Acces to the Website of the Health's Ministry Back to home page home-page
Bathing water quality
Sanitary control organisation
Advices and recommendations
Water and health

Bathing Water Quality and Human Health


Table of contents


Microscopic algae

published on 01/08/2008


In both saltwater and freshwater, the water may sometimes appear coloured – green, red or brown – due to what is commonly referred to as “water blooms” or “algae blooms”. Such phenomena are due to the proliferation of microscopic algae (or phytoplankton) which develop according to the water’s characteristics (amounts of nutritive salts, nutrient salts, nitrogen and phosphorus, temperature…).
Certain bathing areas in saltwater and freshwater are particularly conducive to phytoplankton development at certain times of the year. Some phytoplankton species produce toxins that may impact bather health, with toxicity levels varying according to the toxin quantities produced.
algae bloom in lake
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).
More information:

Health Impacts

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.

Health Recommendations

Do not enter the water if bathing is prohibited in a specific area based on the results of cyanobacterial analysis.