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Higher temperatures exacerbate drought conditions by increasing evaporation rates, reducing soil moisture and altering precipitation patterns. As global temperatures rise as a result of climate change, these effects intensify, leading to more frequent and severe droughts. This link between higher temperatures and drought is particularly evident in sensitive ecosystems like the Amazon rainforest, where reduced rainfall and higher evaporation rates result in significantly lower water levels, threatening biodiversity and human livelihoods. As an example, the serious drought experienced in the Amazon basin in 2023 resulted in a significant decline in fish populations. Elevated water temperatures, reaching up to 38°C, led to mass mortality events, because these temperatures surpass the thermal tolerance of many Amazonian fish species. We know this because our group has collected data on critical thermal maxima (CTmax) for various fish species over multiple years. Additionally, warmer waters can cause hypoxia, further exacerbating fish mortality. Thus, even Amazon fish species, which have relatively high thermal tolerance, are being impacted by climate change. The Amazon drought experienced in 2023 underscores the urgent need for climate action to mitigate the devastating effects on Amazonian biodiversity. The fact that we have been able to link fish mortality events to data on the thermal tolerance of fishes emphasizes the important role of experimental biology in elucidating the mechanisms behind these events, a link that we aim to highlight in this Perspective.

Amazonia is home to 15% (>2700, in 18 orders) of all the freshwater fish species of the world, many endemic to the region, has 65 million years of evolutionary history and accounts for 20% of all freshwater discharge to the oceans. These characteristics make Amazonia a unique region in the world. We review the geological history of the environment, its current biogeochemistry and the evolutionary forces that led to the present endemic fish species that are distributed amongst three very different water types: black waters [acidic, ion-poor, rich in dissolved organic carbon (DOC)], white waters (circumneutral, particle-rich) and clear waters (circumneutral, ion-poor, DOC-poor). The annual flood pulse is the major ecological driver for fish, providing feeding, breeding and migration opportunities, and profoundly affecting O2, CO2 and DOC regimes. Owing to climate change and other anthropogenic pressures such as deforestation, pollution and governmental mismanagement, Amazonia is now in crisis. The environment is becoming hotter and drier, and more intense and frequent flood pulses are now occurring, with greater variation between high and low water levels. Current projections are that Amazon waters of the near future will be even hotter, more acidic, darker (i.e. more DOC, more suspended particles), higher in ions, higher in CO2 and lower in O2, with many synergistic effects. We review current physiological information on Amazon fish, focusing on temperature tolerance and ionoregulatory strategies for dealing with acidic and ion-poor environments. We also discuss the influences of DOC and particles on gill function, the effects of high dissolved CO2 and low dissolved O2, with emphasis on water- versus air-breathing mechanisms, and strategies for pH compensation. We conclude that future elevations in water temperature will be the most critical factor, eliminating many species. Climate change will likely favour predominantly water-breathing species with low routine metabolic rates, low temperature sensitivity of routine metabolic rates, high anaerobic capacity, high hypoxia tolerance and high thermal tolerance.