Shallow lakes are typically found in one of two stable states, a macrophyte-dominated clear water state or a turbid state due to excessive phytoplankton and suspended sediment. Whether shallow backwater lakes in large river floodplains exhibit similar alternate stable states is less understood. This study considers mechanisms, interactions and feedbacks associated with a shift in environmental conditions and biotic community structure in backwater lakes of a hydrologically dynamic floodplain river system. We use long-term data from backwater lakes to show an increase in submersed aquatic vegetation, improved water quality, and resulting shifts in the community structure of aquatic vegetation and fish following a 4 year period of summer low water discharge on the Upper Mississippi River. Backwater lakes in our study span a gradient of environmental conditions. Backwater lakes located in the upper reach of our study area were chronically turbid and support only sparse aquatic macrophytes, whereas those downriver exhibited clearer water and abundant vegetation. An increase in submersed aquatic vegetation in the lower backwater lakes resulted in a fish community shift to more vegetation-associated species. A lesser response in submersed aquatic vegetation abundance and fish community shift was observed in the upper, more turbid backwater lakes. The combination of vegetative cover and turbidity were key environmental variables associated with fish community structure in lower backwater lakes. Turbidity was the key environmental variable associated with submersed aquatic vegetation in both upper and lower backwaters. Providing further insight into the physical, chemical and biological interactions associated with ecological shifts will help guide management and restoration decisions towards more resilient, macrophyte-rich floodplain backwaters.

中文翻译：

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大型洪泛区河流回水湖的年代际趋势和生态变化：密西西比河上游

浅水湖通常处于两种稳定状态中的一种，一种以大型植物为主的清水状态或由于浮游植物和悬浮沉积物过多而导致的混浊状态。大型河流泛滥平原中的浅回水湖泊是否表现出类似的交替稳定状态尚不清楚。本研究考虑了与水文动态洪泛平原河流系统的回水湖泊中环境条件和生物群落结构变化相关的机制、相互作用和反馈。我们使用来自死水湖的长期数据显示，在密西西比河上游夏季低水位排放后的 4 年期间，沉水植被增加，水质得到改善，从而导致水生植被和鱼类群落结构发生变化。我们研究中的死水湖泊跨越了环境条件的梯度。位于我们研究区上游的死水湖泊长期混浊，仅支持稀疏的大型水生植物，而下游的湖泊则表现出更清澈的水和丰富的植被。低回水湖中沉水植被的增加导致鱼类群落转向更多与植被相关的物种。在上游、更浑浊的回水湖中观察到沉没水生植被丰度和鱼类群落变化的响应较小。植被覆盖度和浊度的组合是与低回水湖泊鱼类群落结构相关的关键环境变量。浊度是与上回水和下回水的沉水水生植被相关的关键环境变量。