Methane oxidation in African and European rivers depends on stream size and wetland connectivity

Rivers are large natural sources of methane (CH₄) resulting from the net balance of inputs from methanogenesis and removal by methane oxidation (MOX). Here, we use an extensive dataset collected from African and European rivers to investigate spatial patterns and causes of variability of MOX. The MOX rates were highest in African streams draining flooded forests and increased with river catchment size. In large rivers, MOX represented the more important pathway of dissolved in-stream CH₄ removal compared to diffusive degassing to the atmosphere; but this relative share was minimal in small rivers and was, on average, higher in African streams (37%) than European streams (9%). The relationships between MOX rates and potential drivers such as total suspended matter, pH, or dissolved nutrient concentrations did not follow the patterns expected from controlled experiments reported in the literature and reflected the broad spatial patterns across and within the studied river networks driven by stream size and wetland connectivity.