Freshwater jellyfish observed in Upper Sûre Lake; Credit: GSAQ-CGDIS / Jean-Luc Linster

Luxembourg's Nature and Forest Agency (Administration de la nature et des forêts - ANF) and Water Management Authority (Administration de la gestion de l'eau - AGE) have announced the detection of jellyfish and cyanobacteria (blue algae) in the Upper Sûre Lake in north-western Luxembourg.

At the beginning of September, the authorities received reports of freshwater jellyfish in the Upper Sûre Lake; divers from the CGDIS sent photographs of these jellyfish to the ANF and AGE. This observation confirmed the presence of Craspedacusta sowerbii or peach blossom jellyfish in the lake. Unlike other species, the freshwater jellyfish poses no health risk to humans.

Native to Asia, the freshwater jellyfish has mostly spread to other parts of the globe due to the introduction of aquatic plants and fish. This invasive alien species does not exceed 25mm in diameter and prefers calm, stagnant and clean waters that warm up considerably in summer. Since these jellyfish needs temperatures often exceeding 20 °C over a long period of time, they tend to appear from July to October with a peak observed from late August to early September. These jellyfish have also been reported in other neighbouring regions.

Anyone who spots jellyfish can report them to the following email address:

In addition, the AGE reported a worsening of the situation at the Upper Sûre and Weiswampach lakes with regard to blue algae. Measurements carried out on the ground as well as water analyses at the beaches of these two lakes revealed the presence of toxic cyanobacteria in high densities and the presence of toxins which can cause health problems in humans and animals. Consequently, the authorities have issued a ban on swimming and all water sports with immediate effect for the whole of Upper Sure Lake and Weiswampach Lake. The ban on nautical activities in the Remerschen lakes also remains in place.

The public is advised to prevent pets from drinking water from the lake and to avoid eating fish caught in these areas. In case of health problems experienced after swimming, contact a doctor and mention the possible contact with toxic cyanobacteria. The authorities assured that there is no risk associated with the consumption of drinking water from the Upper Sûre Lake as the raw water is withdrawn at a depth (25 m) where the concentration of cyanobacteria is very low and the treatment system in place can effectively remove toxins from cyanobacteria.

The authorities also stressed that the presence of blue algae is a phenomenon totally independent of the presence of freshwater jellyfish.