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The seaweed that strikes down surfers and fishermen in Dakar

All describe identical symptoms, similar to those of a bad flu: sneezing, sore throat, red eyes, sometimes fever, headache and even bleeding in the most severe cases. It is a disease that has remained mysterious for a long time and which strikes both surfers and fishermen in Dakar, all used to spending long hours on the waters of the Atlantic surrounding the capital of Senegal, but also simple walkers and residents of beaches. . The phenomenon has been known since the early 2000s – perhaps it started earlier, no one remembers a precise date – and always occurs in the hot season, from July to the end of October or the beginning of November, when the water temperature is around 25 degrees Celsius.

“A lot of people stopped surfing around this time”deplores Abdoulaye Diallo, himself a surfer, who fell ill at least once, in 2019. “People are more and more wary about going surfing”, abounds Babacar Thiaw, professional of this sport and president of the Senegalese branch of Surfrider Foundation, an international NGO; he remembers being knocked down three or four times, about three days each time. Dakar nevertheless brings together the ideal conditions for the practice of water sports: it is here that the waves of the Atlantic come to break. Pointe des Almadies, the westernmost point of the peninsula where the Senegalese capital is located (the peninsula of Cape Verde) but also of the entire African continent, would be a paradise if the air were not , at regular intervals, its share of patients.

Waters flooded with nutrients

Fishermen on Ouakam beach, a ten-minute drive away along the sea to the south, experience the same paradox. The waters of this area, blessed by the currents that flood it with nutrients, are full of fish – even if the oldest, concerned about competition from huge European and Asian trawlers, complain of having to go ever further offshore to fill their fillets of thiofs (a type of grouper), sea bream and octopus. Above all, there is this disease, which affected several dozen of them, at least in August 2019, 2020 and then 2021, not always with the same magnitude. Three years ago, “almost everyone who came to the water’s edge got sick”, remembers Mamadou Sarr, secretary general of the local fishermen’s association, who recorded all the occurrences of the disease on his big calendar. He’s been living there for nearly forty years, at the foot of the Mosque of the Divinity, below the city, and he’s never seen anything like it. Amadou Diallo, 46, also felt the effects of the disease for a few days in 2019, as did his two brothers. But he hasn’t given up on fishing, his job since forever: he’s been at sea, from 6:00 a.m. to 2:00 p.m., every day – except Friday, the day of prayer – since 1994. It’s his life.

Mamadou Sarr, secretary general of the Ouakam fishermen’s association, has recorded all occurrences of the disease on his big calendar (credit: Simon Carraud).

The episodes have become so recurrent, the testimonies so consistent, that they could not be the result of psychosis. Some have accused the area’s chemical factories of poisoning the locals. Others have blamed this unexplained flu on chicken farms or wild releases from passing cargo ships. The rumor of a curse cast by who-knows-who to punish local customs also ran. Faced with the accumulated clues, marine ecosystem specialists for their part suspected, as early as 2016, a toxic microalgae, a track that is more easily verifiable empirically. “We reviewed all the hypotheses, bacteria or others and even chemical pollution”remembers François Galgani, researcher at Ifremer (he is responsible for the station located in Bastia, Corsica) and world-renowned specialist in the effects of pollution on marine environments. “We discussed it and thehypothesis ofOstreopsis arrived on the table.” Ostreopsis, a genus of algae, some species of which release harmful toxins for marine life but also for human health. It was then necessary to wait for fieldwork to definitively elucidate the cause of the infections.

Ultra fast efflorescence

This is where the Senegalese biologist Waly Ndiaye, from the Senegalese Institute for Agricultural Research (Isra) and the Frenchman Patrice Brehmer, from the Research Institute for Development (IRD) come in. In July and then in August 2021, they received reports from inhabitants of two distinct areas, on the tip of Almadies, and immediately went to the indicated beaches to take samples of sea urchins, algal blooms and seaweed. A pleasant task in appearance for these researchers abandoning, for a few hours, the blouse for the diving suit. But voluntary. And not without risk since Waly Ndiaye develops, following this outing, flu-like symptoms for three days.

Waly Ndiaye in his laboratory at the Senegalese Agricultural Research Institute; on his computer, a microscopic observation ofOstreopsis cf. ovata (credit: Simon Carraud).

