Ocean & Marine Degradation

The forgotten forests of the sea

The most extensive marine vegetated ecosystem on Earth has been overlooked and is disappearing. A landmark legislation in Argentina shows what protecting it could look like.

The bay is lit up by the high sun of a June day in Wales. Tiny wavelets glitter across the sea surface, from a faint southerly breeze. If you were to stand here, in the shallows, you would see bright sand glowing through glass-green water and high cliffs surrounding the bay. The rocky shore is swathed in heaps of dark intertidal seaweeds exposed by the ebbing tide. Their potent odour is lifted landward in the breeze, conjuring the smell of the seaside. Anchored to the rock, they split their time between air and water, like amphibians of the plant world. But dipping below the waterline, lies a hidden wonder: a tangle of brown straps forming a canopy above the seabed. This is the upper level of a tangle kelp forest, Laminaria hyperborea, that lives permanently submerged. Common with the other 100 or so kelp species globally, this is a forest of the cool temperate seas.

Scientists estimate that forests like this across the world are being lost at a rate four times faster than tropical rainforests and two times faster than coral reefs. They are among the fastest-disappearing major ecosystems on Earth. Despite huge restoration efforts in some regions, they are almost absent from international biodiversity laws.

Underwater engineers

Like plants on the land, kelp captures energy from sunlight, absorbs carbon dioxide, and releases oxygen through photosynthesis, forming the foundation of marine food webs. It is what ecologists call an ecosystem engineer, a species that physically reshapes its environment, creating conditions other species depend on. The term was first introduced in 1994 with textbook examples of beavers, corals, and earthworms. Kelp sits on this eclectic list, producing a complex framework that dampens wave energy, dims the light reaching the seabed, and provides habitat for thousands of other species.

The kelp forest is a three-dimensional metropolis similar in structure to a land forest. Instead of branches and leaves each kelp has a frond, and a stipe in place of a trunk. They lack roots and instead are fixed to the rock by a branched, claw-like holdfast, bonded with a glue strong enough to withstand the tug of surging waves in winter storms. Each kelp plant provides living space for other marine life; the frond is furred by colonial bryozoans, and grazed by grey top shells — sea snails that sweep it clean of tiny algae to eat. The stipe provides a vertical post for red seaweeds to attach that flutter like festival bunting in the swell, and the holdfast houses an entire mini ecosystem of sea anemones, crabs, barnacles, mussels, worms and even juvenile fish.

Kelp is a term to describe large brown seaweeds or macroalgae (large algae). Although they resemble plants, with leafy fronds, a wood-like stipe and rooty holdfast, kelp have no flowers, vascular tissue or roots. They are classified in the order Laminariales, encompassing thirty genera worldwide that include giant kelp (Macrocystis) in the Americas, bull kelp (Nereocystis) in the north Pacific, sugar kelp (Saccharina) and oarweed (Laminaria) in the north Atlantic, and bamboo kelp (Ecklonia) in the southern hemisphere.

Mature kelps form a canopy near the surface, an understory of smaller seaweed species below, and a forest floor of crustose algae and animals anchored to the bedrock, similar to the structure of forests on land. They host vertical communities of fish, invertebrates and mammals, providing food and shelter to their inhabitants.

Kelp is one of the fastest-growing plants on the planet. The giant kelp Macrocystis pyrifera can grow up to 60 cm/day in optimal conditions. Per square metre, kelp forests are among the most productive ecosystems on Earth, comparable in places to tropical rainforests.

Along almost a third of the world's coastlines, in cold to temperate waters: Norway, Iceland, Greenland, Canada, the United States, Chile, Argentina, South Africa, Namibia, Australia, New Zealand, Japan, and South Korea, to name a few. Sustained sea temperatures above 20°C are not tolerated by kelp. They do not grow in the tropics, which explains why ocean warming and heatwaves push them polewards.

Hundreds of species and thousands of individuals in a single hectare. In California, sea otters and Garibaldi damselfish. In Norway, wolffish, cod and the harbour seal. In South Australia, the weedy seadragon and the giant cuttlefish. In Argentina, southern right whales calve nearby. Scientists recorded more than seven hundred species associated with a giant-kelp forest in California's Channel Islands. Lose the kelp, and we lose these species too.

