Sheet 6: Decomposition and Sargassum storage

Algae decomposition

Decomposition is a natural phenomenon of degradation of an organic or chemical substance, followed by putrefaction, a process that involves an array of micro-organisms such as fungi or bacteria. The process takes more or less time depending on the material concerned, and follows specific stages over time.

Degradation (or aerobic) in the first instance is carried out by micro-organisms and requires the presence of oxygen. The carbon present in the algae is used as energy and is transformed into carbon dioxide (CO2) – the most common mode of plant degradation. The degradation does not lead to the formation of gases, and is therefore not the cause of the hydrogen sulphide released by the Sargassum.
Next comes fermentation (or anaerobic), a complex process that takes place in the absence of oxygen, during which many compounds are reduced to organic acids, ammonia or sulphur compounds. It is fermentation that produces gases such as methane and hydrogen sulphide.

The beached Sargassum does degrade in the open air and therefore finds the necessary oxygen, but the dry crust that forms during degradation, slows the exchange down between the micro-organisms at work and this essential oxygen. A change in conditions can therefore rapidly be observed in the algae decomposition. Degradation gives way to an intense fermentation, and the oxygen that was trapped under the dry crust is drawn off and transformed during the process into gases. These same gases are characteristic of Sargassum and are harmful, causing issues on an environmental, sanitary and economic scale.

Where to store the collected sargassum?

The problem of how to collect stranded Sargassum in the shortest period possible and most efficiently and respectfully remains, but there is also the problem of where to store the Sargassum once collected. The massive events highlight the lack of storage sites throughout the Caribbean. The seaweed sometimes contains heavy metals and releases harmful gases when decomposing, and then poses a major health and environmental problem.

On average, 90% of harvested Sargassum is spread on backshore sites in layers. The gases emitted by the spread Sargassum during the decomposition process are more diffuse and less harmful, and fermentation is less active. Although secure storage sites are provided for this purpose in the French Antilles, the massive arrival of Sargassum, combined with varying management strategies according to the country concerned, mean that much of it is stored on unofficial sites, sometimes near risky environmental zones (mangrove swamps, waterways, houses, etc.). In Sainte-Anne in Guadeloupe in 2018, the Sargassum quantity was such that it was stored on backshore sites close to mangroves, subsequently degraded by its decomposing (water pollution, smothering of vegetation etc.).

Although they are generally too few compared to the quantity of seaweed, Sargassum storage sites remain safe areas, both environmentally and sanitarily. They can be managed by communities, or privately as it is generally the case in the Dominican Republic or Mexico. However, collecting Sargassum is expensive, regardless of the technique- the seaweed transport amounting to a third of total clean-up operations cost. Also, many of these storage sites must be located close to the collection ones. Martinique, Guadeloupe and French Guiana are focussing on former landfills, far from the risky areas. In other Caribbean islands and in the United States, backshore and burial sites are favoured.

Spraying (usually agricultural) is the most common technique across the Caribbean since it is the quickest and cheapest method, although it remains risky. These environmental and health risks (as Sargassum can contain heavy metals and arsenic) can be mitigated if the Sargassum arrivals are analyzed, allowing the quantity of Sargassum per hectare to be foreseen. According to the first studies (carried out within Carib’Agro in 2016), the level of heavy metals is not worrying since it is present in small quantities and therefore complies with the standards (arsenic is not taken into account in this study). Attention should be paid to the sodium present in the algae, which can lead to serious salinisation of the soil. Storing Sargassum awaiting spreading is not recommended, as the gases and juices produced during fermentation are likely to contaminate the areas concerned.

For the Dutch islands of Bonaire, Saint Eustatius and Saba, for example, the government strongly recommends dewatering and compacting the Sargassum at monitored storage sites. If the Sargassum is used as a spread, the layers should not be more than 10 centimeters thick. But again, this is not an obvious solution – it would take the equivalent of 10 football stadiums each year to spread the Sargassum to a layer that thick. The greater the amount of Sargassum, the more complicated it becomes to meet this standard.

Sources

CNRS
Etude sur l’impact environnemental de l’épandage des Sargasses en Guadeloupe,
Bureau de recherches géologiques et minières (BRGM)
http://www.saint-barth-saint-martin.gouv.fr/
Rapport de l’ANSES : Expositions aux émanations d’algues sargasses en décomposition aux Antilles et en Guyane

Ministère de l’Agriculture
ADEME : Algues sargasses, prévention des échouages et perspectives de valorisation
Dutch Caribbean Nature Alliance – Prevention and clean-up of sargassum