Contrasting thermal and hypoxic responses of species from blackwater and whitewater rivers

In the Amazon basin, climate change is increasing water temperatures and CO₂ concentrations, which intensify environmental stressors for fish by increasing natural thermal regimes, reducing dissolved oxygen and pH levels. Extreme droughts, such as those recorded in 2023 and the new record set in 2024, have exacerbated these conditions and have led to widespread fish mortality. Amazonian rivers, which are characterized by contrasting whitewater and blackwater systems, provide unique habitats with distinct pHs and temperatures that shape biodiversity and species adaptations. With regional water temperatures projected to increase by 2.2–7.0°C by the end of the century, understanding the expected thermal vulnerability of Amazonian fish has become crucial. This study aims to investigate the influence of water type (blackwater vs. whitewater) on both thermal and hypoxia tolerances and hypoxia responses in different fish species following the extreme drought in 2023 and analyse these responses on individual and taxonomic levels. The results revealed significant variability in both thermal and hypoxia tolerances among species, with fish displaying a lower thermal safety margin in blackwaters. The contrasting thermal and hypoxia tolerances observed between blackwater and whitewater fishes highlight the complexity of their adaptive strategies in response to environmental stressors and are fundamental for predicting future conditions of the Amazonian environment under the current climate change scenario.