Joint Effects of Water Quality Degradation and Non-Native Fish Invasion on Estuarine Biodiversity and Ecosystem Stability

Water quality degradation and biological invasions represent major drivers of global ecosystem degradation, yet their interactive effects remain poorly understood, especially in anthropogenically disturbed estuarine ecosystems. Here, we employed environmental DNA (eDNA) metabarcoding to characterize multi-trophic biological communities in a subtropical estuary and to assess how water-quality degradation influences the invasion success of non-native fishes. The results show, firstly, the relative sequence abundance of non-native fish, rather than their species richness, emerged as the primary factor negatively associated with native fish diversity. Based on the observational dataset, we identified a statistical breakpoint, and the transition from low to high invasion pressure corresponds to a unimodal (hump-shaped) diversity pattern. Secondly, non-native fish primarily compete for food resources, thereby constraining the ecological niches of native species. Thirdly, a strong interactive effect exists between water quality degradation and non-native fish invasion. Water quality degradation acts not merely as a direct environmental stressor, but also indirectly weakens native communities. Collectively, these findings provide new insights into how declines in water quality interact with biological invasions and highlight the need for integrated invasion management strategies that simultaneously improve water quality and mitigate the impacts of invasive species in estuarine ecosystems.