(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.
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.
Hascoet told CNN that authorities need to do more to investigate the cause of death when people die in areas where the sea lettuce, or ulva lactuca, is present.
"There are 600 deaths per year related to swimming, so we don't say that every time it's because of algae, but we're angry because it's a hidden phenomenon," he said.
"We ask that every time someone dies, we need an investigation."
Anniet Laverman, a microbiology researcher at the French National Center for Scientific Research (CNRS) explained that more of the plants are observed in areas where there is a lot of human activity, including agriculture and the disposal of waste water in the sea.
"Algae when they are alive, they are plants and they do not pose a problem," Laverman told CNN. "But when they are dead, they degrade."
When the sea lettuce breaks down it releases hydrogen sulphide, or H2S, she explained, a toxic gas with a smell that has been compared to rotten eggs.
"People can smell the smell of H2S but are not aware that it's a very toxic gas smell," said Laverman.
Ines Leraud, an investigative journalist who recently published a book on the algae problems in Brittany, told BFMTV the problem is worsening due to climate change.
Normally dead algae is collected from the beaches every morning but there is so much this year that it's not possible to keep up, she said.
Leraud believes that there is a lack of transparency around the issue.
"There are several taboos in this story: there is a taboo on the origins of the algae and there is a taboo on the effects of the algae," she said.
CNN has attempted to contact the Saint-Brieuc mayor's office for comment.