C-DRONE GUIDE · 7 SEPTEMBER 2026

Sargassum strandings in the French Caribbean: what a drone actually measures on the foreshore

Since 2011, the coasts of Guadeloupe, Martinique, the Northern Islands and part of the French Guiana shoreline have been hit in waves by rafts of pelagic sargassum — Sargassum natans and Sargassum fluitans — drifting in from the transatlantic belt described in the journal Science in 2019. On the foreshore, these brown algae decompose within days and release hydrogen sulphide and ammonia; that is why France's national Sargassum plan sets a target of collection within 48 hours. For a mayor, an inter-municipal authority or a collection contractor, the problem is not knowing that sargassum has landed: it is knowing how much, where first, and whether yesterday's clean-up actually worked. That is precisely where a drone flight replaces an eyeball estimate with a figure — and precisely where its competence stops.

Published on 7 September 2026, reviewed on 7 September 2026 — regulations in force as of September 2026.

From the Atlantic belt to the foreshore: what satellites see, and where they stop

The sargassum reaching the French Caribbean does not come from coastal seabeds: these are pelagic species, floating freely in surface rafts. Offshore monitoring has long relied on optical remote sensing. A study by Gower and King published in 2011 in the International Journal of Remote Sensing produced the first full mapping of the distribution and movement of floating Sargassum in the Gulf of Mexico and the western Atlantic using the MERIS sensor, on 2002-2008 data (see the study on Google Scholar). In 2019, a team led by Mengqiu Wang and Chuanmin Hu described in the journal Science the Great Atlantic Sargassum Belt, the algae band that in some years stretches from West Africa to the Gulf of Mexico (see the study on Google Scholar).

On that basis Météo-France (Antilles-Guyane interregional directorate) has produced, since 2020 and as an institutional mission since 2022 under the national plan, a stranding surveillance and forecast bulletin for Martinique, Guadeloupe, the Northern Islands and French Guiana, issued once or several times a week. Detection combines MODIS (Aqua and Terra) and VIIRS (NOAA-20, Suomi-NPP) at 1 km resolution, OLCI (Sentinel-3A/3B) at 300 m, and MSI (Sentinel-2A/2B) at 10-30 m as expert support. Those resolutions are enough to spot a raft offshore and model its drift from currents and wind; they are no longer enough to say how many cubic metres have piled into a 300-metre cove, or how thick the algae mat is at the loader's wheels. The drone takes over at exactly that scale: the one where resource decisions are made.

The 48 hours, H2S and the thresholds: what the drone informs, and what it cannot measure

The framework was built in two stages. Sargassum Plan I, announced in June 2018 with €13 million over two years, set out crisis-management measures and the objective that has become the reference for the whole system: putting in place collection solutions able to intervene in under 48 hours to prevent the algae from decomposing. The wording matters: this is a resourcing objective of the national plan — ten million euros of state funding were earmarked to equip the four territories concerned, Guadeloupe, the Northern Islands, Martinique and French Guiana — not a legal obligation enforceable against a municipality. Sargassum Plan II, presented on 14 March 2022 for the 2022-2025 period, raised state funding to nearly €36 million over four years, an increase of around 30%, around 26 measures, and created GIP Sargassum bodies in Guadeloupe and Martinique to concentrate resources on a single operator per territory. The assessment drawn up by the Directorate-General for Overseas Territories in summer 2025 is mixed: 10 measures fully delivered, 9 partially, 7 not started. A Sargassum Plan III was announced at the interministerial sea committee of 26 May 2025 and is under local consultation, with 2026 remaining a budgetary transition year.

On the health side, management thresholds come from an opinion of the Haut Conseil de la santé publique dated 8 June 2018, made public on 6 July and adopted by the prefectures. Expressed as 24-hour averages for hydrogen sulphide: between 0.07 and 1 ppm, vulnerable people are advised to keep away from affected areas; between 1 and 5 ppm, the advice extends to the general population; above 5 ppm, access to at-risk areas is strongly discouraged and collection workers must carry a personal alarm detector. An associated threshold of 8.3 ppm applies to ammonia. The HCSP notes that H2S production stays limited if removal is early, since the aerated structure of sargassum does not favour anaerobic degradation: that is the whole logic of the 48-hour target.

