湖泊表层水体“甲烷悖论”现象研究进展

Research progress on “methane paradox”phenomenon in lake surface waters

传统观点认为,甲烷(CH4)产生于严格的厌氧环境,在有氧环境中容易被氧化,但许多湖泊表层有氧水体出现了CH4过饱和现象,这种现象被称为"甲烷悖论"现象。为了解释湖泊"甲烷悖论"现象,本文根据湖泊表层CH4的来源,归纳出"外来假说"和"自产假说"。"外来假说"假说认为,岸边浅水区底泥或消落区土壤产生CH4向湖心表层水体横向扩散传输(FL),这种假说适应于岸边富含有机质的小型浅水湖泊。"自产假说"认为,湖心表层水体中产甲烷古生菌原位产生CH4(P),这种假说适应于山区大型深水湖泊。此外,湖泊表层有氧水体中CH4的来源还有湖泊周围河流的输入(FR)、沉淀物或次表层水体的CH4垂直向上湍流扩散(FZ)、气泡CH4溶解在表层水体中(FD)等,而湖泊表层有氧水体中CH4的损耗有"水-气"界面上气体排放(E)、CH4氧化(O)等。在厘清湖泊表层水体中CH4收支的基础上,建立CH4质量收支平衡模型,有助于客观认识湖泊表层水体中CH4的来源。实际上,湖泊表层水体中过饱和甲烷的来源与湖泊的环境特性有关,但数据分析方法、取样时段、湖泊环境条件等差异,容易造成"外来假说"和"自产假说"之争。

The traditional paradigm in aquatic sciences holds that microbial methanogenesis happens primarily in the anoxic conditions.The widely reported paradoxical supersaturation of CH4 in oxygenated surface waters challenges such traditional paradigm.Here,I summarized two new concepts in literature,i.e.,external source hypothesis and in-situ produced hypothesis,to explain the"methane paradox"phenomenon.The former supports that the lateral input of CH4 is from the surrounding littoral zone(FL),which could be important for small lakes with an organic-rich littoral zone.The latter believes in-situ production in the oxic surface waters(P),which accounts for large,deep lakes in the valleys.The external inputs from the upstream rivers(FR),turbulent diffusion from the sediments and metalimnion layers(FZ),the dissolution of bubbles(FD)are all contributed to the supersaturated CH4 concentrations in the oxic surface layers.CH4 emission flux at the air-water interface(E)and CH4 oxidation(O)can lead to CH4 loss in the oxic surface layers.The construction of CH4 mass balance model will be helpful for understanding the supersaturation of CH4 in oxygenated surface waters.Actually,the supersaturated CH4 concentration in the surface waters is related to environmental characteristics of lakes.However,differences in data analysis methods,sampling periods,and lake environments can result in the debates of two hypotheses.