A combination of pressures. Marine heatwaves, discrete events of anomalously warm water, have driven catastrophic losses in Tasmania, California and Baja California. Pollution from agriculture, sewage and aquaculture feeds, overgrowth by filamentous algae that smother the kelp. Kelp trawling in Norway and Chile. The removal of predators such as sea otters, sunflower stars, lobsters and large fish favours sea urchin populations that graze the forests bare.

When sea urchins overgraze a kelp forest, bare rock encrusted with pink coralline algae is all that remains. Once urchin barrens form, they tend to persist in a stable state; any kelp recruits are eaten before they can establish. Recovery requires either restoring predators (otters, sunflower stars, wolffish, crabs), removing urchins by hand, or both.

Blue carbon is the carbon captured and stored by ocean and coastal ecosystems such as mangroves, seagrass meadows, salt marshes and seaweed forests. Mangroves, salt marshes and seagrasses are formally recognised in the IPCC's coastal blue-carbon accounting, which makes them eligible for climate finance and national emissions inventories. Seaweeds, including kelp, are not. This is partly because much of the carbon it fixes is exported to the deep sea rather than stored in coastal sediment, and partly because the science of how to quantify that export is still being settled. Whether kelp belongs in the policy framework is one of the open questions of this decade.

Scientists studying the kelp forests of California's Channel Islands have catalogued more than 700 associated species. Similarly biodiverse are the giant underwater forests of Patagonia. From tiny polychaete worms nestling in the holdfasts, to Patagonian squid laying their eggs on the fronds, to Magellanic penguins hunting Fuegian sprats in the understorey, to southern right whales passing through on their way south – a multitude of species benefit from the kelp.

Record breaking growth

Alongside their spectacular biodiversity is their impressive productivity. Kelp grows faster than any other plant on the planet. Macrocystis pyrifera, the giant kelp grows up to 60 cm per day in optimal conditions. With patience and a thick wetsuit, watching kelp grow could pass as a meditative spectator sport, a type of floating blue therapy. In the space of one season a whole forest springs up beneath the waves.

Scientists investigating the global productivity of seaweeds found that forests of brown seaweeds like kelp produced 2-11 times more biomass per square metre than intensively farmed crops like wheat, rice, soybeans and corn.

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These burgeoning forests provide us with a plethora of services. Scientists estimated the global ecological and economic value of their contribution to fisheries, nitrogen removal and carbon storage at $500 billion a year. Not accounted for in this number were recreation, coastal protection, direct harvest and the many other benefits they afford us.

Even small forests are worth a great deal, as demonstrated by a study of a Sussex kelp forest in the south of England, almost entirely lost to bottom trawling. Here scientists estimated that restoring it to its former glory would deliver benefits worth more than £3.5 million.

Hidden in plain sight

Kelp forests are the most extensive marine vegetated ecosystem in the world, according to the United Nations Environment Programme. From the UK to Norway, South Africa, Japan, Australia and the Americas, wherever cold water meets rocky coast, kelp grows. "Every marine coast has a seaweed, but about a third of the world's coastline has a kelp forest," says Aaron Eger, founder and director of the Kelp Forest Alliance. Together, the world's kelp forests cover around 1.47 million square kilometres. That’s an area larger than France, Germany and Italy combined.

Eger suggests that there is ignorance to the importance of kelp and that "...someone living in London might be more familiar with a coral reef than with a kelp forest, even though they're surrounded by kelp". Roughly 60% of the UK coastline is home to kelp forests. Yet the sparkly glamour of a bustling coral reef off a tropical island paradise, often portrayed by the media, has skewed the idea of what underwater looks like. With around 1 in 8 people globally living within 50km of a kelp forest, according to Sunstone calculations, one might begin to question if these large brown seaweeds have a PR problem.

For a start, they grow in the cold seas of the world, meaning a swim to see them is bracing, and the creepy “something just touched my leg” sensation is most likely a seaweed frond wafting in the waves. But put on a diving mask or goggles and you experience another world. Getting to know the magic of these seaweed forests allows us to see them as more than plants, and appreciate their worth as rich ecosystems full of wonder.

Most of the world's great land forests have names. The Amazon, the Boreal, the Daintree, the Sundarbans. Yet few of the world’s kelp forests do. The two exceptions are Australia's Great Southern Reef named in 2016 by marine ecologists, who argued that naming was a conservation act, and South Africa's Great African Sea Forest christened by the filmmakers behind My Octopus Teacher. A third name is emerging now in Argentina, where marine conservation organisation Por el Mar and its partners are calling their newly protected kelp the Sea Forests at the End of the World. Naming these great forests allows us to know them. It changes how we feel about them. While the nameless are easier to forget and lose, the identified become recognised, loved and protected. Every named kelp forest is a forest someone has chosen to fight for.

