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What is the Ostreopsis algae?

Ostreopsis is a micro-algae invisible to the naked eye that thrives in warm, calm waters near rocky shorelines. It first appeared in significant amounts on the Basque coast in the summer of 2021. Two species are present locally: Ostreopsis cf. siamensis and Ostreopsis cf. ovata, the latter being toxic to humans.

What are the health risks?

Symptoms, generally mild, appear within 6 hours of exposure and usually clear up within 3 to 4 days:

  • Cough, sore throat
  • Runny nose and watery eyes
  • Nosebleeds
  • Breathing difficulty
  • Fever, chills, shivering
  • Muscle pain
  • Skin rashes, irritation

Have you been to a beach, with or without symptoms? Tell us about it (anonymous) →

The right reflexes

Shower after the beach

Take a fresh-water shower after swimming or any activity at the water's edge.

Gut locally caught fish

Gut fish caught locally before eating them.

Vulnerable people: be careful

If you have a history of ENT, lung or cardiovascular conditions, or diabetes, avoid the beaches during high concentrations.

See a doctor if symptoms are marked

If symptoms persist or are severe, see a doctor or call 15 (French emergency medical number).

The monitoring programme

A monitoring programme runs year-round on the Basque coast, led by the Communauté d'Agglomération Pays Basque, which takes the water samples whose results are shown on this platform:

  • Year-round: monthly samples at 4 beaches (Port Vieux in Biarritz, Parlementia in Bidart, Erromardie in Saint-Jean-de-Luz, Deux-Jumeaux in Hendaye).
  • In summer (15 June – 15 September): samples twice a week at 5 beaches (Port Vieux, Parlementia, Cenitz in Guéthary, Mayarco and Erromardie in Saint-Jean-de-Luz).
  • Under heightened vigilance: monitoring extended to 10 beaches, from Petite Chambre d'Amour to Erromardie.
  • Each sample undergoes microscope analysis and molecular analysis (PCR).

Since January 2024, a research programme has brought together the GIS Littoral basque, Rivages Pro Tech, Ifremer, the Water Agency (Agence de l'Eau) and the ARS. Results are published via the Kalilo app and on beach notice boards.

The official vigilance levels

  • Green vigilance — 0 to 30,000 cells/L

    Normal situation, swimming without any particular restriction.

  • Yellow vigilance — 30,000 to 100,000 cells/L

    Increased monitoring, public information displayed on the beaches.

  • Red vigilance — more than 100,000 cells/L

    Health risk: possible restrictions decided by the local councils.

The colour bands shown on this citizen platform's map are indicative readability bands, distinct from these official levels — see the methodology.

Source

Content adapted from the official page of the Communauté d'Agglomération Pays Basque: communaute-paysbasque.fr. Contact: Direction Littoral et milieux naturels, [email protected].