To assess that risk, when bathing water suddenly changes colour, a cyanobacterial count is made and, in certain cases, toxin dosing is performed (with the outcome generally stated in terms of microcystine LR quantities).
The ARS offices contribute to gathering information about the presence of cyanobacteria at freshwater bathing sites.
In seawater, algae bloom phenomena are monitored through the Phytoplankton Network (REPHY) set up in 1984 by IFREMER (the French Research Institute for the Exploitation of the Sea). REPHY data can, where appropriate, be complemented by that of various research organisations, university laboratories, or institutions not directly involved in the regulatory process, such as the CERBOM (Centre for Research on Biology and Medical Oceanography in the Mediterranean).
In seawater, blooms are often spectacular to the eye, but few actually hold health risks for bathers. The toxicity affects above all the filtering shellfish that can later become unsuitable for consumption. (
shellfish picking).
In freshwater, risk arises from the toxins released by cyanobacterial algae (often referred to as blue algae). These develop in particular in shallow, warm, still waters with high nutrient content.
Depending on type, the toxins can, at certain thresholds, be the cause of somatic disorders of differing type and intensity, such as acute stomach flu, or even neurological impairment. Such disorders occur upon ingestion or possibly inhalation of contaminated water.
Do not enter the water if bathing is prohibited in a specific area based on the results of cyanobacterial analysis.