(CNN) Six beaches around Saint-Brieuc in the French region of Brittany have been closed to the public due to unmanageable quantities of sea lettuce, which local campaign groups say may be linked to two recent deaths in the area.

On July 6 an 18-year-old oyster farmer was found dead in nearby Morlaix Bay and initial tests showed that he may have drowned, according to the local prosecutor's office.

However local campaign group Halte aux Marée Vertes claims that the victim may have been poisoned by hydrogen sulphide, a toxic gas released as the sea lettuce decomposes, reports CNN affiliate BFMTV.

A spokeswoman for the prosecutor's office told CNN that there are concerns over the algae and its potential effects, but he will wait until the results of ongoing tests on the victim are released before discussing more specific information.

Algae blooms are increasing in size due to climate change, according to one expert.

The recent death of a 70-year-old man in Douarnenez Bay raised similar concerns, according to Jean Hascoet, a member of the NGO Eau et Rivières, which has petitioned authorities to investigate both incidents.

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