Article

Biogeochemistry

, Volume 59, Issue 1, pp 95-119

Methane distribution in European tidal estuaries

  • Jack J. MiddelburgAffiliated withNetherlands Institute of Ecology
  • , Joop NieuwenhuizeAffiliated withNetherlands Institute of Ecology
  • , Niels IversenAffiliated withDepartment of Civil Engineering, Environmental Engineering Laboratory, Aalborg University
  • , Nana HøghAffiliated withDepartment of Civil Engineering, Environmental Engineering Laboratory, Aalborg University
  • , Hein de WildeAffiliated withNetherlands Institute of Sea Research
  • , Wim HelderAffiliated withNetherlands Institute of Sea Research
  • , Richard SeifertAffiliated withInstitute of Biogeochemistry and Marine Chemistry, University of Hamburg
  • , Oliver ChristofAffiliated withInstitute of Biogeochemistry and Marine Chemistry, University of Hamburg

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Abstract

Methane concentrations have been measured along salinity profilesin nine tidal estuaries in Europe (Elbe, Ems, Thames, Rhine,Scheldt, Loire, Gironde, Douro and Sado). The Rhine, Scheldt andGironde estuaries have been studied seasonally. A number ofdifferent methodologies have been used and they yieldedconsistent results. Surface water concentrations ranged from0.002 to 3.6 μM, corresponding to saturation ratios of 0.7 to1580 with a median of 25. Methane concentrations in thefresh-water end-members varied from 0.01 to 1.4 μM. Methaneconcentrations in the marine end-members were close to saturationoffshore and on the order of 0.1 μM in estuarine plumes. Methaneversus salinity profiles in river-dominated, stratified estuaries(Rhine and Douro) appeared rather erratic whereas those in thewell mixed, long-residence time estuaries (Elbe, Ems, Thames,Scheldt, Loire, Gironde and Sado) revealed consistent trends. Inthese systems dissolved methane initially decreases withincreasing salinity, then increases to a maximum at intermediateto high salinities before decreasing again going offshore. Tidalflats and creeks were identified as a methane source to estuarinewaters. The global estuarine flux of methane to the atmospherehas been calculated by combining the median water-air methanegradient (68.2 nmol dm−3) with a global area weighted transfercoefficient and the global area of estuaries. Estuaries emit 1.1to 3.0 Tg CH4 yr−1, which is less than 9% of the global marinemethane emission.

Douro Elbe emission Ems estuaries Gironde Loire methane Rhine rivers Sado Scheldt Thames