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 <3000 m were strongly related to temperature, nitrite concentrations, and elevation (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.淡水湖泊是大气中甲烷（CH4）的重要排放源，但目前对其排放的约束仍较为薄弱，这既源于甲烷产生速率偏高且波动显著，也与全球范围内的观测数据有限且结果不一致密切相关。鉴于高海拔湖泊在全球山地中分布广泛，这类湖泊此前尤其被研究忽视；同时，气候变化引发的升温预计将对高海拔区域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中5个覆盖不同海拔梯度的山地湖泊，测定了其表层甲烷浓度、扩散通量、水温以及无机营养盐的变化情况。通过为期两年的观测，我们发现湖泊甲烷浓度存在显著且稳定的季节周期性：通常在最温暖的月份浓度较高，而在冬季来临前浓度较低。多数单个湖泊的甲烷浓度变化与水温显著相关（r2=0.43~0.81）；跨所有湖泊的分析显示，甲烷浓度变化与海拔的决定系数r2为0.39。海拔低于3000米的湖泊，其甲烷浓度与水温、亚硝酸盐浓度以及海拔显著相关（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磷比以及海拔相关（r2=0.48）。本研究结果加深了学界对甲烷时间变化特征的认知，同时证实了淡水湖泊甲烷浓度与扩散通量存在显著的季节周期性变化。研究表明，在大规模甲烷排放估算中应考虑时间尺度上的变化；同时，湖泊甲烷扩散通量可通过海拔、水温以及营养盐水平进行预测。