The occurrence of drifting and beach-cast macroalgae is a natural process that takes place on many coasts worldwide. These marine macroalgae, also known as seaweeds, accumulate as deposits that play essential roles in coastal ecosystems. They provide nutrients, offer shelter and food for numerous species, and help protect the shoreline from erosion.
However, this natural process has nothing to do with the seaweed tide episodes that are increasingly affecting some coastal regions. These events are caused by the massive proliferation of certain species, favored by environmental changes linked to human activities such as eutrophication and climate change. This produces extraordinary amounts of macroalgal biomass that accumulate along extensive stretches of coastline, causing severe ecological and socioeconomic impacts.
Due to their volume and extent, the large Sargassum seaweed tides reaching Caribbean coasts since 2011 are one of the most significant cases. Let us now examine these episodes in more detail and what causes them.
Pelagic Sargassum.
The seaweed tides that occur in the western tropical Atlantic are caused mainly by the species Sargassum fluitans and Sargassum natans. These species are pelagic, meaning that they live floating freely at the sea surface thanks to small gas-filled vesicles. This distinguishes them from most macroalgae, which usually live attached to rocky seabeds, that is, as benthic organisms, in sunlit coastal waters, known as the photic zone. These pelagic Sargassum species are native to the well-known Sargasso Sea, located in the central North Atlantic. This is a unique sea because it is not surrounded by coasts, but lies in the open ocean, bounded by large ocean currents that act as natural borders. Its name comes precisely from the presence of extensive floating masses of Sargassum, already described in the 15th century during Atlantic crossings by the first navigators. These floating masses form a natural ecosystem of great ecological importance, providing habitat, shelter, breeding areas and food for numerous marine species, including invertebrates, fish, turtles, mammals, birds and sharks, many of them of major commercial interest. These include tuna species and the endangered European eel, which travels to this area to reproduce.
The new Atlantic belt.
The large Sargassum tides, known as brown seaweed tides because of their characteristic color, affect the coasts of many countries in the Caribbean and the Gulf of Mexico, but they do not come from the Sargasso Sea. Their origin lies in a new Great Atlantic Sargassum Belt, which extends from the coasts of West Africa to the Caribbean Sea. This belt is more than 8,000 km long and is estimated to accumulate between 10 and 20 million metric tons of floating Sargassum. The formation of this Great Belt is thought to have begun, on the one hand, with exceptional changes in surface ocean circulation in 2009–2010, which displaced part of the Sargassum towards the tropical Atlantic. In this region, pelagic Sargassum found environmental conditions that favored its massive proliferation, mainly high nutrient concentrations, together with warmer waters and greater light availability. These nutrients are considered to come mainly from human activities, such as agriculture, and also from fires, through inputs from major rivers in Africa and the Americas.
Impacts and uses.
The brown seaweed tides coming from this new Great Atlantic Sargassum Belt can reach enormous dimensions. The accumulations that arrive on the coast are usually estimated at between 2 and 10 million metric tons per year, although in 2025 they exceeded 30 million metric tons. These accumulations cause serious environmental and economic problems for the Caribbean countries affected by them. Their ecological impacts include biodiversity loss and disruption of the functioning of coastal ecosystems. These effects are mainly due to reduced light and oxygen availability and to an excess of nutrients in the sea. Economic impacts include major losses in coastal tourism and fisheries, two essential activities for many Caribbean countries. Accumulations on beaches cause nuisance and bad odors, reducing their tourist appeal. In fisheries, they damage fishing gear and considerably reduce catches. The main management measures to limit the impact of Sargassum seaweed tides focus especially on removing large coastal accumulations, but also on collecting them at sea using boats, which avoids the disturbances caused by removal on the shoreline. These actions represent a very high additional economic cost, estimated in some regions at between 0.3 and 1.1 million dollars per kilometer of coastline. To reduce these costs and turn an environmental problem into a business opportunity, the use of these large accumulations as a resource is being developed for different industrial applications, such as biofertilizers or agricultural biostimulants, animal feed, and the production of biomaterials such as bioplastics, biofuels or biogas, among others.
| Location and extent of the Great Atlantic Sargassum Belt in the tropical Atlantic, the source region of pelagic Sargassum golden tides affecting Caribbean coasts. Countries affected by these events are shown in red. Figure prepared by C. Peteiro, based mainly on data from Debue et al. (2025), published in Marine Pollution Bulletin, and Rodríguez-Martínez et al. (2025), published in Harmful Algae. |
Tenured Scientist (permanent position) affiliated with the Department of Aquaculture and Blue Biotechnology at the Oceanographic Centre of Santander (COST), which belongs to the Spanish Institute of Oceanography (IEO), part of the Spanish National Research Council (CSIC).
His academic background includes a Bachelor of Science in Biology, a Diploma of Advanced Studies in Environmental Biology from the University of A Coruña (UDC), and a PhD from Rey Juan Carlos University (URJC). His doctoral thesis was entitled Open-sea cultivation of commercial kelps on the Atlantic coast of southern Europe.
He has more than 20 years of research experience in marine macroalgae (also known as seaweeds) addressing both fundamental and applied aspects through culture and experimentation. His work focuses particularly on the cultivation and conservation of kelp species (i.e. Laminariales and closely related orders). He leads the research group “Marine Macroalgae Cultivation and Experimentation,” whose main objective is to research, develop and transfer knowledge on the cultivation, sustainable use and conservation of seaweeds.
His research has contributed to the generation of original and relevant knowledge in the field of marine macroalgae, with a particularly significant contribution to kelp biology and aquaculture. His results have been published mainly in international journals with scientific and technological impact, including 30 publications indexed in Web of Science (WoS). Further details on his publications and research outputs are available through his institutional CSIC author profile (https://digital.csic.es/cris/rp/rp14639) and ORCID profile (http://orcid.org/0000-0003-0698-3573).