THE BEACH INVADER

For the past decade, a prolific beach invader has wreaked seasonal havoc on idyllic beaches in the tropical Atlantic.

From the eastern coast of Mexico to the west coast of Africa – and everywhere in between – a potent-smelling seaweed known as Sargassum is devastating not only beaches, but livelihoods too. Moreover, climate change appears to be making it worse.

Back in 2011, little was known about Sargassum. But since then, it has made its presence so prevalent that it has had researchers around the world grappling to understand where it comes from and what to do with the thousands of tonnes that wash up on shores every year.

Emma Tompkins, Professor of Geography, Environment and Development, is part of a team at Southampton focusing on those questions. She also has a personal interest in understanding the seaweed conundrum, as she explained: “My family emigrated to the Cayman Islands 35 years ago. I visited in 2018 and our favourite beach was covered in stinking seaweed, so much so that we couldn’t go into the sea. It was just bizarre.”

This stinking seaweed was, Emma discovered, Sargassum. It was first spotted in extensive blooms across the tropical Atlantic in 2011.

Back in Southampton, Emma worked with Robert Marsh, Professor in Oceanography and Climate, Dr Yue Zhang, Lecturer in Environmental Engineering, and researcher Dr Susan Hanson to understand more about Sargassum, finding out both climate and oceanic drivers are behind it.

Sargassum is a brown, large-leaf seaweed that floats. It has long been found in the Sargasso Sea, off the coast of Bermuda and, until 2011, was always contained to this area.

However, an anomalous weather event in 2010 is believed to be responsible for Sargassum spreading. Seedlings were pushed south to warmer waters, where it is thriving every year. Huge rafts of Sargassum, amounting to millions of tonnes, have been spotted floating anywhere between Mexico and Ghana – some of which have measured the equivalent of 200 football pitches in size.

Emma said: “The mats of Sargassum can be massive, and when it washes to shore it can pile metres high. It can be devastating. As it decomposes, it stinks, can cause skin irritation and it gives off gases that can cause breathing difficulties.

“It’s affecting fishermen’s ability to go out and fish in boats; it’s affecting tourism as people cannot go in the sea or on the beaches because it smells so bad; and it’s a problem for ecosystems – particularly for turtles that go to lay their eggs on beaches in the summer when Sargassum is at its worst.”

Research by Emma and Robert determined that the spread of Sargassum is related to the Atlantic Meridional Mode, which is the way in which the ocean moves. Robert also produces annual forecasts for Sargassum, predicting where it will be 180 days in advance.

They are now looking at whether the spread of Sargassum relates in part to nutrients coming off major river deltas, or to the warming seas associated with climate change, or to the changes in ocean movements, which is also in part due to climate change. This work is ongoing. In 2020, the team won funding from the Global Challenges Research Fund to work in collaboration with the University of Ghana and the University of the West Indies to investigate how the poorest people affected by Sargassum can potentially use it. The SARTRAC (Teleconnected SARgassum risks across the Atlantic: building capacity for TRansformational Adaptation in the Caribbean and West Africa) project is due to conclude this year.

“We have been looking at how Sargassum can be used by a person on the beaches carting it away, rather than looking at industrial-scale utilisation or collection at sea,” explained Emma. “We have also looked at agricultural uses for Sargassum for small-scale farmers.”

One potential use is to support the growth of mangrove seedlings. “We have found that if we mix Sargassum mulch 50/50 with other organic waste, such as pig waste, this improves mangroves’ growth,” said Emma.

A second area of research is whether Sargassum can be used to grow tomatoes and peppers, which are the important crops for poorer communities.

A third potential use is in bioenergy. Emma explained: “In rural areas, bio-digesters are important sources of energy for people with no access, or variable access to a national grid. If you shovel in Sargassum, along with waste from animals, it produces methane. This gas can be used to support the electrification of rural areas.”

While potential uses for Sargassum are on the horizon, the bigger questions remain.

“We are still left with some really big questions that we cannot answer,” concluded Emma. “How much worse will Sargassum inundations become under climate change? And what will be the economic damage to fisheries and tourism sectors caused by Sargassum across the tropical Atlantic?” This remains to be seen.

Find out more www.sartrac.org