) emissions from small rivers and streams, particularly via ebullition, are currently under-represented in the literature. Here, we quantify the methane effluxes and drivers in a small, Northern European river. Methane fluxes are comparable to those from tropical aquatic systems, with average emissions of 320 mg CH₄ m^-2 d^-1. Two important drivers of methane flux variations were identified in the studied system: 1) temperature-driven sediment methane ebullition and 2) flow-dependent contribution suspected to be hydraulic exchange with adjacent wetlands and small side-bays. This flow-dependent contribution to river methane loading is shown to be negligible for flows less than 4 m³ s^-1 and greater than 50% as flows exceed 7 m³ s^-1. While the temperature-ebullition relationship is comparable to other systems, the flow rate dependency has not been previously demonstrated. In general, we found that about 80% of the total emissions were due to methane bubbles. Applying ebullition rates to global estimates for fluvial systems, which currently are not considered, could dramatically increase emission rates to ranges from lakes or wetlands. This work illustrates that small rivers can emit significant methane and highlights the need for further studies on the link between hydrodynamics and connected wetlands.