Large-scale disturbances can profoundly alter aquatic ecosystems by modifying water chemistry, sediment dynamics, and biodiversity across connected freshwater and marine environments. The 2015 Fundão Dam collapse in Brazil provides an unprecedented context to evaluate how multiple aquatic communities respond to intense and prolonged contamination, revealing key mechanisms of community reorganization under chronic anthropogenic impacts. By a multi-taxa monitoring dataset collected from 62 freshwater and marine sites, we analyzed how different communities, comprising phytoplankton, zooplankton, macroinvertebrates, and fish, responded along a gradient of mining tailing exposure. Applying threshold indicator taxa analysis (TITAN), we identified critical environmental variables, capturing tipping points for positive and negative biotic responses to tailing-related environmental gradients. Our results showed that mining-related contaminants, combined with hydrogeochemical alterations triggered by the dam collapse, have driven long-term changes in biodiversity, reshaping ecosystem functioning. The same stressors that caused widespread negative responses in community structure also promoted localized increases in opportunistic, mainly planktonic taxa, indicating a multidirectional community reorganization. In general, small increases in metals and hydrogeochemical alterations (such as turbidity up to 17 NTU in riverine systems and 2 NTU in the marine sites), resulting from the dam collapse have driven persistent changes in biodiversity, ultimately reshaping ecosystem functioning. Integrating community-level thresholds should represent a critical approach to improve the detection of regime shifts and to support the development of habitat-specific recovery strategies for riverine and marine ecosystems affected by mining disturbances.
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