Many inhabitants of the Morbihan and Loire-Atlantique coastlines have noticed a noticeable change in the color of the ocean this summer. Since August 2021, the waters of the Atlantic coast have indeed taken a red to brown color. This coloring of the water would be due to the proliferation of a microalgae. This is not the first time that this phenomenon has occurred.
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It has been several years that we can observe a seasonal change in the color water on the Atlantic coast, and more particularly along the south coast of Brittany.
La microalgue Lingulodinium polyedra responsible for the red color of the waters
During the summer, the ocean water turns red, even brown. This natural and seasonal phenomenon, closely followed by biologists, is linked to the rapid proliferation of the microalgae Lingulodinium polyedra. These unicellular photosynthetic organisms are part of the family of phytoplanctons. They are also bioluminescents and produce a light bluish when the sea is rough. An impressive sight visible at night.
Lingulodinium polyedra is however recognized as being a toxic microalgae. It produces in particular yessotoxins (toxins lipophilic) which will gradually accumulate in the shells. If the presence of Lingulodinium polyedra is apparently safe for swimming, too high a concentration of the toxin in shellfish can pose a risk to consumers.
Analyzes are thus carried out regularly to measure the yessotoxin concentration in shellfish, in particular in areas where the quantity of microalgae is greatest. To date, the concentrations are well below the safety threshold.
The characteristics of summer 2021 favorable to the proliferation of microalgae
This massive proliferation of microalgae occurs when the supply of nutrients (azote and phosphorus mainly), the water temperature and sunshine are favorable. Under these conditions, these organisms then multiply rapidly: we speak of bloom or efflorescence.
Even though this summer has not been very favorable for tourists, it has therefore been rather favorable for Lingulodinium polyedra. In question: the heavy rains which greatly increased the debits of the Loire and the Vilaine, two rivers flowing into the Atlantic Ocean. The water brought by rivers is indeed soft and loaded with nutrients. The high flows this summer have therefore increased the supply of nitrogen and phosphorus in the ocean, promoting the proliferation of microalgae. This explains why the blooms of Lingulodinium polyedra are mainly observed on the coasts at the mouth of these two rivers.
Eutrophication, a problem for aquatic ecosystems
This excessive supply of nutrients thus creates a significant imbalance in the environment. This eutrophication phenomenon is mainly linked to human activities: nitrogen and phosphorus come mainly from agricultural fertilizers and wastewater discharge. The heavy rains this summer, washing away agricultural land, have dragged large amounts of nutrients into the ocean. The proliferation of microalgae in the face of this massive input is not without consequences: it generates a strong disturbance of the aquatic environment. In addition to producing toxins, these microalgae also consume large amounts of oxygen, causing a asphyxia progressive ecosystems aquatic.
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