ABSTRACT

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.

摘要

淡水湖是大气中甲烷 (CH ₄ ) 的重要来源，但其限制很差，这反映了 CH ₄产量高但可变的速率以及世界范围内有限且不一致的测量。高海拔湖泊尤其被忽视，因为它们在世界各地的山脉中丰度高，高海拔地区CH ₄动力学的潜力预计将受到气候变化导致的温度升高的不成比例的影响。我们研究了加利福尼亚内华达山脉跨越多个海拔的 5 个山地湖泊中地表 CH ₄浓度和扩散通量、温度和无机养分的变化。2 年多来，我们发现 CH 具有强烈且一致的季节性₄湖泊浓度；通常在最温暖的月份观察到较高的浓度，而在冬天之前观察到的浓度较低。CH ₄浓度的变化与大多数个别湖泊的温度显着相关（r ²  = 0.43–0.81），并与 整个湖泊的海拔高度（r ² = 0.39）相关。海拔<3000 m 的湖泊中甲烷浓度与温度、亚硝酸盐浓度和海拔高度相关（r ²  = 0.90），而海拔> 3000 m 时，CH ₄与溶解无机氮与溶解无机磷的比率和海拔有关（[R ² = 0.48）。我们的结果扩展了我们对 CH ₄时间变化的理解，并证明了淡水湖中 CH ₄浓度和扩散通量的显着季节性。我们建议在大规模估算 CH ₄排放时应考虑时间变化，这些通量可能是海拔、温度和养分的可预测函数。