Methane Dynamics Associated with Tidal Processes in the Lower Columbia River

Tidally varying methane (CH4) concentrations in estuaries may arise from physical advection and by chemical effects tied to varying exposure to salinity. An investigation of spatial and temporal variability in water-column CH4 was conducted in the lower Columbia River using shipboard surveys and time series data from fixed stations. Peaks in CH4 coincided with ebb tides at multiple sites located along the flank of the estuary adjacent to tidal flats and wetlands. High-resolution measurements taken at the outflow of a shallow lateral bay revealed that these CH4 peaks were positively related to tidal amplitude when the lateral bay was exposed exclusively to freshwater over the tide cycle; in contrast, this relationship was inversed when brackish waters were involved. A positive relationship between tidal amplitude and CH4 is consistent with a mechanism of tidal pumping from bottom sediments in the bay. In the presence of saltwater, however, a higher-than-expected flux of CH4 could occur via suppression of removal processes such as biological oxidation. We present a conceptual model of tidal pumping modified by diurnal inequality in tidal amplitude and effects of salinity on sediment CH4 oxidation to explain CH4 variability on tidal to seasonal time-scales. The combined influences of tides and salinity likely affect CH4 emissions in estuaries worldwide, making sea level rise and estuarine geomorphological change relevant factors for consideration when accounting for estuarine contributions to global methane budgets.