When waves of sargassum – a type of seaweed – washed up on Eastern Caribbean shores seven years ago, people hoped it was a one-off. Matted piles swamped coastlines from Tobago to Anguilla.
"In 2011 it was the first time we'd seen it," says Professor Hazel Oxenford, an expert in fisheries biology and management at the University of the West Indies.
"It came as a complete shock and no-one had a clue what to do with it."
Three years later the seaweed returned, in larger quantities. Over several months, it made its way through the Caribbean to southern Mexico, where its impact on Cancun's beaches made international headlines.
Now it is happening again and everything suggests 2018 could be the worst year yet.
"On satellite images the quantity that's being picked up is greater than ever before," says Prof Oxenford, who is based in Barbados.
"Certainly we've had it for longer and in huge amounts. And some of the islands are getting it for the first time."
Caribbean governments are acknowledging that the seaweed, which impacts on tourism, fisheries and wildlife, could pose a long-term threat.
"The same way we prepare for hurricanes, we have to prepare for sargassum," Antigua's environment minister said recently.
What is sargassum?
- Stringy, brown seaweed that spends its life floating
- Traditionally starts life in the Gulf of Mexico and is pushed by currents out into the North Atlantic to float in the Sargasso Sea
- Forms rafts that serve as vital feeding and breeding grounds for marine life
- Not harmful to humans and in small amounts, it can help nourish beaches
In Barbados, sargassum has been hitting southern and eastern shores since May.
"You see metres high of sargassum piled up on shore but you're also seeing it in the ocean," says Iris Monnereau, a Barbados-based fisheries expert with the UN's Food and Agriculture Organisation (FAO).
"Sargassum mats can be up to seven metres deep."
On shore, as well as blocking beaches and repelling swimmers, the sargassum stinks as it decomposes.
Removal is time-consuming, expensive and can damage the beaches. Incoming rafts smother sea grasses and coral reefs, while fishermen struggle to get in the water.
"The sargassum tangles up their motors, their engines, their nets, their lines," says Ms Monnereau.
Catches are affected – in sargassum years flying fish numbers have fallen significantly, although it is not clear why.
Dolphinfish (or mahi-mahi) are being caught much earlier in their development, prompting fears for the fishery down the line.
There is also the turtle population. Nesting sites can be blocked by sargassum or damaged by removal work – and the sea is no safe refuge.
"For us, the worst year we had for sargassum still remains 2015," says Carla Daniel of Barbados Sea Turtle Project.
"We had a large single raft come in at Long Beach with a foraging turtle population in the water. The raft came in, they became entangled and unable to surface, and over 40 turtles died."
Public perception is key
Then there is tourism. In one extreme case, a resort in Antigua was forced to close its doors until 30 September.
"The issue is that we never know what it's going to be like – we can have a week or two weeks where it's very clear and then all of a sudden overnight it washes in," says Larry Basham of Elite Island Resorts, which runs St James's Club.
"We've spent an ungodly amount of money on tractors, heavy equipment, we've tried a number of different barrier systems on the water, none of which has worked well."
The group has other resorts on the island which are not affected. In fact, many beaches and resorts are clear. The sargassum is mostly affecting southern and eastern coasts and in the Eastern Caribbean, many of the main tourist beaches face west.
A quick glance at sites like Facebook and TripAdvisor, nonetheless, shows tourists are concerned. Public perception is a factor in the impact and some international headlines "shouting that the Caribbean is consumed in rotting seaweed" have not painted an accurate picture, Prof Oxenford says.
The reason southern and eastern shores are worst hit lies in the seaweed's source. In 2011 people initially assumed the Caribbean influx had, for some reason, drifted from its traditional home in the Sargasso Sea.
But now research indicates the sargassum is from a new source – an oval band stretching from the Brazilian coast to West Africa.
James Franks, of the University of Southern Mississippi's Gulf Coast Research Laboratory and co-author of a 2016 study on the subject, says the sargassum circulates through this band, consolidates off Brazil and then is periodically released northwards by currents into the Caribbean.
But he says it is not yet clear what has caused this huge new bloom. "We believe that it initially began as a result of shifts we observed in the climatological and meteorological indices – the waters were extremely warm in 2010; there were some shifts in the currents and winds," says Mr Franks.
He points to nutrients both from falling Saharan dust and pushed up from the sea floor as possible factors, but says more research is needed to establish exact triggers.
One major question is how long the phenomenon will continue.
Satellite images from the University of South Florida's (USF) Optical Oceanography Laboratory currently show historically high monthly levels of sargassum in the region.
"It is likely this bloom will occur until September this year," says Dr Mengqiu Wang of USF.
But it is not clear what will happen next year, or the year after. Mr Franks says prediction strategies will be very important in planning for influxes but they are in "the elementary stage of development at this point".
Challenges and potential solutions
Lessons have been learned since 2011, however. Removal techniques have improved and experts have written management guidelines.
In Barbados, for example, the grabs used to load sugar cane onto trucks have proved good at picking up sargassum without removing sand from beaches.
A company in Guadeloupe has developed a boat which collects it using a conveyor belt.
Commercial uses of sargassum are also being explored.
In St Lucia one entrepreneur is turning it into a plant tonic, while in Barbados a fertiliser project is underway.
There is also talk of bio-gas but Prof Oxenford says this depends on a regular supply of biomass and affordable collection costs. Tests have found some sargassum is high in arsenic, which rules out use as animal feed.
There are opportunities, she says, but they take time to develop.
Meanwhile, sargassum continues to arrive. "It is definitely a very large challenge, and a challenge that goes way beyond the capacity of small island states," says Ms Monnereau of the FAO.
"They need support… they are suffering. But every year they are getting better."