The threats

The list of threats to kelp forests is long. Among them are the climate crisis, overfishing, urchin barrens, nutrient pollution and coastal sprawl, industrial wild harvest, pests, disease and invasive species, and a kind of political invisibility. Their effects rarely act in isolation, often amplifying one another and reducing the ecosystem’s resilience.

Heat is one of kelp’s biggest enemies. Marine heatwaves, described as pulses of anomalously warm water lasting weeks or months, have driven catastrophic losses in three of the world's most studied kelp regions. Off Tasmania, 95 per cent of giant kelp has disappeared since the 1970s. In northern California, 95 per cent of the canopy was lost after the 2014-2018 Pacific heatwave known to oceanographers as 'The Blob'. The same event decimated half of Baja California’s giant kelp forest. Tangle kelp, Laminaria hyperborea, the species that builds the forests of British and Norwegian coasts, can tolerate sustained temperatures of up to about 20 degrees centigrade. Any higher becomes lethal.

Sunstone Institute mapped the shallow coastal areas globally where conditions are right for kelp forests to grow. Kelp has disappeared from many of these areas. Click on the dots on the map below to learn about the current status of kelp forests at each location.

Sea urchin outbreaks have decimated kelp forests globally too, creating vast areas of seabed devoid of life. They are often cited as the biggest threat to kelp forests globally. Natural predators of kelp-grazing sea urchins, such as sea otters, sunflower stars, wolffish, lobsters and cod have been depleted along coastlines around the world. When predators disappear, urchin populations explode. Unchecked they overwhelm the kelp forest in a slow-motion deforestation, as they munch away with their ever-sharpening teeth. The result is what marine ecologists call urchin barrens - bare seabed dominated by hundreds of sea urchins. Any kelp recruits are eaten before they can replenish the forest, and the barrens persist, entering a stable state that can last for many years.

Such ecological imbalances can make kelp more vulnerable to heat stress and vice versa. Norway is a case study in how these pressures layer over time. Hartvig Christie, a marine ecologist and kelp expert who has dived Norway’s kelp forests for more than forty years, describes a moving target, “The Norwegian story has in many ways matched the global story by initial large-scale sea urchin grazing some decades ago,' he says. “Later, increasing temperature and crab predation have reduced the sea urchins and the loss of kelp in some areas, but these areas have even later been challenged by filamentous algae and also other native sea urchin blooms.” The result, he says, is that filamentous algae blooms, called lurv in Norwegian, have extended the threat to areas that had begun to recover. There are still healthy kelp forests along the Norwegian coast. But the areas of loss fluctuate. Christie adds that the pattern is now recognised globally “described from other countries and continents, and for different types of blue forests.”

The photo below is what a pristine tangle kelp forest looked like in northern Norway in July 2025.

The next photo is what a tangle kelp forest overgrown with filamentous algae, called lurv in Norwegian, looked like in northern Norway in March 2026. By blocking sunlight, the blanket of algae hinders photosynthesis, so the kelp is unable to make food stores. As a result, their health falters, and they may eventually perish.

The pattern Christie describes was documented by marine ecologists Karen Filbee-Dexter and Thomas Wernberg in 2018 in a foundational study of what they called the rise of turfs. These short, mat-forming filamentous algae replace kelp under warming and nutrient-loaded conditions, and hinder the return of the forest. From Norway to America and Australia, overgrowth and replacement by filamentous algae now ranks as a global threat alongside ocean warming and overgrazing.

According to a 2023 UN synthesis report, up to 60 per cent of the world's kelp forests have been degraded over the past 50 years.

Protection

The fight for these underwater forests is real. Until recently, the protection infrastructure for seaweeds had been overlooked. In a 2025 review of 18 international biodiversity frameworks, scientists found that just four reference seaweeds, and only three name them specifically. These treaties, conventions and action plans govern how the world's ecosystems are protected. Published in 2023, a separate report found that fewer countries mention kelp in their climate plans than mention mangroves, seagrasses or salt marshes. The most extensive marine ecosystem on Earth was somehow eluding the gaze of the international system.

