Bathing water, drinking water, fish consumption … the risks of contamination associated with eutrophication and cyanobacteria are numerous, and long-term effects still poorly known.

Eutrophication is the enrichment of water by substances (nitrates and phosphates) favouring the development of plants and, in particular, algae. The decomposition of these plants results in a decrease in the dissolved oxygen content with, at times, the release of noxious gases. This can result in a reduction in aquatic fauna, which is very sensitive to dissolved oxygen content.
Eutrophication is therefore the transition from a nutrient-poor stage (this is referred to as a oligotrophic state) to a rich stage (eutrophic state). It is a natural evolution of a lake environment. This aging process is generally very slow and occurs at the geological time scale. Human activities can greatly accelerate it. It is then a dysfunction of the eutrophication process called dystrophisation (or sometimes eutrophication).
To get an idea of the amount of algae present we use a “Secchi disc”, corresponding to a black and white disc dipped in water in order to measure the depth at which it disappears.


Cyanobacteria (or blue algae) are very small algae, which are not individually visible to the naked eye. Their multiplication makes the water murky and episodes of rapid development, the blooms cause the formation of a layer of oily or granular appearance on the surface of the water. Blooms last about a week, usually in late summer but they can happen all year round. The blooms of some species of algae or some species of cyanobacteria may lead to the formation of toxic substances.
The growth of cyanobacteria is lower than that of most algae. It is favoured by high temperatures (above 25 ° C), nutrient-rich waters and high PH.

All algae and cyanobacteria need nitrogen phosphorus and light. If the water contains little nitrogen in solution in the form of nitrate, some cyanobacteria, capable of assimilating atmospheric nitrogen (in the form of dissolved gas), will consume all the phosphorus and cause the other algae to regress. This competition also manifests itself for the level of light. The regression of sea grass bed located at the bottom of the lakes may be due to the decrease in the light that is attenuated by the suspended cyanobacteria in the upper part of the water.
Therefore, although they breeding is slowler than most algae, cyanobacteria have specific characteristics that allow them to quickly colonize water bodies. Their elimination is extremely difficult.
During the night cyanobacteria gather on the surface. If the wind is weak, they are no longer stirred by the waves and form a scum. The scums are therefore rather visible in the morning. The scum can reach several centimetres thick and sometimes exceed 10 cm. All scum are not hazardous and only an analysis is able to determine whether toxic cyanobacteria are the cause.

Concequences and Health Effects

Contamination may occur during bathing, drinking water or by consumption of fish caught in waters containing cyanotoxins. Their high toxicity can rapidly lead to severe symptoms, rarely leading to a fatal outcome in humans. The effects of ingestion of low doses over a long period of time, for example via drinking water, are to be feared although not well known. Only preventive measures have proven effective. The first is to reduce the amount of phosphorus entering lakes and rivers by not losing sight of the fact that the unit of time to measure results is about ten years.

Nitrogen and phosphorus

Nitrogen and phosphorus are substances that promote eutrophication. Nitrogen is harmful to humans from a concentration of 50 mg / L in drinking water in the form of nitrates and 500 μg / L in the form of nitrites. Nitrate causes anemia and methaemoglobinaemia, especially in children under four years of age. An increased risk of cancers of the esophagus and stomach is suspected.
Phosphorus (or more precisely phosphates) is not toxic to humans except at very high concentrations. Its toxicity is indirect, through the development of biomass and the formation of cyanotoxins and trihalomethanes in drinking water production plants.


Some species of cyanobacteria are likely to produce toxic substances: cyanotoxins. They can act on the liver (hepatotoxins), the heart (cardiotoxin), the nervous system (neurotoxins), the genetic heritage (genotoxin) or the skin (dermatotoxins). They may also have carcinogenic effects or cause hormonal disruptions. The neurotoxins work very quickly, in a few minutes. The action of hepatotoxins is slower. Cyanotoxins are dangerous to humans and animals, as well as zooplankton which feeds on these algae. The same substance may have different actions depending on dose and duration. Cancers are often the result of chronic poisoning. We can see that a cyanobacterium can produce several cyanotoxins and that the same cyanotoxin can be produced by several cyanobacteria. This will, of course, complicate the analyzes. Moreover, the cyanotoxins are not evenly distributed within a body of water. They are often more concentrated on the surface as well as in areas where the cyanobacteria are grouped by the wind. Most cyanotoxins are released by cyanobacteria only when they die. The treatment of blooms with algicides has the consequence of disseminating the cyanotoxins making the water more dangerous than it was before.

Indirect Toxicity

With respect to drinking water, phosphate levels above 1 mg / L can interact with the flocculation process in water treatment plants. A high organic loading of water before treatment can react with chlorine and form carcinogenic substances: trihalomethanes.
For bathing water, and independently of the toxins that can be released into the water, the excessive development of cyanobacteria promotes the proliferation of undesirable organisms. In particular the botulism bacillus can develop rapidly in organic sediments.

Observations on the lake of Cazaux – Sanguinet

here is an overview of the observations of blooms on the lake of Cazaux-Sanguinet or on the waters in communication with it.

Photo taken on 10/06/2010 of the
photo taken on 10/06/2010 of “Canalot”, channel communicating with the Lake of Cazaux-Sanguinet.
Cause of the bloom undetermined with certainty, presumably cyanobacteria.
Photos taken on 10/04/2011 of
photos taken on 10/04/2011 of” Canalot “.
Cause of the bloom: presumably Microcystis aeruginosa.
Viewing blooms microscopically
viewing blooms microscopically
observing blooms under the microscope.
viewing blooms under the microscope.


Chlorophyceae are “green algae” present both in freshwater and in sea water. They can live in the form of isolated cells or in groups of cells (colonies).
The Chlorophyceae contain chlorophyll A and B, hence their name. They also contain orange-coloured carotene.

In early September 2016, red-coloured deposits appeared along the beach between Navarrosse and ISPE.
red deposits on the beach between Navarrosse and Ispe
Red deposits on the beach between Navarrosse and Ispe.

microscopically, microalgae clusters (colonies) are observed, measuring about 100 µm in diameter.
Microscopic observations.
microscopic observations
Microscopic observations.
microscopic observations

These observations were forwarded to the Community of Communes and the Irstea. Despite the color of the deposits on the beach, it is probably a “green algae” (chlorophycée): Botryococcus braunii. In fact the color of this algae can vary from green to red through yellow.
It does not appear to be toxic to humans. There are, however, toxic for certain micro-organisms and fish. Blooms are formed in the presence of high levels of dissolved inorganic phosphorus.