outubro, 01 2024

By Fábio de Castro, special for WWF-Brazil 

With extreme drought punishing the Amazon for the second consecutive year, state and federal government agencies have reported that the main rivers in the biome are dropping to critically low levels, resulting in serious environmental, social, and economic impacts. Now, unprecedented data from a platform developed by WWF-Brazil and MapBiomas—monitoring 23 of the more than 60 lakes in the Amazon Basin with similar hydrogeomorphology—reveal that the waters of the lakes connected to these rivers are reaching higher temperatures. This represents a serious risk for aquatic fauna, particularly for dolphins. 

All 23 lakes monitored by the tool have temperatures above the accumulated average for August over the last five years. “The most concerning aspect is that, on September 23, 12 of these lakes already had temperatures exceeding those recorded in 2023, a year when the waters reached extreme temperatures, resulting in a catastrophe for the Amazon dolphin population,” says Helga Correa, a conservation specialist at WWF-Brazil and one of the platform’s coordinators. “These lakes have also experienced 5 to 9 months of average temperatures higher than those observed in 2023, highlighting the physiological stress caused by successive exposure to high temperatures and low water levels.” 

The need to monitor the temperature of the Amazon lakes arose in September 2023, when the biome was experiencing extreme drought. During this period, 330 dolphins of the pink (Inia geoffrensis) and tucuxi (Sotalia fluviatilis) species died in the Tefé and Coari lakes, both connected to the Solimões River in the interior of Amazonas. 

“From that situation, the idea arose to use remote sensing data to obtain information that would allow us to monitor water temperatures, rank areas at greatest risk, and establish priorities for action in light of the serious impacts of the climate emergency on the dolphin population,” highlights Helga. 

According to the Mamirauá Institute, it has been confirmed that the deaths of the dolphins in 2023 were caused by high water temperatures, which reached 40 degrees Celsius at the time. The platform also allows for monitoring the area covered by water in the lakes, as the reduction in quantity is directly linked to warming. 

The researchers' greatest concern at this time is that the levels of the main rivers are well below normal in 2024. On August 30, the Solimões reached a level of minus 94 centimeters, the lowest ever recorded by the Brazilian Geological Survey (SGB) since monitoring began in 1989. The previous record was minus 86 centimeters, set in 2010. The situation continued to worsen, and by September 20, the level had reached minus 206 centimeters. 

“Last year, when the tragedy with the dolphins occurred, we had no data on the variations in water availability and temperature in the lakes. The new tool will allow us to monitor these 23 lakes, which have been identified as vulnerable, track variations by comparing temperature and water availability across different years, and generate alerts to guide emergency actions in the field,” explains Helga. 

According to the results obtained so far from the platform, the increase in temperature compared to the average of the last five years reaches 1.5 °C in locations such as Lago do Rei on the Amazon River. However, the most striking data come from 12 lakes that are even warmer than in 2023, with a difference of up to 0.86 °C observed in Lago Calado, near Manacapuru, AM. 

“This increase may seem small in terms of temperature magnitude, but it serves as a warning of the risk of dolphin mortality, as air temperatures are also high and water quality, due to fires, is reaching critical levels,” says the researcher. 

According to Juliano Schirmbeck, technical coordinator of MapBiomas Água, the platform obtains data from two remote sensing sources: the MODIS sensor from the Terra satellite and the TIRS sensor from the Landsat satellite, which passes less frequently. By combining these two sources, it is possible to track the variations in the average temperatures of the lakes. 

“Landsat has a high spatial resolution, meaning each pixel in the image represents an area of 30 meters. However, it only passes over the region once every 16 days. MODIS, on the other hand, has a lower spatial resolution, with each pixel representing one kilometer. Its temporal resolution is much higher, as it passes every day. By combining the two, we create a robust monitoring tool,” he highlights. 

The team is working with a five-year history of temperature data based on the MODIS sensor. “With this, we calculate the average temperature for each month. Our main reference is the year 2023, which was marked by the critical event of dolphin deaths. This allows us to evaluate current temperatures in relation to the averages and to each month of 2023,” emphasizes the researcher. 

