Methane Dynamics Associated with Tidal Processes in the Lower Columbia River

• Published in: Estuaries and coasts Vol. 42; no. 5; pp. 1249 - 1264
• Main Authors: Pfeiffer-Herbert, Anna S., Prahl, Fredrick G., Peterson, Tawnya D., Wolhowe, Matthew
• Format: Journal Article
• Language: English
• Published: New York Springer Science + Business Media 01.07.2019; Springer US; Springer Nature B.V
• Subjects: Amplitude; Amplitudes; Biological activity; Biological oxidation; Bottom sediments; Brackishwater environment; Chemical effects; Coastal Sciences; Diurnal; Earth and Environmental Science; Ebb tides; Ecology; Environment; Environmental Management; Estuaries; Estuarine dynamics; Fixed stations; Freshwater; Freshwater & Marine Ecology; Geomorphology; Inland water environment; Methane; Organic chemistry; ORIGINAL PAPERS; Outflow; Oxidation; Pumping; Rivers; Saline water; Salinity; Salinity effects; Sea level; Sea level rise; Sediments; Spatial distribution; Surveys; Temporal variations; Tidal amplitude; Tidal flats; Tides; Variability; Water and Health; Water circulation; Columbia River; Methane; Tidal pumping; Biogeochemical fluxes; Estuary
• DOI: 10.1007/s12237-019-00568-4
• ISSN: 1559-2723

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.