Environmental controls on methanotrophic oxidation capacity in the southeastern North Sea

Abstract. Shallow coastal seas contribute disproportionately to marine methane (CH₄) emissions, yet the efficiency of the microbial sink that attenuates them remains poorly constrained. Under first-order kinetics, aerobic methane oxidation (MOx) is the product of ambient CH₄ concentration and the fractional turnover rate k′, the latter representing the oxidation capacity of the methanotrophic community. Here we test whether k′ can be predicted from routinely measured environmental variables, drawing on water samples collected during repeated campaigns between 2010 and 2014 across riverine, estuarine, and marine waters of the Elbe and the adjacent southeastern North Sea. In the estuary and adjacent marine waters, a second-order polynomial model based on temperature, salinity, and nitrate reproduced 80 % of the observed spatiotemporal variability in k′ under five-fold cross-validation, indicating that these predictors captured the main environmental controls on k′ and that nonlinear interaction terms cannot be omitted. Neither ambient CH₄ concentration nor phosphate improved model performance, indicating that k′ is not directly controlled by substrate availability or phosphorus limitation. In riverine waters, by contrast, the same approach systematically underpredicted k′ with large uncertainty, suggesting a distinct regional response regime that is likely linked to river-specific environmental controls. Overall, our results highlight that k′ can be parameterized as a dynamic, environmentally dependent term, opening a route to estimating MOx in coastal systems.