Summer storms boost algal blooms that change long-term lake ecosystem dynamics

An important consequence of climate change is the increasing intensity and frequency of extreme weather events that have the potential to change ecosystem structure and function in the short and long term. Exceptional summer storms can alter the physical structure, biogeochemical conditions and biodiversity; yet, our knowledge of their impacts on underlying mechanisms of lake functioning and whole-system responses mainly stem from unreplicated or observational case studies, with no control over site-specific factors. Due to these methodological limitations, observational studies fall short of isolating effects of physical forcing from secondary impacts commonly accompanying storms, e.g. external inputs of suspended solids, colored DOM and nutrients. To overcome these limitations, we performed an ecosystem-level experiment in a unique large-scale enclosure facility ensuring full replication under realistic field conditions. We demonstrate that physical forcing by an extreme storm in deep clear-water lakes entails disruptive changes in chemical and biological variables persisting for weeks. Specifically, diazotrophic cyanobacteria are boosted by mixing both nutrients and seed communities from deeper water layers into the well-lit upper mixed layer. This strongly increases the likelihood for toxic cyanobacteria blooms and changes in biogeochemistry. Experimental results are consistent with a minimal, dynamical model indicating that extreme summer storms induce cascade effects altering ecosystem structure and functioning of lakes, even those that have been least affected by anthropogenic pressures in the past. As these are lakes of particular conservation value, this represents an additional challenge for future lake management.

气候变化的一项重要后果，是极端天气事件的强度与频率持续升高，这类事件可在短期与长期内改变生态系统的结构与功能。罕见的夏季风暴能够改变水体的物理结构、生物地球化学条件与生物多样性；然而，当前学界对其如何影响湖泊功能的内在机制及全系统响应的认知，主要来自无重复设计或观测性案例研究，且无法控制位点特异性因素的干扰。受限于这些方法学缺陷，观测研究难以区分物理扰动与风暴伴随的次生影响——例如悬浮颗粒物、有色溶解有机物（Colored Dissolved Organic Matter）与营养盐的外源输入。为克服上述局限，我们依托一套独特的大型围隔设施开展了生态系统水平的实验，确保在真实野外条件下实现完全重复。研究表明，深水清水湖泊遭遇极端风暴引发的物理扰动，会引发化学与生物变量的颠覆性改变，且这种改变可持续数周之久。具体而言，风暴将深层水体的营养盐与种群种子库混合至光照充足的上层混合层，从而促进固氮蓝细菌（Diazotrophic cyanobacteria）的大量增殖。这会大幅提升有毒蓝藻水华的发生概率，并改变水体的生物地球化学过程。实验结果与一个简化动力学模型的预测一致，该模型显示夏季极端风暴可引发级联效应，改变湖泊的生态系统结构与功能，即便那些过去受人为压力影响极小的湖泊也难以幸免。由于这类湖泊具有极高的保护价值，该研究结果为未来的湖泊管理带来了新的挑战。