Why the water at Sablettes beach in Menton turned green
Since the start of summer the water in the Sablettes bay in Menton has been taking on an unusual green colour. The cause is a microalga that is actively reproducing in the enclosed bay. It is not dangerous to people, but the authorities are already implementing temporary measures and preparing to rebuild the dam that divides the bay into two basins.
🌊 What is the reason for the green tint?
The water at Sablettes beach changes colour because of the microalga Pyramimonas — a four-flagellated alga that reproduces rapidly in warm, stagnant water with high light levels. The bay’s enclosed geography and underground freshwater springs create ideal conditions for it (France 3 Provence-Alpes-Côte d’Azur).
Main factors contributing to its spread:
- An enclosed bay with limited water exchange.
- High water temperatures.
- Presence of underground freshwater springs.
🧪 Is this alga safe?
According to the Menton municipality, the alga is not dangerous to humans — toxicity tests found nothing. However, its massive proliferation can lead to oxygen depletion in the water and place a burden on the local ecosystem.
«This creates a yellow or bright green sea at very high temperatures», — explains the municipal official of Menton responsible for beach hygiene and safety, Mari-Lor Morgado.
🚜 Temporary measures: mixing the water
In the height of summer the authorities periodically carry out operations to mix the water in the bay using a tug — this saturates the water with oxygen and reduces the intensity of the green tint. Similar measures have been used before and have produced results.
🏗️ Long-term solution: rebuilding the dam
A permanent solution is a 4.1 million euro protection project for the Sablettes bay, led by the SMIAGE Maralpin syndicate together with the Communauté Riviera Française, the Menton town hall, the region and the Alpes-Maritimes department. The first phase — an underwater breakwater and beach replenishment in the northern bay — was completed in April 2026. The second phase starts in October 2026: the central stone section of the dam will be replaced by a pontoon on piles, raised by 45 cm to allow for sea level rise — this should improve water circulation between the two basins of the bay (SMIAGE, Alpes-Maritimes department).
🌍 Environmental consequences and measures
Works at the dam are carried out with regard for protected species that have established themselves in its stone structures — ecologists insist on preserving biodiversity during the reconstruction.
🏖️ What lies ahead for Sablettes beach
After the work is completed the bay should return to its usual blue colour — improved water exchange will reduce the frequency of algal blooms in the future. A similar situation has already occurred on the neighbouring beaches of Menton and on Sablettes beach, which is famed for its Italian charm.
🔹 Note: the combination of a hot summer and poor water exchange in an enclosed bay is the main factor accelerating microalgal blooms along the Côte d’Azur.
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Photo: AXP Photography / Pexels