High resolution sampling of methane transport in the Columbia River near‐field plume: Implications for sources and sinks in a river‐dominated estuary

Examining fluxes of biogeochemical constituents at the mouth of an estuary is necessary for assessing the modification of terrigenous-source materials in the estuary prior to reaching the ocean. In many rivers and estuaries, including the Columbia River estuary (CRE), methane is highly enriched with respect to oceanic concentrations and the equilibrium solubility of the atmospheric gas. We developed a methane budget for the CRE to examine the potential for significant modification of the estuarine methane budget by lateral exchange with peripheral tide flats. We accomplished the challenging task of constraining the net transfer through the estuary-ocean interface using novel instrumentation: a rapid methane analyzer combined with a membrane-contactor interfaced with a pumped-sampling undulating towed vehicle. Transport of riverine methane into the CRE was essentially balanced by losses due to flux to the atmosphere (42%), microbial oxidation in the water column (21%), and transport to the ocean (32%), suggesting limited net effect of lateral tide flat processes on the CRE methane budget. Estimated uncertainty bounds constrained lateral sink/source terms within -30% to +20% of the primary river input. This result contrasts with a number of prior studies of methane cycling in estuaries that reported dominant contributions from lateral sources and relatively minor export to the coastal ocean. The magnitude of lateral supply of methane is a useful indicator of the hydrologic source potential of other related signals of organic matter remineralization from anoxic or suboxic settings in the estuary.