Salinity and nutrient availability dictate methane production and its temperature sensitivity in lakes.

Freshwater and saline lakes are critical yet understudied sources of methane (CH4) emissions. Anthropogenic activities and climate warming can elevate the availability of methanogenic substrates such as acetate and methanol, as well as transient intermediates like formate, in lakes. However, the interactive effects of salinity gradients, nutrient states, and temperature on substrate-driven methanogenesis across lake environments remain poorly understood. This study utilized microcosm experiments with sediments from 10 lakes, encompassing a broad salinity spectrum (0.25-180 g L-1) and varying nutrient states. We investigated CH4 production responses to varying concentrations of formate, acetate, and methanol at controlled temperatures of 8°C and 18°C. In nutrient-rich freshwater lakes, CH4 production rates significantly increased (P<0.05) upon the additions of all tested substrates, with acetate showing the most pronounced effect. Conversely, in nutrient-poor saline lakes, only methanol addition significantly stimulated CH4 production. Moreover, increased acetate and methanol availability demonstrably amplified the temperature sensitivity (quantified by Q10) of methanogenesis in freshwater and saline lakes, respectively. Our analysis revealed that Q10 values for substrate-driven methanogenesis were predominantly influenced by salinity-related factors for formate treatment (50.6%), nutrient-related factors for acetate treatment (60.3%), and methanol concentration for methanol treatment (61.0%). These findings demonstrate that both the magnitude and temperature sensitivity of methane production in lake sediments are intricately modulated by substrate type, salinity, and nutrient states. Consequently, as climate change effects manifest in altered lake temperatures, salinities, and nutrient inputs, CH4 emissions from lakes may exhibit heightened sensitivity to environmental shifts.