Methane Dynamics Associated with Tidal Processes in the Lower Columbia River
2019
Pfeiffer-Herbert, Anna S. | Prahl, Fredrick G. | Peterson, Tawnya D. | Wolhowe, Matthew
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.
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