Theoretical models have spurred empirical testing to understand how anthropogenic disturbances cause sudden shifts between alternative functional states. Most studies are done over short periods of time, making it difficult to determine the occurrence of ecological thresholds or regime shifts. This study used carbon stable isotope ratios (δ¹³C) from museum specimens of fish, mussels, and snails over a timeline of 75+ years to ascertain if trophic structure of two hydrologically modified rivers changed between premodification and postmodification periods. An alternative state model following Scheffer's shallow lake model was developed. As long‐lived species, mussels and snails are time integrators with long tissue turnover time and serve as a surrogate for the balance of benthic and pelagic basal food resource contributions to fish biomass. Hydrological datasets over the period of record for the Upper Mississippi (UMR) and lower Ohio (LOR) rivers were placed in 5‐year intervals, as were δ¹³C data for invertivorous and piscivorous fish. The model indicated changes in δ¹³C of fish relative to changes in mean stage height (UMR) and mean minimum stage height (LOR) over time. Evidence based on the model and empirical data of δ¹³C‐stage height relationship suggests a regime shift occurred during the postdam period in the LOR. Postdam, fish from the UMR revealed little change in δ¹³C of invertivores, whereas postdam decreases in variability of δ¹³C for piscivores shifted from a threshold to an unstable equilibrium. Decreases in hydrological variability led to a loss of resilience, with both rivers becoming metastable due to hydrological modification.

中文翻译：

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水文改良河流内的生态阈值和水位变化：75年以上的回顾性分析

理论模型刺激了经验测试，以了解人为干扰如何导致替代功能状态之间突然转变。大多数研究都是在短时间内完成的，因此很难确定生态阈值或政权转移的发生。本研究中使用的碳的稳定同位素比率（δ ¹³C）从超过75年的时间线中的鱼类，贻贝和蜗牛的博物馆标本中，确定两条经过水文修改的河流的营养结构在前期改造和后期改造之间是否发生了变化。在Scheffer的浅湖模型之后，开发了另一种状态模型。作为长寿物种，贻贝和蜗牛是具有较长组织周转时间的时间整合者，可以替代底栖和中上层基础食物资源对鱼类生物量的贡献。以上记录的用于上部密西西比（UMR）和降低俄亥俄（LOR）河流期间水文数据集分别置于5年的时间间隔，因为是δ ¹³为invertivorous和食鱼鱼C数据。该模型表明在δ变化¹³鱼的C相对于平均阶段高度（UMR）和平均最小阶段高度（LOR）随时间的变化。根据δ的模型和经验数据的证据¹³ C阶高度关系说明期间，在LOR的波茨坦期间发生了政权移。波茨坦，从UMR鱼揭示δ变化不大¹³ invertivores的C，而在波茨坦δ的变异性减小¹³ ℃下piscivores从阈值移位到一种不稳定的平衡。水文变异性的降低导致复原力的丧失，两条河流由于水文改良而变得亚稳。