Aquatic environments may act as sinks or sources for greenhouse gases, such as carbon dioxide (CO2), methane (CH4) and nitrous oxide (N20). Recent studies have shown estuarine environments may emit up to 0.02-0.5 nmol/m-2/day-1. This study focuses on the CH4 cycle in the Western Scheldt estuary. Cruises were carried out during which continuous CH4 concentrations were performed, testing the brand new HISEM sensors. Water was also sampled for both analyses of CH4 concentration and CH4 isotopic composition. The study shows that there is spatial and temporal variability of CH4 concentrations in surface waters, with values in the open estuary and downstream Scheldt & portal area ranging respectively from 40 to 173 nmol/L and from 16 to 216nmol/L. Isotopic analysis, of 13C and deuterium in CH4, from samples coming from the mouth and middle of the estuary support the current view that CH4, produced in estuarine waters, is of biogenic origin. Isotopic signatures of δ13C-CH4 ranging from -61 to -51‰ (vs VPDB) and δD-CH4 ranging from -212 to -150‰ (vs VSMOW) were measured, typical signatures of biogenic pathways. On the other hand, samples coming from the downstream Scheldt and portal area have an isotopic signature enriched in heavy isotopes, with δ13C-CH4 and δD-CH4 values falling in the range of -45 to -25‰ (vs VPDB) and -64 to 110‰ (vs VSMOW) respectively. These are signatures associated with thermogenic CH4. However, thermogenic sources are generally accompanied by high CH4 concentrations, contrary to what was observed. The hypothesis of enrichment in heavy isotopes, through oxidation of methane by methanotrophic bacteria, was dismissed due to calculated ɛ values. Contamination of Scheldt waters remains a possible cause.
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