Hundreds of dolphins found dead in Amazon lake were in water warmer than a jacuzzi, study says

When Dolphins began to appear dead by dozens in Lake Tefé in the Brazilian state of Amazonas, they sent hydrologist Ayan Fleischmann to find out why.

What he and his colleagues discovered was surprising: A brutal drought and extreme heat wave that began in September 2023 had transformed the lake into a steaming cauldron. The lake’s waters reached 41 degrees Celsius or 105.8 degrees Fahrenheit, warmer than most spa baths.

Their findings, published Thursday in the journal Science, highlight the impacts of planetary warming on tropical regions and aquatic ecosystems, and come as the United Nations COP30 climate talks begin in Brazil.

“You couldn’t put your finger in the water,” lead author Fleischmann, of the Mamiraua Institute for Sustainable Development in western Brazil, told News.

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

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

While this study focused on 2023, another record drought occurred in the Amazon a year later. These phenomena are becoming more frequent, severe and long-lasting as a result of human-caused climate change.

In total, the team visited 10 lakes in the central Amazon and found that five were experiencing exceptionally high daytime water temperatures, exceeding 37 degrees Celsius, well above the 29-30 degrees Celsius considered normal.

The most extreme reading came from Lake Tefe, whose surface area shrank by about 75%.

According to the Consumer Product Safety Commission, a hot tub typically has a temperature between 100 and 102 degrees Fahrenheit, or just under 40 degrees Celsius.

Officials with Brazil’s World Wildlife Fund said 153 dolphins were found dead the week of September 23, 2023, including 130 pink dolphins and 23 tucuxi dolphins. Both are listed as endangered on the IUCN Red List and the latter of which are considered “the guardians of rivers and a symbol of good luck,” according to RiverDolphins.org.

“The climate emergency is already here”

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

Using computer models, the team identified four key factors: strong solar heating, shallow water, low wind speeds and high turbidity, a measure of water turbidity.

These factors reinforce each other. Shallow depth increases turbidity, which traps more heat, while weak winds carry away less heat, leaving the water more exposed to clear skies and bright sunlight.

Another stressor for aquatic life was the large swing between maximums and minimums, with a maximum of 41°C in Tefe followed by a nighttime minimum of 27°C.

Although much national and global attention was focused on the more than 200 dead dolphins recorded in less than two months, they represented just the tip of a broader socio-ecological crisis, in which fish also died en masse.

There was even a phytoplankton bloom that turned the lake red when the algae were stressed, the subject of another forthcoming paper that Fleischmann co-authored.

To understand long-term trends, researchers reviewed NASA satellite data from 1990 and found that Amazon lakes have been warming at about 0.6°C per decade, more than the global average.

“The climate emergency is already here, there is no doubt about that,” said Fleischmann.

He added that he would attend the COP30 summit to advocate for long-term monitoring of the Amazon lakes and for greater inclusion of local populations (including indigenous peoples, non-indigenous riverine inhabitants and Afro-descendant communities) in the development of solutions.

Research published in the journal Hydrological Processes found that droughts “have significant implications for extreme river water temperatures,” as radiation during these periods is more intense while water levels are lower and river velocities are slower.

“Rising river water temperatures can have significant and often detrimental implications for aquatic life, impacting both individual species and entire ecosystems,” said study co-author and University of Birmingham professor David Hannah. “Drought conditions often coincide with high atmospheric temperatures and those trends will become more intense and frequent with climate change.”