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Last week, Marigot Bay looked every bit as wild as the mouth of an Amazonian river after heavy rains. The water was dark and brown, with a unique local feature: a large mass of Sargassum seaweed that had washed ashore and was decomposing on the left side of the bay. On Monday, May 4, the Territorial Collectivity posted a statement on social media reporting a “slight exceedance of the reference threshold in the bay (…) with an average 12-hour H2S concentration exceeding 1 ppm.” Not exactly clear-cut information for understanding the situation. But the Territorial Collectivity seeks to reassure the public, writing: “This situation, which is moderate at this stage, is part of a context of decomposing Sargassum (sic), with a high risk of new strandings in the coming days.” Surprising.
H2S is none other than the chemical formula for hydrogen sulfide, a compound consisting of sulfur and hydrogen. It is a flammable, colorless gas with the foul odor of rotten eggs, highly toxic and slightly soluble in water.
In its press release, the Territorial Collectivity states that sensors have been continuously monitoring H2S and NH3 concentrations in Marigot and Anse des Cayes since 2024. This monitoring has been entrusted to Madininair since 2025, “ensuring a more rapid response.” In late February, the Territorial Collectivity told the JSB that “the implementation of regular reporting is also being finalized with a platform for the public.” These reports are currently sent only to the Territorial Collectivity, the Regional Health Agency, and the prefecture. The collected data is therefore not available to the public.
What about the Saint-Jean site?
Furthermore, there is no mention of sensors at the Saint-Jean storage area for the Sargassum collected from the beaches. A site regularly cleared of stored seaweed, but rarely before it has begun to decompose. This results in the release of various gases and the seepage of sargassum juice into the ground—or, at times, the formation of foul-smelling pools.
During a meeting held behind closed doors on Wednesday, May 6, at the Collectivité, with Prefect Cyrille le Vely and the national coordinator of the sargassum prevention and control plan at the Ministry of Overseas Territories, Baptiste le Nocher, it was stated that the Collectivité is working to implement a soil decontamination operation in Saint-Jean and to create a protected storage area. However, according to the minutes published by the neighborhood representatives, no figures regarding the current state of the site were presented, nor was a timeline for implementation provided. In a post dated February 4, Territorial Council President Xavier Lédée specifically mentioned “soil decontamination” at the Saint-Jean site. The question of the resources to be used and the steps involved in this decontamination was raised with the Collectivité in late February, as were the budget and timeline for the operation. Response: “The preliminary studies for bringing the temporary sargassum storage site into compliance were submitted in late November 2025. The chosen approach consists of watertight cells with leachate collection and treatment via aerated planted filters. The construction contracts are currently being drafted by the Environment Department and the Technical Services Division.”
Regarding the project to install diversion dams at the entrance to Marigot Bay, followed by Cul-de-Sac and Anse des Cayes, it is currently underway. The one in Marigot, a “test” dam, is expected to be installed sometime next August. For the others, the timeline ranges from two to three years. This system does not include collection at sea.
Territorial Councilor Alexandra Questel attended the meeting on Wednesday, May 6. She stated: “Administrative delays are significantly hindering the Collectivité’s ability to act quickly. For example, the deployment of the barriers, which we have been working on for a year now, is being severely slowed down by these procedures. As of now, we do not expect deployment until August, and even then, that is in the best-case scenario.”
In its latest forecast bulletin on Sargassum strandings, Météo France emphasizes that “numerous strandings, sometimes massive, will continue over the coming weeks.”
Since the publication of this article, a press release from the Territorial Collectivity of Saint Barthélemy has stated that readings taken in Marigot Bay indicate a return to normal.
