Blue Algae in lake at Remerschen; Credit: Erliefnis Baggerweier

On Tuesday 28 July 2026, Luxembourg’s Water Management Administration issued an alert concerning the presence of cyanobacteria, also known as "blue-green algae", at the upper lake in Weiswampach.

The administration reported that as part of the regular monitoring of bathing water quality, a significant proliferation of cyanobacteria, commonly known as “blue-green algae”, has been detected in the upper lake of Weiswampach, with analyses carried out on Tuesday 28 July revealing the presence of Microcystis aeruginosa, a toxic cyanobacterium, as well as toxin levels exceeding the action threshold.

Due to the health risks involved, a swimming ban has been imposed with immediate effect for lake, in accordance with the amended Grand-Ducal regulation of 19 May 2009 establishing special protection measures and monitoring programmes for bathing water quality.

The administration detailed that health problems linked to contact with, ingestion of or inhalation of these substances vary in nature and severity. They may include, for example:

• headaches;

• skin irritation and/or burning sensations;

• nausea.

If one experiences health problems after swimming, please contact your doctor and mention any possible contact with toxic cyanobacteria.

Furthermore, for bodies of water where significant cyanobacterial growth has been detected, the Water Management Administration, based on the assessment of the Health Directorate, also recommends that the public:

• do not allow dogs or other domestic animals to drink the water;

• avoid any contact with the water;

• avoid all water-based activities (e.g. water sports, fishing).

In particular, when fishing:

• avoid contact with fish;

• do not consume caught fish;

• stop fishing activities if visible cyanobacterial blooms are present.

Any observation of blue-green algae in Luxembourg can be reported via the Bloomin’Algae app and will be assessed by an expert from the Luxembourg Institute of Science and Technology (LIST). All reports can be accessed at cyanowatch.lu.

As of today, swimming remains permitted at the lower lake in Weiswampach.