DataSheet_1_Nearshore subtidal community response during and after sediment disturbance associated with dam removal.docx

Dam removal is used increasingly to restore aquatic ecosystems and remove unnecessary or high-risk infrastructure. As the number of removals increases, there is a growing understanding about the hydrologic, geomorphic, and ecological responses to these removals. Most dam removal studies, however, focus on river and watershed responses to dam removal. The removal of two dams on the Elwha River provided a unique opportunity to characterize the response of nearshore (coastal) ecosystems. We conducted SCUBA surveys between 2011 and 2022 to quantify trajectories of change in a nearshore ecosystem during and after dam removal. We focused on the degree to which the abundances of kelp, benthic invertebrates, and fish changed in response to patterns of sediment fluxes during and after dam removal. Our findings point to two pathways of response depending on the disturbance mechanism and species type. Sites with persistent sediment deposition were characterized by wholesale community changes that did not recover to a before dam removal condition. Instead, the sites were colonized by new species that were largely absent prior to dam removal. Sites that experienced high turbidity but lacked persistent seafloor deposition were primarily characterized by a reduction in the abundance of kelp and other algae during dam removal and a rapid recovery after sediment flux to the nearshore declined. Dam removal influences on invertebrates and fish at these sites were more variable, benefiting some species and disadvantaging others. In addition to dam removal, sea star wasting syndrome and a marine heatwave exerted distinct controls on subtidal communities during the same period. The loss of the predatory sea star Pycnopodia helianthoides was associated with gains in some of its prey species, and kelp community changes reflected regional trends in ocean temperature and kelp abundance. The results presented here have important implications for understanding the response of marine ecosystems to future dam removals and similar sediment perturbation events.

大坝拆除正日益被用于修复水生生态系统，并拆除冗余或高风险的水利基础设施。随着拆除案例数量的增长，学界对大坝拆除引发的水文、地貌及生态响应的认知也不断深化。然而，绝大多数大坝拆除相关研究均聚焦于河流与流域对大坝拆除的响应。埃尔瓦河（Elwha River）两座大坝的拆除，为刻画近岸（海岸）生态系统的响应提供了难得的研究契机。本研究于2011至2022年间开展水肺潜水（SCUBA）调查，以量化大坝拆除期间及拆除后近岸生态系统的变化轨迹。本研究重点探究了海带、底栖无脊椎动物与鱼类的丰度变化幅度，及其对大坝拆除期间及拆除后沉积物通量变化模式的响应。本研究结果显示，生态系统的响应路径存在两类，具体取决于扰动机制与物种类型。持续发生沉积物沉积的区域，其群落发生了整体性更替，且无法恢复至大坝拆除前的状态；取而代之的是，这些区域被大坝拆除前几乎不存在的新物种所定植。经历了高浊度但未发生持续性海底沉积的区域，其主要特征为：大坝拆除期间海带及其他藻类的丰度下降，而当近岸沉积物通量降低后，藻类丰度可快速恢复。大坝拆除对这类区域内无脊椎动物与鱼类的影响则更为复杂，部分物种从中获益，另一部分则受到不利影响。除大坝拆除外，海星消瘦综合征与一场海洋热浪在此期间也对潮下带群落产生了显著调控作用。捕食性向日葵海星（Pycnopodia helianthoides）的消失，与其部分猎物种群的增长存在关联；而海带群落的变化则反映了区域海洋温度与海带丰度的变化趋势。本研究结果对于理解海洋生态系统对未来大坝拆除及类似沉积物扰动事件的响应，具有重要的借鉴意义。