From 07 to 29 October 2017


The ‘Sargasso’ mission, conducted on the Yersin vessel between Mindelo, in Cabo Verde, and Fort-de-France, in Martinique, was part of a programme led by the Mediterranean Institute of Oceanology (MIO). It followed the ‘Sargasso Caribbean’ campaign previously conducted in June-July 2017 to study algae proliferations in the Tropical West Atlantic. The aim of this new mission: supplementing the observations and measurements made during the first campaign between Franch Guiana and the Sargasso Sea, by carrying out sampling for the first time on a transatlantic journey.


The ‘Sargasso’ mission aimed to:

  • perform an additional sampling of Sargassum to:
  • Complement the population genetic and phylogenetic studies,
  • Measure the contaminant levels,
  • Study the bacterial communities associated with the fish.
  • Perform dives in the pelagic environment to improve the knowledge of the ecological role of the Sargassum rafts that have recently appeared in this part of the Atlantic Ocean
  • Compare the measurements at sea and satellite images in order to refine the methods for interpreting these satellite images to locate the Sargassum seaweeds and predict their movements.

Sargassum, what is it?

Sargassum or sargasso weed are common names for brown marine algae in the genus Sargassum. Only two species, Sargassum natans and Sargassum fluitans, live in high seas, with a strictly pelagic cycle. They float on the surface, forming large and dispersed clumps called rafts. They are fully functioning ecosystems which are home to many living species, ranging from bacteria to fish.

There are also 350 other benthic Sargassum species, which live attached to the seabed in coastal areas. They are distributed in several regions of the world.

Worrying proliferations

Sargassum seaweeds proliferate in the tropical Atlantic, causing unprecedented massive strands on the Antilles islands and on African coastlines since 2011. These accumulations on the shores lead to significant ecological, economic and health problems. The sulphurated hydrogen resulting from their decay is a toxic gas for humans and is strongly oxidising. Whereas in small quantities, these algae are beneficial to biodiversity, the same is not true when they proliferate.

For all these reasons, it is obviously urgent and important to discover why Sargassum weeds are multiplying at such an alarming pace.

The transatlantic mission: a rare opportunity

The Sargassum rafts in high seas constitute an ecosystem in its own right and the scientists seek to understand not only why they grow, why they proliferate, but also what are their effects on the high seas.

Given the small number and the cost of properly equipped vessels, expeditions such as that led by Monaco Explorations during this transatlantic crossing are rare opportunities to collect samples in the high seas.

The mystery of the Sargasso Sea

During his journey to the New World, Christopher Columbus had already described these brown algae, near the Bermuda archipelago, which gave their name to the Sargasso Sea. At the same time as the massive strandings observed since 2011, the satellite imagery revealed the new presence of Sargassum in the North Equatorial Current, forming what we could call a ‘new Sargasso Sea’. What are the causes of this phenomenon? And what are the consequences on the pelagic life of the ocean? These are the two main questions to which the team led by Thomas Changeux whishes to obtain answers.


Three questions to Thomas Changeux

The laboratory work

Since their return from the mission, the scientists have been performing genetic analyses of the samples collected to identify the three different morphological forms of Sargassum found during the mission and thus better understand the multiplication phenomenon

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Laboratory work Thomas Changeux ©O.Borde Monaco Explorations


A photography exhibition retracing the highlights of the mission is available. It consists of 12 panels in 110x110cm format and 8 panels in the dimension 110x165cm. Since October 2019, it is exhibited in the hall of the MOI where the students of the Institute can discover it.

Masked booby ©Guillemain

Brown booby ©Sandrine Ruitton

Fin whale ©Sandrine Ruitton

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Observation of animals of the open seas

A transatlantic crossing makes it possible to meet many animals.

Discover here the species encountered by the scientists

A whole ecosystem to be studied

18 stops were made during the crossing of the Atlantic, with at each stop the sampling of fish, zooplankton, Sargassum, etc. These analyses will allow a better understanding of the ecosystem as a whole and will determine whether it is influenced by strictly oceanic inflows or also by inflows from large watercourses such as the Amazon River or other rivers.

Continuation of the research

Thomas Changeux receives funding from the French National Agency for Research (ANR) to continue the programme as well as the analysis of the results from the first two campaigns.


Photo gallery








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The eight scientists on board during the mission.

  • Thomas CHANGEUX, research engineer, IRD, mission leader – diver – sampling of macro-organisms
  • Sandrine RUITTON, university lecturer, AMU, diving supervisor – diver – sampling of macro and micro-organisms – filtration – images
  • Natascha SCHMIDT, doctoral student, AMU – sampling, plastic material
  • Sophie GUASCO, microbiology specialist, CNRS – sampling of micro-organisms, filtration
  • Anouck ODY, post-doc, IRD – remote sensing, satellite image analysis, spectral measurements
  • Hubert BATAILLE, diver, IRD – photo shooting, sampling of macro and micro-organisms,
  • Dorian GUILLEMAIN, biologist, diver, CNRS – sampling of macro and micro-organisms
  • Vincent FAUVELLE, post-doc, AMU – chemist, extraction of contaminants

AMU: Aix Marseille University. IRD: Institut de recherche pour le développement / Research Institute for Development