The State of the World’s Seaweeds: Our algal allies are under threat

A landmark report has laid out the threats facing the world’s seaweeds – and how urgent action is needed to save them.

The State of the World’s Seaweeds report shows that these organisms are a vital part of our planet, forming habitats that are relied on by animals and people alike. They have important roles to play in combatting the climate crisis, plastic pollution and world hunger, but are often underappreciated.

The report’s authors argue that this needs to change in the face of the multitude of threats seaweeds are facing. Climate change, pollution and overharvesting are all pushing seaweeds towards their limits but conservation efforts are sparse.

Dr Sophie Corrigan, the lead author of the report, says that “urgent action” is needed to fully assess how threatened seaweeds are, and that a global effort is needed to help save them.

“Seaweeds sometimes have a bit of a negative reputation as slimy, unpleasant things, but they’re really beautiful and important,” Sophie explains. “Food, medicines and even the cosmetics industry rely on seaweed for the properties they can offer. We still haven’t realised the full potential of seaweeds and their uses.”

“Safeguarding the future of seaweeds will require a global movement to unite governments, researchers, industries, charities, Indigenous Peoples and local communities in protecting seaweeds. This report is the first step in that process.”

Professor Juliet Brodie, a seaweed expert and co-author of the report, says that “seaweeds are extraordinary” and need more research to understand them.

“This is one of the most challenging times for seaweeds but also one of the most exciting for scientists,” Juliet adds. “We’re in a new age of discovery but a lot of it is disappearing before we can really study it.”

“The State of the World’s Seaweeds report brings together our collective knowledge but also highlights what we don’t know.”

How are seaweeds threatened?

Seaweeds are a diverse group of macroalgae that have lived on Earth for more than a billion years. At around 1.6 billion years old the ancestors of red seaweeds are some of the oldest known multicellular organisms, and were later followed by green and brown seaweed species.

Having been present for so long they are an integral part of Earth’s ecosystems. Collectively covering an area almost the size of Australia, they form the planet’s largest vegetated marine habitats which many other species depend on.

Just one square kilometre of kelp forest, for example, is estimated to support fish that collectively weigh nearly 240,000 kilogrammes. Calcified red seaweeds, meanwhile, cement corals together and allow extraordinarily diverse reefs to form.

Seaweeds also play a significant role in cleaning up our world. They take in carbon dioxide at a far greater rate than plants on land and can help to absorb pollution from the oceans.

Unfortunately, seaweeds are currently under pressure from all sides. Climate change is the greatest threat they face as it affects the oceans in a variety of different ways.

“Higher temperatures, marine heatwaves and ocean acidification are making parts of the ocean unsuitable for seaweeds to live in,” Sophie explains. “While some seaweeds may be able to survive by moving to cooler areas, many will not.”

“Kelp forests, which support much of our marine biodiversity in temperate regions, are set to lose a significant amount of their habitat by the end of the century.”

Climate change is also making it easier for disease and invasive species to spread, multiplying their impact on seaweeds.

Other issues for seaweeds include overfishing, which removes the predators that keep sea urchins under control and allows these voracious herbivores to overwhelm seaweed beds. Pollution is also a problem, as it can smother slow-growing seaweeds.

Targeting seaweed recovery

Reversing the ongoing declines in seaweeds is no easy task, especially as there’s so much that scientists still don’t understand about these organisms. Of an estimated 24,000 species, just 12,000 have been described.

“There’re not a lot of scientists who study seaweed taxonomy, and there are also a lot of seaweeds that are difficult to access,” Sophie explains. “We think there are probably many endemic species in areas we haven’t explored yet, especially in our ocean depths.”

“For example, we know about deep-water seaweed habitats that stretch to 200 metres below the surface of the water off Bermuda and in the Gulf of Mexico,” adds Juliet. “There must be other habitats like this around the world, but because they’re hard to study we don’t know about them.”

Even when seaweeds are well-known, protections are limited. Kelp forests are among the best-studied seaweed habitats, but only 16% are in a protected area and just 1.6% in regions with the strictest protections.

For other seaweed habitats the level of protection is unknown but probably minimal.

“While interest in kelp forests is rightfully building, other seaweeds have been left behind,” Sophie says. “This is something we want to change, and we’re hoping that seaweeds will benefit from the 30 by 30 initiative as more protected areas are created or expanded in the coming years.”

It’s something that’s set to be included in the ‘Seaweed Breakthrough’, a set of conservation targets that will be launched at the UN Oceans Conference in June. The State of the World’s Seaweeds report will provide the underlying evidence for these targets, giving the organisms the best chance of survival.