Methane distribution in European tidal estuaries

Methane concentrations have been measured along salinity profiles in nine tidal estuaries in Europe (Elbe, Ems, Thames, Rhine, Scheldt, Loire, Gironde, Douro and Sado). The Rhine, Scheldt and Gironde estuaries have been studied seasonally. A number of different methodologies have been used and they yielded consistent results. Surface water concentrations ranged from 0.002 to 3.6 muM, corresponding to saturation ratios of 0.7 to 1580 with a median of 25. Methane concentrations in the fresh-water end-members varied from 0.01 to 1.4 muM. Methane concentrations in the marine end-members were close to saturation offshore and on the order of 0.1 muM in estuarine plumes. Methane versus salinity profiles in river-dominated, stratified estuaries (Rhine and Douro) appeared rather erratic whereas those in the well mixed, long-residence time estuaries (Elbe, Ems, Thames, Scheldt, Loire, Gironde and Sado) revealed consistent trends. In these systems dissolved methane initially decreases with increasing salinity, then increases to a maximum at intermediate to high salinities before decreasing again going offshore. Tidal flats and creeks were identified as a methane source to estuarine waters. The global estuarine flux of methane to the atmosphere has been calculated by combining the median water-air methane gradient (68.2 nmol dm(-3)) with a global area weighted transfer coefficient and the global area of estuaries. Estuaries emit 1.1 to 3.0 Tg CH4 yr(-1), which is less than 9% of the global marine methane emission.