Brussels Environment has confirmed the presence of toxic blue-green algae, or cyanobacteria, in four ponds across the capital, warning residents to avoid contact with the contaminated waters.
The affected sites are Leybeek pond, the Grand Mellaerts and Petit Mellaerts ponds in Woluwe-Saint-Pierre, and Rouge-Cloître 3 in Auderghem.
Two other ponds — located in Parc de Tercoigne and Parc Roi Baudouin — are under close observation, with signs of possible blooms but no confirmed large-scale growth.
According to Bruno Heuze, head of the “Maillage Bleu” department at Brussels Environment, surveillance has been underway since the start of summer.
Park wardens, gardeners, and local associations conduct visual checks, followed by technical measurements to confirm suspected blooms.
Detection uses a multi-parameter probe to measure chlorophyll a, present in all algae, and phycocyanin, a pigment unique to cyanobacteria.
“Last week, we confirmed cyanobacteria in four ponds. This week, two more have been confirmed, prompting us to install public information signs,” Heuze told The Brussels Times. Once a pond is classified as “cyanosé” — meaning cyanobacteria are present — inspections and readings become more frequent.
Outbreaks are becoming “more frequent and more intense” in Brussels, Heuze noted, driven largely by warmer, longer summers.
The two main drivers are high water temperatures and excess nutrients such as phosphorus and nitrogen, which enter ponds through urban run-off, waste discharges, and decomposing organic matter. Reduced rainfall worsens the problem by slowing water renewal.
Cyanobacteria, while not always deadly, can cause headaches, nausea, diarrhoea, skin rashes, and respiratory irritation in humans.
Children and animals face greater risks, including potential neurological effects at high toxin concentrations. Contaminated fish and birds that prey on them can also be affected.
Brussels Environment advises residents to avoid all contact with water from affected ponds, keep dogs on leads nearby, and refrain from consuming fish caught in these areas.
Warning signs have been placed around contaminated sites, and the agency is working with local authorities to ensure information is widely shared.
If toxin levels become critically high, authorities may fence off ponds or, in extreme cases, drain them entirely.
Long-term prevention strategies include reducing nutrient pollution, restoring pond banks, and trialling bio-remediation methods. However, Heuze acknowledged that budget constraints limit the scope of these measures.
Climate change is intensifying the problem, he warned. “Rising average temperatures and more frequent heatwaves create ideal conditions for cyanobacteria. This makes it all the more urgent to tackle other contributing factors within our control,” Heuze said.
As Brussels faces the dual challenge of safeguarding public health and protecting urban biodiversity, officials stress that vigilance from both authorities and the public will be key in mitigating the impact of harmful algae blooms.
