Methane sources, sinks and fluxes in a temperate tidal lagoon: The Arcachon Lagoon (SW France)

The Arcachon lagoon is a 156 km 2 temperate mesotidal lagoon dominated by tidal flats (66% of the surface area). The methane (CH 4 ) sources, sinks and fluxes were estimated from water and pore water concentrations, from chamber flux measurements at the sediment–air (low tide), sediment–water and water–air (high tide) interfaces, and from potential oxidation and production rate measurements in sediments. CH 4 concentrations in waters were maximal (500–1000 nmol l −1 ) in river waters and in tidal creeks at low tide, and minimal in the lagoon at high tide ( −1 ). The major CH 4 sources are continental waters and the tidal pumping of sediment pore waters at low tide. Methanogenesis occurred in the tidal flat sediments, in which pore water concentrations were relatively high (2.5–8.0 μmol l −1 ). Nevertheless, the sediment was a minor CH 4 source for the water column and the atmosphere because of a high degree of anaerobic and aerobic CH 4 oxidation in sediments. Atmospheric CH 4 fluxes at high and low tide were low compared to freshwater wetlands. Temperate tidal lagoons appear to be very minor contributor of CH 4 to global atmosphere and to open ocean.