Our understanding of ecosystem responses to exceedingly large rare flood events is currently limited. We report the resilience of aquatic community to a 100-year record-high flood, and how it varies depending on levels of water pollution, in a fourth-order gravel-bed river in northern Japan. We used data on riparian landscape structure, channel morphology, and community structure of aquatic fauna, which were collected in sites with and without effluent before (1 month–3 years) and after (10 months) the flood. Carbon and nitrogen stable isotope ratios of consumers and basal resources were measured only before (1 year) the flood. We observed aquatic food web with introduced rainbow trout (Oncorhynchus mykiss) as the top predator, with variable relative contributions of basal resources and their pathways to the rainbow trout, under the effects of water pollution. Biofilm-originating dietary carbon became the more dominant resource, with a slightly shorter food-chain length in the polluted sites. The flood led to a loss of riparian forest and a substantial increase in the proportion of exposed gravel bars (5–24%). While the average river-bed elevation changed a little, the localized scours of river bed down to > 2 m were observed with lateral shifts of channel thalweg. Despite the landscape-level physical and structural changes of ecosystem, aquatic community showed a remarkably high resilience exhibiting negligible changes in abundance, except in the polluted site where only fish abundance showed a slight decrease. This study suggests that the abundance of aquatic organisms in gravel-bed rivers is resilient to a flood of unprecedented magnitude in recent history.

中文翻译：

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水生生物群落对砾石床河中100年洪水的高复原力

目前，我们对生态系统对超大型罕见洪水事件的响应的了解有限。我们报告了日本北部四阶砾石河床中水生生物群落对100年创纪录的高洪灾的复原力，以及其如何随水污染水平的变化而变化。我们使用了有关河岸景观结构，河道形态和水生动物群落结构的数据，这些数据是在洪水发生前（1个月至3年）和之后（10个月）在有无污水的地点收集的。消费者和基础资源的碳氮稳定同位素比仅在洪水发生前（1年）测量。我们观察到了引入虹鳟鱼的水生食物网（Oncorhynchus mykiss）是顶级捕食者，在水质污染的影响下，基础资源的相对贡献及其通往虹鳟的途径具有可变的相对贡献。起源于生物膜的膳食碳已成为更主要的资源，受污染部位的食物链长度略短。洪水导致河岸森林的流失，裸露的碎石块的比例大幅增加（5-24％）。虽然平均河床高度变化不大，但随着河道横向移动，观察到了河床局部冲刷> 2 m。尽管生态系统发生了景观层面的物理和结构变化，但水生社区却显示出极高的复原力，其丰度变化可忽略不计，除了在受污染的地方，仅鱼类的丰度略有下降。