Abstract Mixing dynamics in the bottom boundary layer (BBL) of lakes is of primary importance for mediating mass and heat fluxes across the upper sediment. In lakes with depth of several hundred meters, the BBL mixing is often suggested to be low; however, quantitative information from these depths is extremely rare. We assessed the mixing conditions in the BBL of Lake Iseo, a 256 m deep, meromictic Italian lake, where anoxia and accumulation of phosphorus is a major issue. High-resolution temperature and currents measurements demonstrated regular development of turbulence at 220 m depth, with dissipation rates of the turbulent kinetic energy up to 10−7 W kg−1, characteristic of a shear boundary layer. Analysis of temperature and oxygen dynamics revealed a direct link between the turbulence intensification in the BBL and the passage of a 90-h period basin-scale internal wave. This slow oscillatory motion was attributed to lake-wide internal waves of second vertical mode, whose effect in the upper part of the water column was much less profound. The wave passages were able to increase hypolimnetic velocities above 1 cm s−1, to produce thermal instability across the BBL and to significantly enhance turbulent mixing in the deepest waters. During periods of high velocity bursts, the chemical stratification in the BBL was effectively eroded while direct sediment resuspension was unlikely. The new results reveal the turbulent character of the bottom boundary mixing in deep lakes, highlighting the direct link to wind-driven motions and important effects on the water–sediment exchange of solutes.

中文翻译：

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大型分流伊塞奥湖近底湍流混合的来源和尺度

摘要 湖泊底部边界层 (BBL) 的混合动力学对于调节上层沉积物的质量和热通量至关重要。在数百米深度的湖泊中，BBL 混合通常被认为是低的；然而，来自这些深度的定量信息极为罕见。我们评估了伊塞奥湖 BBL 的混合条件，这是一个 256 m 深的意大利等离子湖，其中缺氧和磷的积累是一个主要问题。高分辨率温度和电流测量表明，在 220 m 深度处有规律的湍流发展，湍流动能的耗散率高达 10-7 W kg-1，这是剪切边界层的特征。温度和氧气动力学分析表明，BBL 中的湍流增强与 90 小时周期盆地尺度内波的通过之间存在直接联系。这种缓慢的振荡运动归因于第二垂直模式的全湖内波，其在水柱上部的影响要小得多。波浪通道能够将下水流速度提高到 1 cm s-1 以上，从而在整个 BBL 中产生热不稳定性，并显着增强最深水域的湍流混合。在高速爆发期间，BBL 中的化学分层被有效侵蚀，而直接沉积物再悬浮不太可能。新结果揭示了深湖底边界混合的湍流特征，