Warmer oceans have brought a sudden growth in vast floating mats of seaweed, smothering coral reefs, seagrass and fish populations, according to new research.
A study in Mexico, led by marine scientist Dr Ian Hendy, at the University of Portsmouth, found climate change has opened up new territories to giant mats of sargassum blooms, threatening the survival of other species and turning the ocean brown.
Dr Hendy says the risks the spread of sargassum poses to other plants and animals are extreme, and conservation management and protection is urgently needed.
It is the first study to directly examine the direct impact of sargassum on ocean biodiversity.
Sargassum blooms are causing a coastal dead zone.
Sargassum is abundant in parts of the Atlantic. The Sargasso Sea is named after it. But since 2011, increasing amounts of it have begun washing up on the coastlines of Brazil, the Caribbean, Gulf of Mexico, and the east coast of Florida.
In addition to the devastating impact on flora and fauna, growth in the number and size of blooms is estimated to have cost millions in lost tourism revenue.
Mexico earns £7.4 billionn a year in coastal tourism.
Dr Hendy said the sargassum blooms pose an immeasurable threat to all plant and animal life in the region, particularly seagrass, an ecological 'hero' for its ability to store CO2.
He said: "Not only is the spread of sargassum worrying in terms of it blotting out the light to other marine creatures, smothering their ecosystems, it also leaches a toxic fluid when it decomposes.
"It is causing a coastal dead zone."
Corals and seagrass are what we'd call ecosystem engineers, they provide habitats and structure for countless organisms. There's only so much disruption they can stand.
The researchers studied an area off the north west coast of Mexico where seagrass was estimated to cover about 400,000 hectares.
They found sargassum blooms had brought a 4.5 centigrade rise in water temperature, a 73 per cent decrease in light, lower oxygen levels, and a seven-fold reduction in seagrass growth.
"It's a cocktail of immeasurable devastation for other species," Dr Hendy said.
The spread of sargassum mats is not just posing a risk to the complex biodiversity of the ocean, when it washes up, beaches are covered in deep bands of toxic rotting seaweed.
Dr Hendy said: "The spread of sargassum mats is creating a collapse in flora and fauna. We found a distinct difference in almost every measurable factor of ocean health in the areas we studied.
Where sargassum blooms, the surrounding plants and sealife are suffering, as is the billion-pound tourism industry
"Corals and seagrass are what we'd call ecosystem engineers, they provide habitats and structure for countless organisms. There's only so much disruption they can stand. If you take these out of commission, you also take out the young and vulnerable species that thrive because of them.
"Managing this will be complex."
Possible mitigation would likely involve a web of people and groups, and require oversight further afield to control of the 'pinch points' that help create the problem, such as reducing eutrophication outfall – where the sea becomes enriched with nutrients – from the Amazon River.
The study is published in Climate Change Ecology.
