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.

中文翻译：

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中营养湖泊含氧水中过量甲烷的双重稳定同位素表征

为了确定在好氧过量甲烷的来源，通常在淡水环境中发现的表面水的列，我们测量了在原位浓度和稳定同位素组成（δ ¹³ C和δ ² H）在琵琶湖，一个中营养湖在日本甲烷。除了中上层带水柱中的那些，还确定了沿岸带和湖底沉积物的值。此外，我们进行了孵育实验，以测量微生物的氧化速率和甲烷同位素特征的变化。我们发现在中上层带测量的甲烷的原位浓度和稳定的同位素组成都存在明显的垂直和季节性变化。我们得出的结论是活性微生物氧化是为变化主要负责在δ ¹³C和δ ^2个在上层水柱甲烷h的值。其结果是，我们定义了一个新的指标Δ（2,13）来表征溶解的甲烷的源极，其中，变化都δ ¹³ C和δ ²甲烷氧化H期间已被纠正。在含氧的地表水柱中，过量的甲烷表现出的Δ（2,13）值与沿岸带的相似。我们得出的结论是，中上层带表面的过量甲烷是通过侧向运输从沿海带产生的。靠近沉积物和流入河流的缺氧导致沿海区水体和中上层带地表水柱中的甲烷富集。