The samples are sent to the Ifremer laboratory in Nantes (Loire-Atlantique), where the algae are cultured and examined under a microscope, then to Concarneau (Finistère), where DNA analyzes are carried out. Verdict: the organisms sampled belong to the genus Ostreopsis and have all the characteristics ofOstreopsis ovata, a species first spotted in Asia and then in the Mediterranean, in the bays of Villefranche-sur-Mer (Alpes-Maritimes) or Genoa (north-west of Italy), then off Morocco and Brazil. Dissemination favored by sea currents and probably, at a faster rate, by transport ships. Genetically, we cannot distinguish theOstreopsis cf. ovata (in biology, “cf.” is used to designate a specimen that looks like a known species without having all the characteristics; it can be a close species or a variety or a simple atypical form, editor’s note) of the Mediterranean from that whichfound in Senegal. For us, itis the same speciesexplains Philipp Hess, of theIfremer, a specialist in toxins produced by algae, which took part in the identification work.

“It is a species that proliferates rapidly when the conditions are right: when there are enough nutrients and the temperature is high enough”, summarizes Waly Ndiaye. The individuals of this species, which line the seabed at shallow depths, then experience an ultra-rapid efflorescence (a “bloom”), which lasts about a week, with a peak after 48 hours. The rapid proliferation of these unicellular organisms allows them to reach impressive concentrations, up to more than a million cells per liter of water! Sufficient to cloud the sea water when the swell lifts the algae from their rock. The wind then takes care of spreading the toxin released during this “bloom” into the air. This is when the symptoms appear, including in people who have not been in direct contact with the sea. Often bothersome, they have never, fortunately, been fatal – no deaths in any case was listed.

On the trail of the culprit

It remains to know the reason for the proliferation ofOstreopsis cf. ovata at this exact location. First, the surroundings of the Dakar peninsula offer an ideal habitat: the temperatures are mild, the light favorable and the nutrients available in large quantities for the ecosystems. “Senegal is known as theone of the most productive areas in the world for marine life in general”, points out Philipp Hess. Every medal has its reverse. But there may be something else. Waly Ndiaye and Patrice Brehmer think they have found a culprit: waste water spills, themselves due to galloping demography in this part of Dakar and poorly controlled urbanization. “It is likely that the lack ofwater purification is an aggravating factor”, believes Patrice Brehmer, who describes himself as a “researcher in development” rather than a laboratory man. It is not difficult, in fact, to find a canal, in Ngor (one of the nineteen communes in the district of Dakar), from which flow a continuous flow of dirty water, without prior treatment. In the absence of an adequate sanitation system, local residents connect their own pipes directly to it. From there could come an increase in nitrogen, calcium and phosphorus, which would explain the rise of seaweed in Dakar. “A good working hypothesis”which still needs to be substantiated, according to Philipp Hess.

Uncontrolled sewage disposal channels in the Atlantic like this one could explain the rise of algae in Dakar (credit: Simon Carraud).

Global warming may also have something to do with the spread ofOstreopsis cf. ovata : with rising temperatures, this algae from the tropics would more easily find habitats where the water is to its liking. Thus she could have established herself permanently in the Mediterranean and traveled to distant regions of Asia. In short, its range seems destined to expand.

What to do then? It seems impossible to eradicate microalgae but, if Waly Ndiaye and Patrice Brehmer’s intuition is correct, better wastewater treatment could help reduce the frequency of “blooms”. This is why they alerted, in 2021, the public authorities of Senegal by means of a political note. An alert for the moment remained unanswered, in a country where the Constitution guarantees, however, since 2016, the “right to a healthy environment”. In the meantime, fishermen and surfers continue to set sail.

Simon Carraud and Alexandra Combe, special correspondents in Dakar (Senegal)

To produce this report, Simon Carraud and Alexandra Combe, journalists, received a grant from the European Journalism Center of the Bill and Melinda Gates Foundation, which finances investigations relating to health and the environment in developing countries. developing.

Front image: Observation of “water flowers” in an underwater view, characteristic of Ostreopsis blooms (credit: Patrice Brehmer / IRD).

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