Harmful blooms of the filamentous cyanobacteria Planktothrix rubescens have become common in many lakes as they have recovered from eutrophication over the last decades. These cyanobacteria, capable of regulating their vertical position, often flourish at the thermocline to form a deep chlorophyll maximum. In Lake Zurich (Switzerland), they accumulate during stratified season (May–October) as a persistent metalimnetic thin layer (~2 m wide). This study investigated the role of turbulent mixing in springtime layer formation, its persistence over the summer, and its breakdown in autumn. We characterised seasonal variation of turbulence in Lake Zurich with four surveys conducted in April, July and October of 2018 and September of 2019. Surveys included microstructure profiles and high-resolution mooring measurements. In July and October, the thin layer occurred within a strong thermocline (\(N \gtrsim 0.05\) s\(^{-1}\)) and withstood significant turbulence, observed as turbulent kinetic energy dissipation rates (\(\varepsilon \approx 10^{-8}\) W kg\(^{-1}\)). Vertical turbulent overturns –monitored by the Thorpe scale– went mostly undetected and on average fell below those estimated by the Ozmidov scale (\(L_O \approx 1\) cm). Consistently, vertical diffusivity was close to molecular values, indicating negligible turbulent fluxes. This reduced metalimnetic mixing explains the persistence of the thin layer, which disappears with the deepening of the surface mixed layer in autumn. Bi-weekly temperature profiles in 2018 and a nighttime microstructure sampling in September 2019 showed that nighttime convection serves as the main mechanism driving the breakdown of the cyanobacterial layer in autumn. These results highlight the importance of light winds and convective mixing in the seasonal cycling of P. rubescens communities within a strongly stratified medium-sized lake.

丝状蓝细菌Planktothrix rubescens的有害花朵在过去的几十年中，它们从富营养化中恢复过来，在许多湖泊中已变得很普遍。这些能够调节其垂直位置的蓝细菌通常在温床上旺盛，形成最大的叶绿素。在苏黎世湖（瑞士）中，它们在分层季节（5月至10月）内以持久的金属动力薄层（约2 m宽）堆积。这项研究调查了湍流混合在春季层形成中的作用，其在夏季的持久性以及在秋季的分解。我们通过在2018年4月，7月和10月以及2019年9月进行了四次调查来表征苏黎世湖的湍流季节变化。调查包括微观结构剖面和高分辨率的系泊测量。在7月和10月，薄层发生在强温跃层（\（N \ gtrsim 0.05 \）  s \（^ {-1} \））并经受了显着的湍流，以湍动能耗散率（\（\ varepsilon \约10 ^ {-8} \）  W kg \（ ^ {-1} \））。垂直湍流倾覆（由索普标度监测）几乎未被发现，平均水平低于奥兹米多夫标度所估计的水平倾角（\（L_O \约1 \） 厘米）。一致地，垂直扩散率接近分子值，表明湍流通量可以忽略不计。这种减少的金属电磁混合解释了薄层的持久性，该持久性随着秋天表面混合层的加深而消失。2018年的双周温度曲线和2019年9月的夜间微观结构采样表明，夜间对流是推动秋季蓝细菌层分解的主要机制。这些结果强调的光风和对流混合的重要性中的季节性循环P.冬凌草强分层中型湖泊内的社区。