Methane dynamics of high-elevation lakes in the Sierra Nevada California: the role of elevation, temperature, and inorganic nutrients

Freshwater lakes are important but poorly constrained sources of methane (CH4) to the atmosphere, reflecting high but variable rates of CH4 production as well as limited and inconsistent measurements worldwide. High-elevation lakes have been particularly overlooked given their high abundance in mountain ranges around the world, and the potential for CH4 dynamics at high elevations is expected to be disproportionally impacted by increases in temperature due to climate change. We examined variations in surface CH4 concentrations and diffusive fluxes, temperature, and inorganic nutrients in 5 montane lakes spanning multiple elevations in the Sierra Nevada of California. Over 2 years, we found strong and consistent seasonality in CH4 concentrations in lakes; higher concentrations were typically observed in the warmest months and lower concentrations right before winter. Changes in CH4 concentrations were significantly related to temperature in the majority of the individual lakes (r2 = 0.43–0.81) and related to elevation (r2 = 0.39) across lakes. Methane concentrations in lakes at elevations r2 = 0.90), whereas at elevations >3000 m, CH4 was related to dissolved inorganic nitrogen to dissolved inorganic phosphorus ratios and elevation (r2 = 0.48). Our results expand our understanding of temporal variations in CH4 and demonstrate substantial seasonality in CH4 concentrations and diffusive fluxes in freshwater lakes. We suggest that temporal variation should be considered in large-scale estimates of CH4 emissions, and these fluxes may be a predictable function of elevation, temperature, and nutrients.

淡水湖泊是大气甲烷（CH4）的重要排放源，但其排放通量却缺乏可靠约束：一方面湖泊甲烷生成速率较高但波动显著，另一方面全球范围内的相关观测数据稀缺且一致性欠佳。全球各山地中分布广泛的高海拔湖泊，其甲烷循环过程长期以来被研究忽视；而气候变化引发的升温，预计将对高海拔湖泊的甲烷动态产生不成比例的显著影响。本研究针对美国加利福尼亚州内华达山脉内5座海拔梯度各异的山地湖泊，开展了为期两年的原位观测，分析了湖泊表层甲烷浓度、扩散通量、水温以及无机营养盐的变化特征。两年观测期间，所有湖泊的甲烷浓度均表现出显著且稳定的季节变化规律：暖季甲烷浓度较高，而冬季来临前浓度降至最低。多数单座湖泊的甲烷浓度变化与水温呈显著相关（决定系数r²=0.43~0.81）；跨湖泊的整体分析显示，甲烷浓度变化与海拔高度的决定系数为r²=0.39。海拔低于3000米的湖泊，其甲烷浓度仅与水温相关性极强（r²=0.90）；而海拔高于3000米的湖泊，甲烷浓度则与溶解态无机氮（dissolved inorganic nitrogen）与溶解态无机磷（dissolved inorganic phosphorus）的比值及海拔高度相关（r²=0.48）。本研究结果深化了对甲烷浓度时间变化规律的认知，证实淡水湖泊的甲烷浓度与扩散通量均存在显著的季节波动特征。研究表明，在大尺度甲烷排放估算中应当纳入时间变化的影响因素；同时，湖泊甲烷扩散通量可通过海拔、水温与营养盐条件实现可预测性。