Boats and houseboats stranded in Lake Tefé in the Brazilian Amazon, which was severely affected by drought, on October 3, 2023. (Photo Credit: Bruno Kelly/Reuters via CNN Newsource)

(CNN) — Lakes in the Amazon became hotter than the maximum temperature recommended for hot tubs as unprecedented heat and drought in 2023 turned them into “shallow, simmering basins,” according to a new study — and it proved a death sentence for endangered dolphins.

The world’s lakes are considered sentinels of climate change and are warming dramatically as global temperatures rise. Research, however, has tended to focus on temperate lakes rather than those in the tropics, despite their huge vulnerability to intense warming.

Scientists in Brazil are trying to fill some of these gaps. They used satellite data to analyze 10 Amazon lakes during a devastating climate change-driven heat wave and drought in September and October 2023, which pushed them to the lowest water levels on record at the time.

In half the lakes, daytime temperatures were “exceptionally high,” above 37 degrees Celsius (98.6 Fahrenheit), according to the research published Thursday. These kinds of temperatures are rare in large bodies of water — even in tropical regions. Daytime temperatures in Amazonian lakes are more typically around 30 degrees Celsius (86 Fahrenheit).

Lake Tefé, a roughly 37-mile long lake in northwestern Brazil, endured especially intense heat.

Daytime temperatures in the lake spiked to 41 degrees (105.8 Fahrenheit) in October 2023 — hotter than the average spa bath, and exceeding the 104 degrees Fahrenheit limit the US Centers for Disease Control and Prevention recommends for hot tubs.

Strikingly, this heat was not just on the surface but was also detected at depths of more than 6 feet, leaving nowhere in the lake for fish and dolphins to take refuge.

A dead dolphin at Lake Tefé which flows into the Solimões River, in the city of Tefé, Amazonas state, Brazil, October 1, 2023. (Photo Credit: Bruno Kelly/Reuters via CNN Newsource)

Such high temperatures to these depths are “likely unprecedented, though, we lack long-term observations” to understand what happened in the past, said Ayan Fleischmann, a report author and hydrologist at the Mamirauá Institute for Sustainable Development in Brazil.

Many of the lakes also shrunk dramatically. Lake Tefé’s surface area decreased by 75%, while another lake, Badajós, shrunk by 92%.

The impacts of the extreme heat were severe. The temperatures went “beyond the thermal tolerance of many aquatic animals,” Fleischmann told CNN. It led to an unprecedented mass die off of Amazon river and tucuxi dolphins, the report found.

At Lake Tefé, more than 200 bodies of freshwater dolphins were found between late September and October 2023. It’s not fully known why the dolphins stayed in the hot lake and didn’t seek cooler water downstream towards the Amazon River, but it’s possible the intense heat affected their brains, the study notes.

The temperature spikes also affected wild and farmed fish in the lake, with 3,000 fish dying at one aquaculture pond.

River communities were heavily affected during this period. Thousands of people were left isolated and without proper access to food, water and medicine, the study found. “When aquatic ecosystems are disrupted, Amazonian communities are disrupted,” Fleischmann said.

Children play next to floating houses on the dry banks of Lake Tefé on September 17, 2024, during the most intense and widespread drought Brazil has experienced since records began in 1950. (Photo Credit: Jorge Silva/Reuters via CNN Newsource)

There were multiple factors behind the remarkable heat: extreme drought leading to very low lake levels, murky water able to absorb more of the sun’s energy, intense sunlight, and low winds, which meant less heat was lost through evaporation and nighttime cooling.

While the study focused on 2023, the central Amazon endured another extreme drought in 2024, again pushing lakes to record low levels and causing severe water heating.

A lack of long-term monitoring makes it difficult to understand exactly how fast Amazon lakes have been warming over the decades.

The study authors used remote sensing data for 24 lakes across the region to estimate warming trends. They found average surface water temperatures have increased at 0.6 degrees Celsius (about 1 degree Fahrenheit) a decade over the past 30 years.

“Amazon lakes are facing a long-term warming process, likely associated with climate change and global warming,” Fleishmann said.

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