Meuse Turns Green in Liège: Scientists Solve the Mystery

Residents of Liège who noticed striking green reflections on the Meuse river in recent days finally have their answer: the unusual discoloration was caused by a significant proliferation of cyanobacteria, a phenomenon scientists call a “bloom.” Researchers at the University of Liège (ULiège) confirmed the finding after analyzing water samples collected along the river’s quays.

A Rare Phenomenon in the Belgian Meuse

Water samples were analyzed by the laboratory of Professor Annick Wilmotte, a cyanobacteria specialist at ULiège. Microscopic examination revealed the presence of two genera—Microcystis and Planktothrix—both of which are potentially toxin-producing cyanobacteria, according to the university’s official communication.

“This particular color is due to the presence of a bloom of cyanobacteria,” Professor Wilmotte explained. “The origin of the bloom is probably to be found in tributaries where the current is weaker, but further studies of the watershed would be needed to know for sure.”

Scientists emphasize that this is an unusual event for the river. “The presence of planktonic cyanobacteria in rivers remains a rare phenomenon,” noted Jean-Pierre Descy, professor at UNamur and scientific collaborator at ULiège, as reported by L’Avenir. “In the 1990s, there was no Microcystis in the Belgian Meuse.”

Heat and Climate Change as Drivers

The bloom occurred against the backdrop of an exceptional heatwave that has gripped Belgium throughout July 2026. Scientists point to the elevated water temperatures as the primary explanation for the proliferation, with climate change playing an increasingly influential role. Chlorophyll levels of 20 µg/L were measured in the samples—a concentration considered indicative of a bloom, according to Professor Dail Laughinghouse of the University of Florida, who collaborates in Wilmotte’s laboratory.

Green patches were also observed near Visé, where cyanobacteria presence is suspected but not yet confirmed. The exact concentration of cyanobacteria in the river has not yet been determined.

Health Risks and Precautions

While the sight of a green river may be startling, the health risks for humans remain limited. “These algae can release toxic substances that can cause health problems in humans—rare, because we don’t drink Meuse water—and in animals, particularly dogs, which can be poisoned if they drink where a bloom is present,” the ULiège statement noted.

However, the RTBF reported that Professor Wilmotte warned of broader consequences: “In certain conditions, when waters are rich and temperatures are high, they proliferate anarchically and sometimes produce cyanotoxins. We have quite a few problems with, for example, animals. Also, we can no longer fish in waters where there are microcystis. And for humans, recreational waters are banned because children might ingest the bloom, which can be dangerous.”

Beyond the visible green color, the bloom signals a dysfunction of the aquatic ecosystem. Cyanobacteria are part of the normal phytoplankton, but their excessive proliferation indicates an imbalance that can affect irrigation, livestock watering, and aquatic life.

Broader Context: Drought Stresses Wallonia’s Rivers

The cyanobacteria bloom comes as Wallonia grapples with severe drought conditions. Kayak traffic has been banned on the Lesse and Semois rivers since July 9 due to insufficient water flow, and the Lesse at Gendron was flowing at just 1.12 cubic meters per second, as L’Avenir reported. Water restrictions are in place in at least eight communes across the region.

Citizen Science and Future Research

In response to growing concerns about cyanobacteria, the interuniversity network Sciences.be launched the Bloomin’ Algae project at the beginning of summer 2026—the first participatory census of cyanobacteria in French-speaking Belgium. The app allows citizens to photograph blooms, which are then evaluated by experts.

Meanwhile, ULiège is hosting the 23rd Symposium of the International Association for Cyanophyte/Cyanobacteria Research from August 3-7, bringing together international experts on cyanobacteria. The “Bloom Day” on August 6 will be open to the general public, offering an opportunity for Liège residents to learn more about the organisms that turned their river green.

What to Watch For

As the heatwave continues and climate change accelerates, scientists caution that such blooms may become more frequent in Belgian rivers. The presence of cyanobacteria in the Meuse—historically rare—serves as a visible reminder of the pressures facing aquatic ecosystems. Researchers will continue monitoring the river’s condition, with further studies of the watershed needed to fully understand the origins of this unusual event.