What he and his colleagues discovered was startling: a brutal drought and extreme heat wave that began in September 2023 had transformed the lake into a steaming cauldron, its waters reaching 41 degrees Celsius -- hotter than most spa baths.

Their findings, published Thursday in the journal Science, spotlight the impacts of planetary warming on tropical regions and aquatic ecosystems, and come as the UN's COP30 climate talks kick off in Brazil.

"You couldn't put your finger in the water," lead author Fleischmann, of western Brazil's Mamiraua Institute for Sustainable Development, told AFP.

He recalled the "psychological impact" of seeing the carcasses of Amazon River dolphins and tucuxis, another freshwater dolphin species.

It's an "overlooked problem," he said, adding that tropical lakes, essential for the food security and livelihoods of local communities, have been far less studied than those in Europe and North America, and were assumed to be relatively stable.

While this study focused on 2023, another record-breaking drought happened in the Amazon a year later. Such events are becoming more frequent, severe, and longer-lasting as a result of human-caused climate change.

In all, the team visited 10 central Amazonian lakes, finding that five experienced exceptionally high daytime water temperatures exceeding 37C, far higher than the 29-30C considered normal.

The most extreme reading came from Lake Tefe, which saw its surface area shrink by about 75 percent.

- Socio-ecological disaster -

What made it even more remarkable, said Fleischmann, was that the same temperature was found not just at the surface but throughout the two-meter-deep water column.

Using computer modeling, the team identified four key drivers: strong solar heating, shallow waters, low wind speeds, and high turbidity -- a measure of water haziness.

These factors reinforce one another. Shallowness increases turbidity, which traps more heat, while low wind carries less heat away, leaving the water more exposed to clear skies and intense sunlight.

Another stressor for aquatic life was the large swing between highs and lows, with the peak of 41C in Tefe followed by a nighttime low of 27C.

Though much of the national and global attention focused on the more than 200 dead dolphins recorded in under two months, they represented only the tip of a wider socio-ecological crisis, with fish also dying in droves.

There was even a phytoplankton bloom that turned the lake red as the algae came under stress -- the subject of another forthcoming paper Fleischmann co-authored.

To understand longer-term trends, the researchers combed back through NASA satellite data beginning in 1990, finding that Amazonian lakes have been warming at roughly 0.6C per decade, higher than the global average.

"The climate emergency is here, there is no doubt about it," said Fleischmann.

He added he would be attending the COP30 summit to advocate for long-term monitoring of the Amazon's lakes and for greater inclusion of local populations -- including Indigenous peoples, non-Indigenous riverine dwellers, and Afro-descendant communities -- in developing solutions.