Eelgrass ecosystem collapse and social-ecological regime shift driven by hydropower development and climate change.

Global declines in coastal habitats and wildlife have devastating consequences for Indigenous Peoples. In some places, hydropower development has environmental impacts that contribute significantly to coastal habitat loss. As development is rejuvenated for the carbon-neutral transition, mitigation of further coastal habitat impacts depends on assessing the relative and interactive effects of climate change and hydropower development. Here, we follow a detection and attribution framework to understand the collapse and limited recovery of a social-ecological system including eelgrass (Zostera marina, shikaapaashkw), geese (Branta spp., nisk and iyiwaapuwaau), and Cree (Eeyou) in subarctic eastern James Bay, Canada (Eeyou Istchee). Informed by historical observations, Indigenous knowledge, and scientific research, we attribute the collapse of eelgrass and shifts in the associated social-ecological system to ecological effects of hydroelectric development (1978-1996), which degraded local environments causing eelgrass declines. Climate-driven extreme events (1998 onward) compounded losses and ecological feedbacks extended eelgrass declines all along the coast. Eelgrass recovery today appears limited by poor water clarity, altered sediments, hydrologic changes from development, and a new climate regime. Eeyou have adapted by shifting their relationships with the coastal habitat. Comprehensive assessments for hydropower developments should explicitly consider coastal social-ecological dynamics and the possible compounding impacts of climate change.