When dozens of dolphins began washing up dead on Brazil’s Lake Tefé in Amazonas state, hydrologist Ayan Fleischmann was sent to investigate—and what he found was alarming.
A brutal drought and an extreme heatwave that started in September 2023 had turned the lake into a virtual cauldron, with water temperatures soaring to 41°C—hotter than most spa baths. The findings, published Thursday in Science, highlight the severe impact of climate change on tropical regions and freshwater ecosystems, just as the UN’s COP30 climate talks are underway in Brazil.
“You couldn’t even put your finger in the water,” said Fleischmann, of the Mamirauá Institute for Sustainable Development in western Brazil. He described the psychological toll of seeing Amazon River dolphins and tucuxis, another freshwater dolphin species, dead along the shores.
Tropical lakes like Tefé, critical to local food security and livelihoods, have long been understudied compared to their European and North American counterparts, and were previously thought to be relatively stable. Yet this study shows they are increasingly vulnerable to climate extremes.
During fieldwork in 10 central Amazonian lakes, the team found that five recorded daytime water temperatures exceeding 37°C, far above the 29–30°C norm. Lake Tefé was the most extreme, shrinking to just 25% of its surface area, with high temperatures persisting throughout the two-meter-deep water column.
Computer modeling identified four key factors driving the intense heating: strong solar radiation, shallow waters, low wind speeds, and high turbidity. Each factor reinforced the others—shallow waters trap heat, turbidity increases water heating, and low wind prevents cooling—creating deadly conditions for aquatic life.
The thermal stress was accompanied by wide temperature swings, with daytime peaks of 41°C dropping to 27°C at night. While the deaths of over 200 dolphins in under two months drew global attention, the event reflects a broader ecological crisis: fish populations also plummeted, and stressed algae caused a red phytoplankton bloom, set to be detailed in another forthcoming study.
Long-term analysis of NASA satellite data dating back to 1990 revealed that Amazonian lakes are warming at roughly 0.6°C per decade, faster than the global average.
“The climate emergency is here, there is no doubt about it,” said Fleischmann, who will attend COP30 to advocate for long-term monitoring of Amazonian lakes and for the inclusion of Indigenous, riverine, and Afro-descendant communities in shaping solutions.
