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A bird among sargasso seaweed in the Caribbean
Sargassum is a large brown seaweed that constitutes an important habitat for fish and wildlife. But it can proliferate for miles, and become invasive. AFD is a partner of the Sarg'Coop project led by the Regional Council of Guadeloupe, which aims to mitigate the damage caused by sargassum seaweed. The initiative could have striking economic, environmental and health implications.

Sargassum is an invasive brown seaweed, which forms natural floating mats, causing it to drift on the surface of the sea and expand, regularly forming masses of several tens of kilometers along the Caribbean coastline. This is not a new phenomenon, but these seaweed blooms have increased considerably in volume over the past ten years, to the point where they can now be found from the mouth of the Amazon to the Gulf of Mexico.

In the Atlantic Ocean, Sargassum reproduces by fragmentation, which results in rapid growth. It naturally stores arsenic from the seabed while seeking out sources of phosphate, which it needs to survive.

Antithesis of biodiversity

Unsurprisingly, these algae have terrible consequences for biodiversity: when offshore, they form stretched, chain-mail-like structures that can be healthy, but which are often broken up by passing motor boats, causing them to proliferate.

It is when they accumulate on the shore that they become stressed outside of their usual habitat and the situation becomes critical. They die and consume all the oxygen in the surrounding environment, while releasing hydrogen sulphide (H2S), a toxic and potentially deadly gas. From a health perspective, H2S and ammonia (NH3) emissions produced by decomposing sargassum can cause headaches, nausea, and vomiting. 

They can also accelerate the deterioration of electronic and household appliances and frighten away tourists who prefer to stay in unpolluted areas. These golden tides are becoming an increasingly common occurrence but there are growing efforts to curb the problem. “We need to use a floating boom system to redirect the masses of sargassum before they wash up on the shore, to areas where we have the resources to collect them via track systems linked to the coast,” says Damien Devault, lecturer at the University Training and Research Center in Mayotte.”

Economic potential

Sargassum has several industrial uses: alginate can be extracted from it, an essential component of products packaged in tubes such as gouache and toothpaste. It can also be used to make fertilizer. However, a stable level of production cannot be ensured because sargassum strandings are difficult to model and predict. Moreover, sargassum that is covered in sand is unusable, and collecting it after every tide exacerbates coastal erosion – as the sand will eventually run out – and requires an expensive clean-up process.

Few initiatives of this type have been implemented in Overseas French Territories or in foreign Caribbean states. For its part, Agence Française de Développement is providing funding for the Sarg’Coop project with a grant of €300,000 from the Overseas France Fund, which will be used to support the roll-out of a common air quality measurement network to several Caribbean countries. The data collected via this network will in turn be monitored from Guadeloupe. The findings will then help determine how to limit the damage done by the seaweed, while putting it to good use.