Severe drought and soaring temperatures are causing lakes and rivers in the Amazon to reach dangerously high temperatures, threatening species like the Amazon river dolphin, according to a recent study’s preprint.
In 2023, the Amazon experienced its worst drought in recorded history, coupled with the hottest dry season on record. The extreme climate caused the surface area of Brazil’s Tefé Lake to shrink by approximately 75%, with vast sections of the lake reduced to less than 1 meter (3.3 feet) in depth. As air temperatures peaked at 39.1° Celsius (102° Fahrenheit), the little water that remained in the lake quickly overheated. The average surface water temperature for tropical lakes is around 30°C (86°F), but in 2023, Tefé Lake reached 41°C (105°F).
Using advanced hydrodynamic modeling, the researchers found that the entire water column, up to 2 meters (6.6 feet) deep, reached 40°C (104°F), leaving no refuge for animals seeking cooler temperatures.
The study focused on Tefé Lake because of the mortality event observed the same year, 2023, when more than 200 endangered Amazon river dolphins (Inia geoffrensis) and tucuxi dolphins (Sotalia fluviatilis) perished. The researchers say the deaths were likely caused by the extreme water temperatures that year. The scientists caution that with ongoing climate change, temperatures that exceed heat tolerances for aquatic species will become more common.
Tefé Lake is not an anomaly. Five out of the 10 lakes examined in the study had exceptionally high daytime temperatures. The findings reflect a broader, long-term warming trend in Amazonian waters. Satellite data show that the region’s lakes have warmed by an average of 0.6°C (1.08°F) per decade over the past 30 years, the researchers found.
This decadal temperature rise “is a notable increase compared to global averages for lake warming,” Iestyn Woolway, an independent research fellow with Bangor University, U.K., told Mongabay in an email. “This data is critical as it provides a regional perspective that has often been overlooked in broader climate studies.” Woolway was not involved in the study.
Woolway added that the models used to simulate the conditions on Tefé Lake are robust, but on-site data collection would be more precise, a limitation the study’s authors also acknowledge in the paper.
“Overall this study focuses on a specific event in a particular region, and while the data is compelling, more research across different tropical freshwater systems is needed to understand their broader implications,” Woolway said.
The study has been submitted for peer review and preliminary data were presented at the annual meeting of the Brazilian Society for the Advancement of Science.
Banner image: A researcher examines a dolphin that likely died from extreme heat in Tefé Lake. Image courtesy of Miguel Monteiro/Mamirauá Institute.
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