In Lake Tefé, for example, where 209 dolphins died in 2023, temperatures are 0.8 °C above the average of the last five years and 0.2 °C above those of 2023, according to data obtained by the platform. “The problem is that we still have two more months of dry season ahead of us in the Amazon, and these numbers indicate that at the peak of the heat, between September and October, we will experience temperatures well above those recorded last year,” says Schirmbeck. 

Field action alerts 

Mariana Paschoalini Frias, senior conservation analyst at WWF-Brasil and a participant in the platform's development, emphasizes that one of the tool's purposes is to complement the continuous monitoring of dolphins, which has been conducted since the 2023 crisis by a task force led by the Mamirauá Institute and ICMBio (Chico Mendes Institute for Biodiversity Conservation). Additionally, the goal is to expand the platform's use to areas that are still unmonitored and lack any prior information, as this will help scale the problem and assess the level of response needed. 

“The Mamirauá Institute and ICMBio play leading roles in this emergency response group dedicated to monitoring, studying, and rescuing dolphins. The objective of the remote sensing monitoring carried out by the platform is to provide data that allows alerts to be issued to teams directing actions in the field,” says Mariana. “This will also enable other decision-makers at the state and national levels to structure coordinated actions.” 

According to her, although mass deaths of dolphins have only been recorded in the Tefé and Coari lakes, an analysis conducted by researchers from the Mamirauá Institute identified a total of 23 lakes where dolphins are present, which are considered vulnerable areas for water overheating. 

“All 23 lakes have similar geomorphology, and there is potential for the same type of phenomenon to occur. These are elongated lakes that originate in the interior of the forest, widening over large, very shallow areas until they reach a narrower channel where they connect with the Solimões River, draining this superheated water. These characteristics contributed to the deaths of dolphins in the Tefé and Coari lakes, and for this reason, they were chosen as priority areas for monitoring,” explains the researcher. 

Whenever the satellites detect a significant increase in the water temperature of one of these lakes, an alert will be triggered so that teams can move to a specific location and implement the emergency protocol under the direction of ICMBio. “These alerts are essential for giving us enough time to act,” says Mariana. 

Temperatures can rise quickly 

With the current very low levels of the Middle Solimões River, temperatures are expected to rise dangerously in the coming weeks, according to an analysis conducted by the Mamirauá Institute using data collected up to September 2nd . 

According to Ayan Fleischmann, coordinator of the Mamirauá Institute's Geosciences Group, this data was obtained from automatic sensors installed on two floating vessels in Lake Tefé, which measure temperature every 10 minutes at different depths, as well as from monthly monitoring at 24 points on the lake. 

“The level of Lake Tefé is 6.35 meters, about 1.6 meters above the minimum reached on October 23, 2023. When the level is below 7 meters, we observe that the water begins to heat up very quickly during the day. We have already identified that the lake has started to warm in recent weeks. On August 25, it reached 33 degrees at all depths—the highest temperature of the year,” says Fleischmann. 

The overheating of the Amazon lakes is the result of a combination of several factors, including the reduction in water volume, excess solar radiation, and excessively turbid water, which facilitates the diffusion of heat within the lakes, according to the researcher. 

“It has rained a lot in recent days, resulting in a more favorable scenario. As a result, the temperature has not yet reached 40 degrees, which is the dangerous threshold for aquatic mammals, but we are concerned about the coming weeks. Last year, we observed that just 3 to 6 days of intense sun are enough for the temperature to rise rapidly, turning the lake into a real trap for the dolphins,” explains Fleischmann. 

But the risk is not limited to the increase in water temperature. “With water stress—low volume and reduced size of the lake—the dolphins are more vulnerable and exposed to conflicts with fishermen and other negative human interactions,” warns oceanographer Miriam Marmontel, leader of the Amazonian Aquatic Mammals research group at the Mamirauá Institute. “In the last week, at least one dolphin or tucuxi death was recorded each day in Lake Tefé, resulting from interactions with fishing activities, navigation, and direct retaliation,” she adds. 

Find out more:   

The platform developed by WWF-Brazil and MapBiomas is open for consultation and can be accessed by scientists, scholars, and interested institutions through the following link: https://ciencias-wwf.projects.earthengine.app/view/lagos-amazonicos