To determine the source of excess methane in oxic, surface‐water columns often found in freshwater environments, we measured the in situ concentration and stable isotopic compositions (δ13C and δ2H) of methane in Lake Biwa, a mesotrophic lake in Japan. The values from the littoral zone and lake‐floor sediments were determined, besides those in the water column of the pelagic zone. Furthermore, we conducted incubation experiments to measure microbial oxidation rates and alterations in the isotopic signatures of methane. We found significant vertical and seasonal variations in both in situ concentrations and stable isotopic compositions of methane measured in the pelagic zone. We concluded that active microbial oxidation was primarily responsible for the variation in δ13C and δ2H values of methane in the pelagic water column. As a result, we defined a new indicator Δ(2,13) to characterize the sources of dissolved methane, in which variations in both δ13C and δ2H during methane oxidation had been corrected. The excess methane in oxic, surface‐water columns exhibited Δ(2,13) values similar to those in the littoral zone. We concluded that excess methane at the surface of the pelagic zone originated from the littoral zone via lateral transport. Anoxic near sediments and inflowing rivers were responsible for methane enrichment in water of the littoral zone and in the surface water columns of the pelagic zone.
An Erratum to this paper has been published: https://doi.org/10.1134/S0021364022380015
To clarify the sources and fate of CH 4 enriched in coastal seawaters, we determined the distribution of both the concentrations and dual stable isotope compositions (δ 13 C and δ 2 H) of dissolved CH 4 in the bays of Ise and Mikawa in Japan during five sampling campaigns from 2013 to 2020, together with those in the major inflows of the Kiso, Nagara, and Yahagi Rivers. Excess CH 4 were found in the surface layer of Ise Bay, and their δ 13 C and δ 2 H values were close to those of CH 4 enriched in the major inflows, but deviated from those of CH 4 in the sedimentary layer at the bottom of Ise Bay. The oxidation rates of CH 4 in the water columns were negligibly small during the incubation experiments. In conclusion, the excess CH 4 in the surface layer of Ise Bay was derived from the inflows. The CH 4 enrichment in the freshwater sediments of the inflows showing up to four orders of magnitude higher CH 4 concentrations than those in the sediments of Ise Bay supported this conclusion. Similar results were obtained in Mikawa Bay. The total emission flux of CH 4 from the estuary area of Ise Bay was larger than the influx of CH 4 into Ise Bay via the inflows, suggesting that the CH 4 dissolved in the inflows was emitted into the atmosphere immediately after inflowing into the bay water.
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