Green algae

Green algae are part of Brittany’s marine ecosystem. They are commonly known as “sea lettuce” because of their salad-like appearance and the fact that they are edible. In spring and summer, their development in large quantities can cause major strandings, commonly known as “green tides”.

For the comfort and safety of visitors, local councils can restrict or even temporarily close access to beaches (for swimming, shore fishing, other leisure activities, etc.) by means of appropriate municipal by-laws while they are being treated and cleaned. To find out more about environmental issues, we suggest you consult the information below: Why does it happen? Where does it occur? Is it dangerous to health? What can be done about it? All the answers can be found on the website:


  • What is a green tide?

    Green algae grow in suspension in seawater, in sandy bays with a gentle slope, where all the right conditions for their growth are present, including :

    • shallow waters, as green algae need plenty of light ;
    • water rich in nitrogen and phosphorus, the two main nutrients for these plants;
    • water that is calm enough to hold the algae and their nutrients in place.

    Over the last thirty years or so, some sites on the Breton coast, as in many other parts of the world, have seen an excessive growth in green algae, which washes up on the shore. This “green tide” phenomenon is linked to an excessive abundance of nutrients, mainly of agricultural origin (fertilisers) and urban origin (phosphorus), carried by rivers to the sea.

  • Where can you find green algae in Brittany?

    The proliferation of green algae began in Brittany in the 1970s. This complex phenomenon depends on numerous parameters (sunshine, temperature, rainfall, tides, storms, sea currents, etc.) that cannot be anticipated with certainty, and whose extent cannot be predicted.

    For example, heavy rainfall in the spring can encourage the transfer of nutrients into rivers and the sea, resulting in a green tide in areas where algae had not proliferated in the drier previous year. Conversely, heavy winter storms can disperse the algae that remained in the autumn and delay their development the following year. The same is true when the end of winter is cold and sunny. This was the case in 2018, resulting in a significant delay in the growth of algae and particularly late strandings.

    It is therefore impossible to predict whether a beach affected one summer will be affected again the following year. However, data does allow us to identify the sites that are most exposed to seaweed growth. There are eight bays where green tides have occurred every year since 1998, varying greatly in scale: the bays of Saint-Brieuc, Douarnenez, la Forêt and la Fresnaye, la Grève de Saint Michel and the coves of Locquirec, l’Horn-Guillec and Guisseny. It should be noted, however, that in recent years the volume of seaweed stranded has fallen overall. This phenomenon is particularly noticeable in the Lieue de Grève bay, near Lannion.

    (Source: Schéma Directeur d’Aménagement et de Gestion des Eaux du Bassin Loire-Bretagne 2010-2015)

  • Are green algae toxic?

    Whether at sea or freshly washed up on the shore, green algae poses no danger to humans. However, when they have been washed up for several days in large quantities and form a layer of several centimetres thick, they begin to decompose, like all organic matter. This process generates ammonia fumes and another highly toxic gas in high concentrations, hydrogen sulphide.
    This is why the public authorities have introduced a range of risk prevention measures. In bays where large quantities of green algae have been washed up in the past, a reinforced surveillance system means that the green algae can be collected as soon as they are deposited on the beach, thus avoiding any decomposition phenomena and any risk of subsequent gaseous emissions.

  • Is it dangerous to swim in the presence of green algae?

    Whether at sea or freshly washed up on the shore, green algae presents no danger to humans.

    Furthermore, the quality of bathing water is regularly monitored by the Agence Régionale de Santé (ARS). As a result, of the 581 bathing sites inspected and classified in Brittany in 2021, 98.5% have water that meets current quality requirements.

    For more information, visit the Water quality page.

  • Is it dangerous to walk where green algae has washed up?

    Once deposited by the sea on a beach or rocks, green algae piles up, dries out and begins to ferment under a crust that forms on the surface. Like all organic matter, their decomposition produces ammonia and another highly toxic gas, hydrogen sulphide.

    Stepping on these algae and breaking through the surface crust will suddenly release these gases into the atmosphere, posing a real health risk due to their concentration. As soon as you smell the strong rotten egg odour characteristic of the presence of hydrogen sulphide, it is advisable to quickly move away from the area.

  • How can you tell whether beached green algae is still fresh and therefore harmless, or whether it is decomposing and dangerous?

    The smell is the best indicator of the freshness of stranded green algae. If the seaweed is decomposing, it produces a number of potentially highly toxic gases, notably hydrogen sulphide, the strong rotten egg smell of which will alert you immediately.

    In any case, if you notice an abnormally high presence of stranded seaweed on a public beach, we advise you, as a precaution, to move to another location while the equipment is being collected. If you notice a green tide or large piles of green algae on an unsupervised beach, or while walking along the coast, we recommend that you move away from the area and, on your return, alert the tourist office or town hall where you are staying.

  • What should you do in the event of a green tide?

    If you notice an abnormally high presence of stranded seaweed on a beach, we advise you to move to another location as a precautionary measure until the collection equipment has finished.
    If you identify a green tide on an unsupervised beach or while walking along the coast, we recommend that you move away and alert the tourist office or town hall where you are staying.

  • When can I go to a Breton beach with my children without risk?

    Every day of the year! In autumn and winter, there is no proliferation of green algae. In spring and summer, public beaches are systematically cleaned to ensure public safety.

  • What measures are being implemented today?

    Brittany’s local authorities are organising the systematic collection of seaweed from accessible beaches and rocky areas to limit the amount of seaweed at source, so as to avoid any inconvenience to tourists who choose to visit Brittany.

    However, if a beach were to be covered in seaweed during a tide, it would be up to the mayor of the commune concerned, in coordination with the prefect, to temporarily block access as a precautionary measure, while it is cleaned up.

  • How can the phenomenon of green tides be eradicated?

    A “plan to combat the proliferation of green algae” was launched in 2010 and renewed in 2017. As well as stepping up preventive measures, the public authorities and farmers in Brittany are committed to in-depth reform of the use of fertilisers, which are responsible of the proliferation of green algae. Only this preventive approach, which involves farming that is more respectful of the environment, will put an end to the proliferation of green algae. Without the excessive presence of nitrogenous nutrients, the algae will return to a normal rate of development and their growth will be naturally regulated, as it has always been the case in Brittany.

    Find out more about the Plan to combat green algae

Official website of tourism in Brittany