Part of the problem is geographic. “There's been a little bit of conservation imperialism,” Eger says. “Coral reefs tend to occur in tropical countries, which on average are less developed than the temperate countries that tend to have the kelp forests. A lot of these countries and organisations are more willing to go solve the problem somewhere else, as opposed to investing in addressing the issues that are occurring in their own backyard.” But, he adds, “recognition of kelp forests in international frameworks and biodiversity frameworks is increasing.”

Enter Argentina

At the furthest reaches of South America a giant underwater victory has been won. Roughly 90 per cent of Argentina's wild giant kelp is now under statutory protection. That’s around a third of the world's remaining Macrocystis growing in ‘one of the largest, and least disturbed, kelp forest systems left on the planet’. The framework was built in three steps over three years and across the country's two southernmost provinces.

The first layer was the Mitre Peninsula Natural Protected Area in Tierra del Fuego, ratified in December 2022 after a three-decade community campaign. The second was a Tierra del Fuego provincial law passed unanimously on 11 December 2024, banning wild kelp extraction across the whole province, both inside and outside protected areas. The third was a Santa Cruz provincial law, passed on the same date a year later, completing the framework.

In the water

Active restoration is the other half of the conservation effort, and it is slow, expensive work. “You're always going to want to prioritise protection over restoration,” says Aaron Eger of the Kelp Forest Alliance, “because it's much easier to protect and manage an existing kelp forest than to restore one.'’ Almost 19,000 hectares of kelp forest have been actively restored worldwide since the late 1950s. This is roughly two per cent of the Kelp Forest Alliance's target of one million hectares restored by 2040.

The most ambitious state-led effort is in South Korea, where the government invested $280 million between 2009 and 2019, recovering 21,500 hectares of kelp forest. Korea now aims to regreen 75 per cent of its coastline, using a combination of professional restoration crews and community marine-gardening days. The model whereby restoration is funded and treated as public infrastructure, has no global equivalent. Korea's intervention is also a sign of how far the system had to fall before being rebuilt. “South Korea,” Eger says, “is heavily urbanised, populated, exploited, utilised. They're past the point where protection alone would be sufficient.”

Elsewhere, smaller-scale projects develop experimentally. Scientists in Tasmania grow heat-tolerant giant kelp in laboratories and outplant it onto warming reefs. Volunteer divers in Norway and California cull sea urchins from former kelp forests one by one using hammers, and researchers in California reintroduce sunflower stars to take up the predator role. Meanwhile in Sydney, Operation Crayweed has restored seaweed at 16 sites along the city's coastline. From Eger's standpoint, “Being able to outplant kelp successfully, effectively, at a low cost would be revolutionary from a restoration perspective.”

None of these are quick wins, and most depend on the kind of protection Argentina has just enacted. Restored forests remain vulnerable to the same pressures that depleted them in the first place. Revealing the hidden wonders of kelp forests, and giving them an identity, could be a first step towards their protection. In the words of writer Richard Louv, “We cannot protect something we do not love, we cannot love what we do not know, and we cannot know what we do not see.”

The recent victory in Argentina began with someone's introduction to kelp – a first dive, a documentary film, a scientific study – and blossomed into a thirty-year campaign for protection. As of December 2025, when the Santa Cruz legislature passed the final piece of the framework, ninety per cent of Argentina's wild giant kelp is under statutory ban from harvest. Three of the world's great underwater forests now have names. Many more are waiting.

Hidden in plain sight

The threats

The photo below is what a pristine tangle kelp forest looked like in northern Norway in July 2025.

According to a 2023 UN synthesis report, up to 60 per cent of the world's kelp forests have been degraded over the past 50 years.

The forgotten forests of the sea

The forgotten forests of the sea

Why kelp matters

Underwater engineers

Kelp forests explained

Record breaking growth

Enjoy this story?

Hidden in plain sight

The threats

Protection

Enter Argentina

In the water

Hidden in plain sight

The threats

Protection

Enter Argentina

In the water

The forgotten forests of the sea

The forgotten forests of the sea

Why kelp matters

Underwater engineers

Kelp forests explained

What is kelp?

Underwater forests?

How fast do they grow?

Where in the world do kelp forests grow?

Who lives in a kelp forest?

What's killing them?

What are urchin barrens?

Are kelp forests blue carbon?

Record breaking growth

Enjoy this story?

Interested in our sources?

Hidden in plain sight

The threats

Protection

Enter Argentina

In the water

Hidden in plain sight

The threats

Protection

Enter Argentina

In the water