Cyanobacteria Bloom Caused Meuse’s Green Color in Liège
Researchers at the University of Liège (ULiège) have solved the mystery of the Meuse River’s striking green color in Liège this week. Tests on water samples confirmed the unusual transformation was caused by a significant proliferation of cyanobacteria — a phenomenon known as a “bloom” — according to La Libre Belgique.
A Rare Phenomenon
The green tint appeared on several stretches of the Meuse along the Liège quays during the week of July 27–31, drawing the attention of walkers and residents. Cyanobacteria — formerly called blue-green algae — are among the oldest organisms on Earth and occur naturally in aquatic environments, but under the right conditions they can multiply rapidly, discoloring the water.
Such blooms remain rare in the Meuse. “The presence of planktonic cyanobacteria in rivers remains a rare phenomenon. In the 1990s, there was no Microcystis in the Belgian Meuse,” noted Jean-Pierre Descy, a professor at UNamur and scientific collaborator at ULiège, as reported by DH Net.
What the Analysis Found
Water samples were collected and analyzed by the laboratory of Prof. Annick Wilmotte, a cyanobacteria specialist at ULiège. Microscopic observation identified two genera — Microcystis and Planktothrix — both potentially capable of producing toxins. Researchers measured a chlorophyll level of 20 micrograms per liter, a threshold considered indicative of a bloom, although the exact quantity of cyanobacteria in the river has not yet been determined.
The proliferation signals a dysfunction of aquatic ecosystems, ULiège said, rather than a simple aesthetic curiosity.
“This particular color is due to the presence of a bloom (or proliferation) of cyanobacteria,” Wilmotte said in a ULiège communication quoted by Qu4tre. “The origin of the bloom is probably to be found in tributaries where the current is weaker, but more in-depth studies of the watershed would be needed to know this.”
Heatwave and Broader River Stress
Scientists attribute the bloom primarily to the high temperatures recorded in recent days, with climate change cited as a factor that favors such events. In an interview with RTBF, Wilmotte explained that when waters are nutrient-rich and temperatures are high, cyanobacteria proliferate and can produce cyanotoxins, leading to restrictions on fishing and recreational waters. The toxins can target animals, fish and birds, and when a bloom decomposes it can create zones without oxygen in lakes and ponds. She added that it is not certain the bloom will disperse quickly with the first rain.
The episode is part of a broader pattern of stress on Wallonia’s waterways this summer. Drought has left river levels exceptionally low, with water restrictions imposed in several communes, and the Sambre in Namur also turned green in late July under similar conditions, as L’Avenir reported.
Health Risks and Precautions
Cyanobacteria can release toxic substances that may cause health problems. For humans, the risk is considered low because Meuse water is not used for drinking at the affected urban stretches. However, contact can cause eye or skin irritation, and in some cases gastric or intestinal problems.
Dogs face a more serious threat: they can be poisoned if they drink in areas with high concentrations of cyanobacteria. The bloom can also disrupt aquatic ecosystems and affect irrigation and livestock watering. Green traces have also been observed near Visé, where ULiège suspects cyanobacteria are present but has not yet confirmed it.
What’s Next
Researchers say further studies of the watershed are needed to pinpoint the bloom’s origin. The episode serves as a visible reminder that rising temperatures are increasing pressure on major rivers, and scientists will be watching whether such blooms become more frequent in the Meuse in the years ahead.