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Green seaweed
Update 2 September 2011: Two beaches are currently closed to the public:Saint-Maurice beach in Morieux (Côtes-d'Armor) and Baie des Trepasses (Finistere).
Green seaweed is part of Brittany’s marine ecosystem. The common name for it is ‘sea lettuce’ because it looks like a large lettuce and is edible. In spring and summer, in certain bays, located mainly in the north of Brittany, an abnormal increase in its growth can cause what are known as ‘green tides’. Why does this phenomenon occur? Where does it happen? Is it dangerous to health? What can we do about it?
- Q1: What is a ‘green tide’?
Green seaweed grows in suspension in seawater, in gently sloping sandy bays where the conditions are present that enable it to grow. These are:
- Shallow water, because green seaweed needs a lot of light,
- Water with a high content of nitrogen and phosphorus, the two main nutrients for these plants,
- Water that is sufficiently calm to keep the seaweed and its nutrients in the same place.
Over about the last thirty years abnormal growth of green seaweed that then washes up on the shoreline has been observed at certain sites on the coastline of Brittany, as it has all over the world. This ‘green tide’ phenomenon is linked with the presence of excessive amounts of nutrients in the water, mainly of agricultural (fertilisers, manures) and urban (phosphates) origin, washed down to the sea by the rivers.
- Q2: Where do you find green seaweed in Brittany?
Proliferation of green seaweed appeared in Brittany in the 1970s. This complex phenomenon depends on many parameters (amount of sunlight, temperature, amount of rainfall, tides, storms, marine currents, etc.) that cannot be anticipated with any degree of certainty and the extent of which cannot be predicted.
For example, high rainfall in spring may encourage agricultural fertilisers to pass into the rivers and towards the sea causing a green tide where, the year before, when it was drier, seaweed did not proliferate. On the other hand, major winter storms may alter the contours of the seabed in a bay in such a way that seaweed does not grow there any longer.
So it is impossible to say whether a beach that has been affected one summer will be affected again next year. Despite this, historical perspective enables us to identify the sites most likely to be favourable to seaweed growth.
Eight bays have been identified where green tides, which have varied enormously in scale, have occurred every year since 1988: the bays of Saint-Brieuc, Douarnenez, La Forêt and La Fresnaye, the strand at the Grève de Saint Michel, and Locquirec, L’Horn-Guillec and Guisseny coves.
(Source: Schéma Directeur d’Aménagement et de Gestion des Eaux du Bassin Loire-Bretagne 2010-2015 [Water Management and Development Master Plan, Loire-Brittany Basin 2010-2015])
- Q3: Is green seaweed toxic?
As long as it remains in the sea or has just been washed up onto the beach, green seaweed presents no danger to human beings. However, once it is washed ashore in large quantities and has been there for several days, forming a layer several centimetres deep, it begins to decompose, like all organic material. This process then causes the release of ammonia and another gas that is very toxic in high concentrations, hydrogen sulphide.
This is why the authorities have put in place a set of measures to prevent risks. In bays where large deposits of seaweed have been observed in the past, an enhanced system of surveillance means that green seaweed is collected as soon as it is deposited on the beach. This prevents any process of decomposition taking place and therefore avoids the danger of any subsequent release of gases.
In addition, collection at sea will be trialled this year in the bays of Saint-Brieuc and Lannion to restrict proliferation of the seaweed and prevent it being deposited on the shoreline.
- Q4: Is it dangerous to walk in places where green seaweed has washed ashore?
Once it has been washed ashore on a beach or on rocks, green seaweed piles up, dries out and begins to ferment beneath a crust that forms on its surface. Just as with any other organic material, the process of decomposition then produces ammonia and another very toxic gas, hydrogen sulphide.
Walking on this seaweed and breaking through the crust that has formed on its surface will cause the gases to be released suddenly into the atmosphere, where they present a real danger to health because of their concentration. As soon as you smell the strong odour of rotten eggs that is characteristic of hydrogen sulphide you should quickly move away from the affected area.
- Q5: How can I tell whether green seaweed washed up on a beach is still fresh and therefore safe or decomposing and therefore dangerous?
The odour is the easiest way to tell how fresh washed-up green seaweed might be. This is because when it is decomposing it produces certain gases that are potentially very toxic, in particular, hydrogen sulphide, which has a strong smell of bad eggs that should provide an immediate warning.
In any case, if you notice an unusually large quantity of seaweed washed up on a public beach, we would advise you, as a precaution, to go somewhere else while the machines carry out a clean-up operation. If you notice a ‘green tide’ or large amounts of green seaweed on an unsupervised beach or while you are walking along the shoreline, we would recommend that you keep away from it and, when you get back, alert the tourist office or the town hall of the place where you are staying.
- Q6: What should I do if I come across a ‘green tide’?
If you come across an abnormal quantity of seaweed washed up on a beach, we would advise you to take the precaution of leaving the beach while the machines carry out the clean-up operation.
If you come across a ‘green tide’ on an unsupervised beach or while you are walking along the shoreline, we recommend that you move away from it and, when you get back, alert the tourist office or town hall of the place where you are staying.
- Q7: Is it dangerous to go in the sea where there is green seaweed?
No, not at all, when it is fully in the water green seaweed is not harmful.
In any case, the quality of water for bathing is regularly monitored by the DDASS. So, for example, in 2009, of the 548 beaches monitored in Brittany, only 4% were classified as “possibly polluted for short periods” and none had to be closed (source: Ministry of Health).
For further details, see the “Water Quality” page in this same “Environmental Information” section.
- Q8: When can I visit a beach in Brittany with my children without risk?
Every day of the year! In autumn and winter green seaweed does not proliferate and during spring and summer public beaches will be systematically cleaned.
- Q9: What measures have been implemented today?
The system for routinely collecting green seaweed washed up on public beaches, which was strengthened once again in 2010, together with the trialling of collection in the water to limit the amounts of seaweed at source ought to make it possible to avoid any inconvenience to tourists choosing to visit Brittany.
If, however, a tide were to cover a beach with large quantities of seaweed, then it would be the duty of the mayor of the affected municipality, in collaboration with the Prefect, to close off access temporarily, as a precaution, for the time necessary to carry out a clean-up of the beach.
- Q10: How can the ‘green tide’ phenomenon be eradicated?
An ‘Action Plan for Combating Green Seaweed’ was put forward by the government on 5th February 2010. Over and above the strengthening of preventive measures, local authorities and Breton farmers are working together to make major changes to the use of fertilisers and manures that are causing the proliferation of green seaweed.
