Methane (CH₄) produced in anoxic sediments plays a significant role in the carbon economy of many lakes and reservoirs. CH₄ released from sediments first crosses the bottom boundary layer (BBL), the layer of water overlying the lakebed where currents are slowed by friction with the sediments below. Physical and biogeochemical conditions in the BBL, which can fluctuate hourly to daily with basin-wide internal waves (seiches), likely influence CH₄ transport from sediments into the hypolimnion. In this study, we estimated CH₄ fluxes across the BBL of a eutrophic lake using a novel in situ flux gradient approach adapted from marine applications. For 2–6 h periods throughout the spring and summer, we estimated CH₄ fluxes across the BBL using simultaneous measurements of CH₄ concentrations, turbulent mixing, and thermal stratification. Sub-daily variation in CH₄ fluxes was high, and CH₄ fluxes sometimes changed several-fold within hours. These rapid shifts in BBL fluxes were likely influenced by fluctuations in seiche-driven variations in the intensity of BBL turbulent mixing. Fluxes increased from spring to summer, concurrent with the development of lake stratification, and fueled an accumulation of CH₄ below the thermocline. Throughout the summer, CH₄ flux across the BBL exceeded CH₄ accumulation below the thermocline, suggesting significant methanotrophy in the hypolimnion, consistent with incubation-based oxidation rates. Our results are the first to demonstrate sub-daily and seasonal variability in the timing and magnitude of CH₄ fluxes within a lake BBL, and highlight a need to quantify such variability in other lentic systems.

中文翻译：

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原位通量估计揭示了富营养化湖泊底部边界层的甲烷通量变化很大

缺氧沉积物中产生的甲烷 (CH ₄ ) 在许多湖泊和水库的碳经济中发挥着重要作用。从沉积物中释放出来的CH ₄首先穿过底部边界层 (BBL)，即覆盖在湖床上的水层，由于与下方沉积物的摩擦，水流会减慢速度。BBL 中的物理和生物地球化学条件可能会随着盆地范围内的内波 (seiches) 每小时到每天波动，这可能会影响 CH ₄从沉积物到下层的迁移。在这项研究中，我们使用一种适用于海洋应用的新型原位通量梯度方法估计了富营养化湖泊 BBL 的CH _4通量。对于整个春季和夏季的 2-6 小时，我们估计 CH ₄通过同时测量 CH ₄浓度、湍流混合和热分层的 BBL 通量。_{CH 4}通量的次日变化很大，CH ₄通量有时会在数小时内变化数倍。BBL 通量的这些快速变化可能受到 seiche 驱动的 BBL 湍流混合强度变化波动的影响。从春季到夏季，通量增加，同时湖泊分层的发展，并推动了温跃层以下CH _{4的积累。}整个夏季，穿过 BBL 的CH _{4通量超过 CH 4}在温跃层下方的积累，表明在低温层中有显着的甲烷营养作用，这与基于孵化的氧化率一致。我们的结果首次证明了湖 BBL 内 CH ₄通量的时间和幅度的次日和季节性变化，并强调需要量化其他静息系统中的这种变化。