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Geochemistry of Methane and Sulfide Sulfur in the Bottom Sediments of Small Lakes in Southern Russia
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DOI：10.3390/w18161981.png)
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Abstract
En 中文
Small lakes are widespread, yet their biogeochemistry, particularly regarding greenhouse gases, remains insufficiently studied. This article presents the findings from an investigation of six small lakes in the Southern European part of Russia, conducted from September to October 2024. The primary aim was to examine the coupled distribution of methane (CH4) and sulfide sulfur (a key metabolite of H2S) in the lake sediments, in relation to geochemical parameters such as pH, Eh, sediment density, moisture, and the contents of sulfate ions (SO42−), organic matter, and granulometric composition. The studied sediment layers, reaching depths of up to 110 cm, consist primarily of silty clay. The lakes studied represent both freshwater (0.2–0.7 g/L) and brackish (1.3–24.2 g/L) systems. During the study period, the water column exhibited temperatures of 10.4–22.1 °C, pH values of 7.36–8.53, and dissolved O2 concentrations ranging from 3.16 mg/L (34% saturation) to 11.79 mg/L (125% saturation). Methane concentrations in the water varied widely, from 1.6 µL/L to 37,380 µL/L. The lowest values were found in the highly mineralized Lake Bolshoy Tambukan (1.6–2.0 µL/L), while exceptionally high concentrations were detected in the bottom waters of the thermally stratified freshwater Lake Staroe. In the shallow, productive freshwater lakes, a significant portion of the organic matter undergoes limited mineralization in the water column and settles to the sediments as partially decomposed remains of sand- and coarse-silt-sized organisms. The subsequent degradation of this labile organic matter reduces bottom-water oxygen, triggering intense anaerobic processes in the upper sediment layer. In these freshwater sediments, where sulfate concentrations are relatively low, sulfate reduction is typically suppressed. Combined with an abundance of labile substrates, this condition fosters intensive methanogenesis, resulting in maximum CH4 concentrations (33–179 µg/g). Under stable thermal stratification, such high CH4 concentrations can also accumulate in the bottom water (e.g., up to 37. 4 mL/L in Lake Staroe), posing a risk of significant pulse emissions during autumn mixing. Conversely, the brackish Lake Bolshoy Tambukan exemplifies the crucial role of sulfate reduction, which is stimulated by sulfate-dependent anaerobic oxidation of CH4. This process acts as a powerful natural biogeochemical barrier that curtails the emission of a major greenhouse gas. The sediments of this lake exhibited minimal CH4 content (0.14–0.57 µg/g) alongside maximal sulfide sulfur concentrations (1.06–8.57 mg/g). Overall, this theoretical and experimental analysis demonstrates that sulfate reduction is a key determinant of redox potential, acid–base conditions, and the vertical distribution of CH4 in the anaerobic sediments of small lakes in Southern Russia. Given the projected salinization of lakes in steppe and arid landscapes under climate change, a reduction in CH4 emissions to the atmosphere is likely due to the enhanced sulfate-dependent anaerobic CH4 oxidation associated with sulfate reduction in the sediments.
Keywords:
small lakes
water
bottom sediments
particle size distribution
physical and chemical parameters
methane
sulfide sulfur
sulfate reduction
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W
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3
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3.1W
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7.4W
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