📋 Tender Overview
As part of regular monitoring of the quality of bathing water, a significant proliferation of cyanobacteria, commonly called blue-green algae, was noted in the upper lake of Weiswampach. Analyzes on July 28, 2026 revealed the presence of Mycrocystis aeruginosa, a toxic cyanobacteria, and toxins beyond the action threshold. Given the health risks, a swimming ban is pronounced with immediate effect for the upper lake of Weiswampach in accordance with the amended Grand-Ducal regulation of May 19, 2009 determining special protection measures and monitoring programs for the state of bathing waters. Indeed, certain species of cyanobacteria produce toxins which can lead to a health risk for humans, aquatic fauna, domestic animals and livestock. As a result, the Luxembourg Institute of Science and Technology (LIST) was mandated to monitor the proliferation of cyanobacteria in official bathing areas. Massive blooms of cyanobacteria were seen during this monitoring on July 28, 2026 (see the national cyanobacteria alert plan). The disorders linked to contact, ingestion or inhalation of these substances are of variable nature and intensity and can for example include headaches, skin irritations and/or burns or even nausea. If you experience health problems after swimming, please contact your doctor, mentioning possible contact with toxic cyanobacteria. Furthermore, for bodies of water in which a significant proliferation of cyanobacteria has been noted, the Water Management Administration, based on the assessment of the Health Directorate, also recommends to the public: not to let dogs and other domestic animals drink the water, to avoid any contact with the water, to avoid any water activities (e.g. water sports, fishing). And in particular when fishing: to avoid contact with fish, not to consume caught fish, to give up fishing activity if cyanobacteria blooms are visible. Any observation of blue-green algae in the Grand Duchy of Luxembourg can be reported via the BloominAlgae app and will be assessed by a LIST expert. All reports can be viewed on cyanowatch.lu. To this day, swimming remains permitted on the lower lake. The current situation regarding the opening or closing of official bathing sites can be followed on the website of the Water Management Administration: www.waasser.lu. Additional information(s): How do blue-green algae blooms form? Planktonic cyanobacteria need light, heat and nutrients to grow. In Luxembourg, cyanobacteria generally proliferate between August and October, in calm, nutrient-rich waters such as lakes, ponds and the Moselle. In general, episodes of proliferation of cyanobacteria are observed more and more frequently on all continents. Planktonic cyanobacteria blooms occur primarily in stagnant waters (very slow-moving water bodies and rivers) in which there is an excessive supply of nutrients, leading to plant overgrowth, oxygen depletion, and ecosystem imbalance. To develop, they need high concentrations, especially phosphorus, the inputs of which can have multiple origins: livestock effluents, compost, sludge from wastewater treatment plants, fertilizers spread on the soil, discharge of insufficiently treated wastewater, soil leaching during heavy rainy periods. Reducing phosphorus and nitrogen inputs into surface waters remains today the only sustainable way to protect and/or restore these ecosystems from proliferations of planktonic cyanobacteria. This is the aim of the measures planned in the third management plan for the Luxembourg parts of the international river basin districts of the Rhine and the Meuse. The modernization of sewage treatment plants, the reduction of the use of fertilizers in agriculture and the planting of riparian strips along agricultural plots are among the measures helping to reduce the input of nutrients into waterways. The factors and processes regulating cyanobacteria proliferations being particularly complex, these phenomena are often difficult to predict.