What the drone does not do. It does not measure H2S or ammonia: those concentrations depend on fixed sensors and standardised measurement run by accredited air-quality monitoring networks, not on an onboard camera. Nor does it predict a raft's arrival — that is the job of the Météo-France satellite bulletin and drift models. And it in no way replaces regulatory health monitoring or beach-closure decisions, which remain with the prefecture. An operator selling you an "H2S-detecting drone" is selling a capability that does not exist in this context. The drone documents the material: its area, its volume, its location, its change over time.

Area, volume, clean-up efficiency: the deliverable you can actually use

A low-altitude photogrammetric flight over a beach produces three things. First an orthophoto at a few centimetres per pixel, on which covered area is measured in square metres and broken down by sector. Then a digital surface model: by comparing the loaded foreshore with the same foreshore once cleaned — or with an off-season reference survey — you get an estimate of thickness and then volume, the figure most often missing when ordering truck rotations or justifying a request for resources. Finally a dated, defensible record of the same site before and after collection: this is what lets you verify that a clean-up contract was performed as invoiced, rather than take it on trust.

The feasibility of this approach is documented. Javier Arellano-Verdejo and Hugo E. Lazcano-Hernandez published in 2024 in the journal PeerJ a high-resolution sargassum mapping method based on drone imagery and deep learning, applied to the beaches of Mahahual and Puerto Morelos in Mexico's Quintana Roo (see the study on Google Scholar). Flights were flown at 56.2 m altitude for a 2 cm per pixel resolution, on a set of 15,268 images segmented into three classes (sargassum, sand, other) by a pix2pix model: mean F0.5 score of 0.8637, precision 0.8926, recall 0.8469, with a slight tendency to underestimate sargassum cover rather than overestimate it — a conservative bias that is reassuring for a contracting authority, but one to know about before signing up to a volume figure.

Two derived uses are worth ordering at the same time. The first is monitoring the erosion caused by collection: machines scraping the foreshore also export sand, and a repeated photogrammetric survey quantifies that unintended removal, using the same method described in our guide to coastal erosion monitoring by drone. The second is prioritisation: on a territory with ten or fifteen sites hit at once, a morning of flights ranks beaches by volume and proximity to housing, which beats arbitrating over the phone. In shallow bays where rafts linger before stranding, a drone bathymetric survey usefully completes the file when sizing a floating boom.

Who commissions this kind of mission, method and 2026 prices

The clients are first of all coastal municipalities and inter-municipal authorities in the French Caribbean and Guiana, the GIP Sargassum bodies and state services (prefectures, DEAL) handling resource requests, joint authorities and natural-area managers, hotels and resorts on the windward coast arbitrating between private collection and beach closure, collection contractors wanting to document their performance, and environmental consultancies commissioned for baseline studies or multi-year monitoring.

Field constraints are real and show up in the quote. The trade wind, near-permanent on windward coasts, shortens flight windows and endurance; salt aerosol requires a freshwater rinse and a motor check after every coastal flight, on pain of rapid corrosion; heat and humidity cut battery life and fog the optics as the aircraft leaves an air-conditioned vehicle; flying over water means keeping the aircraft in sight without ground visual references and without an emergency landing area. On top of that come the CTRs of the Caribbean airports — Pointe-à-Pitre Le Raizet, Martinique Aimé Césaire at Le Lamentin, Cayenne Félix Eboué — near which some frequently affected beaches sit: the procedure is the one described in our guide to drone missions inside an airport CTR. The overseas regulatory framework, identical to mainland France but applied to a completely different terrain, is detailed in our guide to drones in Guadeloupe, Martinique, French Guiana, Réunion and Mayotte.

Ranges observed in France in 2026, excl. VAT, for an operator already based in the territory (mobilising from mainland France changes the economics of the mission entirely):

These amounts cover the flight, photogrammetric processing and delivery of orthophotos, surface models and area and volume calculations; they exclude H2S measurement, health analysis and the collection work itself. The subject is close to, but distinct from, Brittany's green tides, covered in our guides to drone monitoring of green algae strandings and mapping green algae strandings: different species, different regulatory framework, different decomposition kinetics. For a municipality, an inter-municipal authority, a GIP, a hotel or a collection contractor, request a quote stating the number of sites, their approximate length of shoreline and the desired frequency of flights.

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Put it into practice

• Drone security & surveillance: rates and cities covered from €600
• Security & surveillance in Bourg-en-Bresse Auvergne-Rhône-Alpes
• Security & surveillance in Nevers Bourgogne-Franche-Comté
• Security & surveillance in Paris Île-de-France

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