Seaweeds: A hidden habitat under threat

Fossils of some of the earliest relatives of modern-day seaweeds date back 1.6 billion years.

Seaweeds have been exceptionally resilient organisms, outlasting the dinosaurs – which first appeared 230 million years ago – and surviving other mass extinction events.

But rising ocean temperatures are putting some species at risk.

Why are seaweeds in decline?

Kelps are structurally complex seaweeds, thought to have first appeared about 23 million years ago. They grow in large, dense groups, which form underwater forests.

These large brown algae tend to grow in cold, rocky seafloors and are abundant in shallow waters around Britain’s coastlines. They’re incredibly important, providing for a whole ecosystem.

However, in recent years there’s been a decline and shift in distribution of kelps and other wild seaweed species. These changes are believed to be the result of climate change.

“We know the water is getting warmer,” says Professor Juliet Brodie, our seaweed expert. “It’s gone up on average by two degrees centigrade in British coastal waters over the last 40 years. That might not seem like a lot, but we know that it’s having a marked effect on some species of seaweed.”

There’s also evidence that the sea is becoming more acidic, because it absorbs a large portion of carbon dioxide. Although the long-term effects are unknown, this could be damaging as it may begin to corrode the exterior of some calcified seaweeds.

We’re not just losing seaweeds in Britain. For example, rising sea temperatures and extreme heatwaves in 2011 caused Western Australia to lose 43% of its kelps.

There was also a 93% decline in canopy-forming kelps in northern California from 2008 to 2014, as seen from aerial surveys by the California Department of Fish and Wildlife.

In 2025, the authors of the State of the World’s Seaweeds report concluded that we need to take urgent action in order to fully assess how threatened seaweeds are. They also believe that a global effort is needed to help save them.

Why is losing seaweed a problem?

Seaweeds play a crucial role here on Earth.

Juliet explains, “kelp forests are incredibly important to our coastline because they break the flow of water, and this helps to prevent coastal erosion.”

Declines in seaweeds leave Britain and other coastlines open to the damaging effects of the sea, which reduce the stability of the land. Inhabitants of the seaweed forests, like fish, are also at risk of losing their habitat.

However, public interest and awareness of these important algae needs to be increased to help protect the coast.

“I think people see it as more of a nuisance,” says Juliet. “But you have to look with fresh eyes and understand their value to us, our coastlines, our fisheries – to the whole ecosystem.”

“Without them, we’d really be in a very bad place.”

Not only do we need seaweeds to keep marine ecosystems functioning and protect our coastlines, we also rely on them for food. The properties in some seaweeds make them valuable for medicines and cosmetics too.

New species and changing distributions

Some non-native species of British seaweed made their way onto the coast over 100 years ago. However, they’ve been arriving in increasing numbers in recent years.

Some of these species remain where they arrived, whereas others have been spreading around Britain’s coast.

Certain species of seaweed in southern Britain, such as the golden kelp Laminaria ochroleuca, are currently at the northern edge of their range, but they’re expanding even further north.

Researchers have been helping us understand these changes by extensively mapping seaweeds – though this project comes with its challenges.

“It’s difficult to get long-term data,” explains Juliet. “But evidence suggests that quite a few of our species are changing in distribution.”

“On the south coast some of our big kelps are in decline, but if you go further north, they’re tending to increase in distribution.”

Join the search for seaweeds

Our scientists are dedicated to understanding key environmental issues such as the rising sea temperature and arrival of non-native species.

Since 2009, and in partnership with the Marine Conservation Society since 2016, the Big Seaweed Search has been an opportunity for everyone to contribute to ongoing research.

Thanks to thousands of people identifying and reporting the seaweed species they find on beaches around Britain, scientists such as Juliet have been able to build up a picture of how the British coast is changing, and what that could mean for the future. The records even fed into the first ever IUCN Red List assessment of British seaweeds – this assessment informs conservation work and identifies how threatened species are.

Want to know which species you might see at your nearest beach? You can now explore an interactive map of the seaweeds spotted around the UK.

What started as a project focused on the UK’s shoreline has now inspired similar surveys around the world, including in the Falkland Islands and Mexico.

Find out more

You can see a stunning collection of seaweeds is in our Hintze Hall. Discover how Juliet and her team recreated a kelp forest for this display.