Isotopic tracing of methane in water and exchange with the atmosphere

Sediments of Lake Constance are rich in bacterial methane (yield CH4 < 130 mu g CH4/g wet sample; delta(13)CH(4) > -85 parts per thousand). However, methane flux from the sediment into the water was not detected. Deep water (below ca. 50 m) has ''background'' signature (yield < 50 nL/L; -40 < delta(13)CH(4) < -30 parts per thousand). Surface water is enriched in methane relative to atmospheric equilibrium, most probably the methane is produced by methanogens in the surface layers. The surface water is a source for atmospheric methane; calculated flux rates exceed high values reported for ocean water. Deep water of the Bay of Bengal is generally low in methane (''background'' signature). At two locations at the continental slope, bacterial methane (delta(13)CH(4) < -50 parts per thousand seeps from the sediments into the water. Methane concentration in the surface water is close to atmospheric equilibrium in the equatorial area but higher in the Ganges/Bramaputra mouth, where methane flux to the atmosphere reaches up to 30 kg km(-2)yr(-1).