"It's hard to look at all this and remember: the carcasses of the dolphins, scattered along the shores of Lake Tefé." This is the moving account of environmental engineer Ayan Fleischmann, upon observing the photograph of a dolphin's carcass being removed by a backhoe from the lake area.

The incident occurred in 2023 and was not limited to the death of only one dolphin in Lake Tefé, in the municipality of the same name in the state of Amazonas. More than 200 pink and tucuxi dolphins were found dead on the shores of the lake. The cause? The warming of the water, caused by climate change.

The discovery comes from a group of researchers from various institutions, led by Fleischmann of the Mamirauá Institute for Sustainable Development, based in Tefé, in the region of Middle Solimões river. The results of the investigations were published this month, in the journal Science, in which they explain the combination of factors that led to the deaths of the dolphin species, both of which are considered endangered, as well as the socio-environmental consequences involved.

According to the study, during the 2023 drought, the surface area of Lake Tefé decreased drastically, with 75% of its area being reduced. From 379 km² it went down to only 95 km², with large areas turning into mere puddles 50 centimeters deep. Between September and October of that year, the temperature throughout the entire water column, which was about two meters deep, reached 41°C on some days, far exceeding the thermal tolerance capacity of many aquatic animals, such as dolphins and fish.

The recorded temperature was well above the average surface water temperature in similar tropical lakes, which is around 29 to 30°C. “Under normal conditions, deeper water is usually colder, serving as a kind of refuge for animals to escape very high temperatures. In the extreme drought of 2023, however, this refuge simply did not exist,” Fleischmann points out.

ANSWERS

Upon noticing the alarming mortality rate of river dolphins in Lake Tefé, the researchers began to wonder what was happening. After all, the causes could be various, such as contamination. But it soon became clear to the researchers what was occurring: along with record temperatures in the air and oceans, the freshwater of the Amazon was also warming up, in an unprecedented way.

The study monitored, in field, not only Lake Tefé, but also nine other lakes in Central Amazonia with similar characteristics, during the same period. Water warming was recorded in all of them, with half reaching temperatures above 37°C and impacting the entire ecological chain. River dolphins are at the top of the food chain in Amazonian lakes, which means that, if they are impacted, the entire ecosystem will likewise be. That year, the death of thousands of fish was also recorded.

According to the research, several factors have contributed to the overheating: in addition to intense solar radiation and low cloud cover, the low wind speed reduced the natural heat loss from the water. With the extreme drought, the waters became more turbid, which causes greater heat absorption, and very shallow, preventing refuge in deeper and colder areas. According to the researchers, the lack of wind was a more decisive factor than the increase in air temperature, which highlights the complexity of the climate system.

Water is getting warmer by an average of 0.6°C per decade

In addition to field studies, researchers sought to expand their investigations using satellite imagery and by considering longer monitoring periods. Although 2023 and 2024 were years with more extreme droughts – with all the impacts this caused to ecosystems – experts say these were not isolated cases, but rather a long-term trend driven by climate change.

“Analyzing temperature data collected by satellites from 24 lakes in the Amazon between 1990 and 2023, we also discovered a clear long-term trend. The waters in the region are warming, on average, 0.6°C per decade, reaching up to 0.8°C per decade in some lakes. Furthermore, above-average warming events have become more frequent in the last decade, warning of a future trend towards increasingly extreme and common events in the region,” warns Ayan. Fleischmann.

The Mamirauá Institute also works in partnership with an initiative of WWW-Brazil and MapBiomas , a platform called “Lagos Amazônicos” [Amazon Lakes]. “These are, for the most part, the same lakes that we study, with some additions. But the idea is to look at a larger scale, and provide real-time answers about what is happening to these lakes. Will they warm up again in an extreme way when a new drought comes? It is important to have monitoring strategies that allow us to provide answers in real time,” says Fleischmann.

PLATFORM

According to Mariana Frias, a Conservation Analyst at WWF-Brazil, the "Lagos Amazônicos" Platform emerged as a response to the 2023 event. "It's a way for us to act quickly, so that we don't have events of high dolphin mortality like the one in that year," she explains.

The platform created monitors 23 Amazonian lakes called ria lakes. “They originate from small tributary rivers, open into a large lake, and flow into the main rivers, which are the Solimões, the Madeira, the Amazon, and the Negro. These systems tend to absorb heat. During the 2023 event, the combination of drought, lack of clouds, and intense solar radiation increased the temperature of these lakes in such a way that they did not have enough time to cool down. Therefore, they turned into a large soup bow, causing the death of the river dolphins, which are endothermic animals, just like us humans. They do not control their body temperature, like alligators, for example, which can leave the water and exchange heat for a higher temperature. Mammals have a basal temperature between 36 and 37°C, and the lakes reached 41°C,” he recalls.

According to Mariana, the platform retrieves data from Google Earth Engine, that is, satellite images that are processed and made available in an online data cloud. “This remote monitoring system can help predict the scenario from one drought to another, observing temperature, depth, water flow, and days of higher radiation. In addition, we also see fire outbreaks, which can contribute to increased local temperatures and disperse ash, which contains a lot of organic matter and can favor, for example, the proliferation of algae, which has a pathological effect already detected,” the analyst points out.

Monitoring allows for quick responses

Ayan Fleischmann recounts that, faced with the dolphin mortality rate in 2023, teams from the Mamirauá Institute, the Chico Mendes Institute for Biodiversity Conservation (ICMBio), and other institutions, with the support of the riverside communities themselves, attempted to save the remaining animals. “On September 28, 2023, we found 70 dolphin carcasses on the shore of Lake Tefé. This had never been seen before. Those who were there that day remember seeing the animals agonizing and dying. It was a tragedy. We tried to understand what was happening and concluded that it had to do with thermal stress. So, we devised a strategy to guide them through the eight-kilometer channel that connects Lake Tefé to the Solimões River, where the temperature was 34°C. It was a mega-operation,” he recalls.

For Mariana Frias, having permanent monitoring like that of the Amazon Lakes platform allows for quick responses from the institutions involved. “We have a decision-making chain already in place, involving various entities, on how to activate emergency and rescue protocols. And at the forefront of all this is the Mamirauá Institute, which has been working with Amazonian aquatic mammals for over 40 years and is a major center of knowledge. What happened in 2023, and also in 2024, wasn't worse only because there is already a solid capacity installed there by the Institute,” says the analyst.

RIPARIANS

Extreme droughts and overheating lakes have also had a severe impact on the population living in the region. Since fish are the basis of the local diet and economy, the high mortality rate of these species has further aggravated the crisis faced by those living along the riverbanks.

“When fish die, whether due to temperature, lack of oxygen, or because the lake dries up, it provokes a huge impact on the food security of these populations. Furthermore, it also affects their income, because fishermen depend on these resources in their daily lives. It is necessary to think about solutions to support these populations, who are increasingly vulnerable to these crises,” Fleischmann indicates.

“In 2023, we had the ecological tragedy, with the death of the fauna in the lakes, but in 2024 the social tragedy was even greater, because the drought lasted longer. So, extreme droughts have brought truly unprecedented socio-ecological impacts. The populations report that they had never seen anything on that scale. It's alarming,” laments the researcher.

INSTITUTIONAL PARTNERSHIP
The production of Liberal Amazon is one of the initiatives of the Technical Cooperation Agreement between the Liberal Group and the Federal University of Pará. The articles involving research from UFPA are revised by professionals from the academy. The translation of the content is also provided by the agreement, through the research project ET-Multi: Translation Studies: multifaces and multisemiotics.