Abstract
We collected water samples from the Scheldt estuary during December 2015 and November 2016 for methane (CH4) concentration and isotopic composition (δ13C and δD values) analyses, to investigate the origin of the excess dissolved CH4, which is a common feature in estuaries. The Scheldt estuary is a eutrophic, heterotrophic tidal estuary, located at the border between Belgium and the Netherlands. The gas chromatography and mass spectrometry analyses revealed (1) variable dissolved CH4 concentrations reaching up to 302.6 nM in surface waters of the Port of Antwerp, which fits within the higher range of values reported for European estuaries, and (2) the presence of surprisingly high isotopic signatures in the upper estuary. While microbial CH4 production dominates in the lower part of the estuary, we observe a clear trend towards isotopically heavier CH4 upstream where isotopic signatures as enriched as − 25.2‰ for carbon and + 101‰ for hydrogen were measured. We conclude that microbial oxidation of most of the CH4 pool could explain such enrichments, but that the origin of riverine CH4 enriched isotopic signatures remains to be explained. This study identifies peculiar features associated with CH4 cycling in the Scheldt estuary, paving the way for a more thorough biogeochemical quantification of various production/removal processes.
Highlights
The distribution of dissolved CH4 in estuaries is governed mainly by riverine inputs, diffusion from the sediments, microbial production in micro-environments, leakage from oil and gas local industries, groundwater discharge, efflux to the atmosphere and microbial consumption (Scranton & McShane, 1991; Middelburg et al, 2002).Given the complexity of estuarine systems, the relative contribution of these processes to the dissolved CH4 pool is subject to wide spatial and temporal variations (Abril & Borges, 2004).CH4 produced in anaerobic aquatic environments can be of thermogenic or microbial origin
In December 2015, the dissolved CH4 concentrations ranged between 15.9 and 94.7 nM in surface waters, with lowest values found in the upper estuary (Fig. 2a and Table S1)
The signatures most enriched in heavy isotopes were measured near the confluence of the Scheldt river and the Rupel and in the Port of Antwerp, at salinities lower than 10. This spatial segregation was apparent in the hydrogen isotopic signatures, with values ranging between − 212 and − 150‰ in the lower estuary and between − 64 and up to + 101‰ in the upper estuary (Fig. 2e)
Summary
The distribution of dissolved CH4 in estuaries is governed mainly by riverine inputs, diffusion from the sediments, microbial production in micro-environments, leakage from oil and gas local industries, groundwater discharge, efflux to the atmosphere and microbial consumption (Scranton & McShane, 1991; Middelburg et al, 2002).Given the complexity of estuarine systems, the relative contribution of these processes to the dissolved CH4 pool is subject to wide spatial and temporal variations (Abril & Borges, 2004).CH4 produced in anaerobic aquatic environments can be of thermogenic or microbial origin. The distribution of dissolved CH4 in estuaries is governed mainly by riverine inputs, diffusion from the sediments, microbial production in micro-environments, leakage from oil and gas local industries, groundwater discharge, efflux to the atmosphere and microbial consumption (Scranton & McShane, 1991; Middelburg et al, 2002). Given the complexity of estuarine systems, the relative contribution of these processes to the dissolved CH4 pool is subject to wide spatial and temporal variations (Abril & Borges, 2004). CH4 produced in anaerobic aquatic environments can be of thermogenic or microbial origin. The two main microbial production pathways are acetate fermentation Whiticar, 1999): Communicated by Carolyn A.
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