In situ observations of biophysical interactions in natural waters typically focus on physical mechanisms influencing biological activity. Yet, biological activity can also drive physical processes in aquatic environments. A community of photoautotrophic, motile and heavy bacteria—Chromatium okenii, which requires light, sulfide, and anoxic conditions to perform anoxygenic photosynthesis, accumulates below the chemocline of the meromictic Lake Cadagno (Switzerland). Upward vertical migration drives bioconvection, which modifies the physical environment of the bacteria‐populated water to create a deep and homogeneous mixed layer of up to 1 m thickness. Continuous convection within the mixed layer and diapycnal diffusivity from its adjacent stratified surroundings determine ecologically relevant gradients. The daytime vertical migration that induce convective instabilities is well‐established. It consists in bacteria swimming upward towards light and accumulating at the upper part of the anoxic layer, leading to a locally‐unstable density excess. However, nocturnal activity has not yet been analyzed. An intensive 48‐h survey was conducted in August 2018 using standard and microstructure profilers, as well as a moored high‐resolution current meter coupled with temperature and turbidity sensors deployed across the mixed layer depth. This survey revealed a persistent mixed layer also during nighttime hours. Using a mixed layer shape model, vertical velocity observations and turbulent dissipation estimates, we conclude that photoautotrophic bacteria continue their vertical migration at night. This nocturnal activity thereby drives “dark bioconvection” and maintains the subsurface mixed bacterial layer in Lake Cadagno throughout the diel cycle.

中文翻译：

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分层湖中生物对流引起的混合层的持久性

对天然水域中生物物理相互作用的原位观察通常集中在影响生物活性的物理机制上。然而，生物活性也可以驱动水生环境中的物理过程。光合自养，运动和重度细菌群落—色木它需要光，硫化物和缺氧条件才能进行产氧的光合作用，其累积量位于茂盛的卡达格诺湖（瑞士）的化学趋化线以下。向上的垂直迁移驱动生物对流，从而改变了细菌聚集水的物理环境，从而形成了厚度达1 m的深而均匀的混合层。混合层内的连续对流以及其相邻分层环境的对流扩散率决定了生态学上相关的梯度。引起对流不稳定性的白天垂直迁移是公认的。它包括细菌朝着光向上运动并聚集在缺氧层的上部，导致局部不稳定的密度过剩。但是，尚未分析夜间活动。2018年8月，使用标准和微结构轮廓仪以及系泊的高分辨率电流表以及在混合层深度范围内部署的温度和浊度传感器进行了48小时的密集调查。这项调查显示，夜间也存在持续的混合层。使用混合层形状模型，垂直速度观测值和湍流耗散估计值，我们得出的结论是，自养细菌在夜间继续垂直迁移。因此，这种夜间活动驱动了“暗生物对流”，并在整个diel周期内维持了卡达尼奥湖的地下混合细菌层。这项调查显示，夜间也存在持续的混合层。使用混合层形状模型，垂直速度观测值和湍流耗散估计值，我们得出的结论是，自养细菌在夜间继续垂直迁移。因此，这种夜间活动驱动了“暗生物对流”，并在整个diel周期内在卡达尼奥湖中维持了地下混合细菌层。这项调查显示，夜间也存在持续的混合层。使用混合层形状模型，垂直速度观测值和湍流耗散估计值，我们得出的结论是，自养细菌在夜间继续垂直迁移。因此，这种夜间活动驱动了“暗生物对流”，并在整个diel周期内维持了卡达尼奥湖的地下混合细菌层。