The Role of Seaweed in Coastal Ecosystems: What It Really Does

Seaweed covers rock along most of the world's coasts. It sways in the surf, grips against the waves, and feeds the animals living around it.

It looks like a plant. It is not one.

The earlier version of this post listed five things seaweed does for the coast and explained none of them. It also credited seaweed with three jobs that belong to other organisms.

This rewrite keeps what holds up in the research and fixes what does not.

Agiorgio. Seaweed on a rock in the surf. There is no soil here and no root below. The whole organism takes what it needs straight from the water flowing past it. CC BY-SA 3.0, via Wikimedia Commons.

What Seaweed Is, and What It Cannot Do

Seaweed is the common name for thousands of species of large marine algae. They come in three main colors: green, brown and red. None of them has roots, stems or leaves.

What they have instead is a body called a thallus. It is simple tissue with no vessels inside it. Water and nutrients enter through the whole outer surface, directly from the sea.

A holdfast is not a root

A seaweed grips rock with a holdfast. It looks like a root ball and works nothing like one. Its only job is to hold on. It absorbs no water and no nutrient.

That fact settles two claims from the earlier version at once. Seaweed cannot pull anything out of the sediment, because it never reaches the sediment. And it cannot hold sediment in place, because nothing it grows through binds sand or mud together.

The organisms that hold sediment in place are rooted plants. Seagrass is a flowering plant with roots and rhizomes under the mud. Salt marsh grasses and mangrove roots do the same at the shoreline.

Feature	Seaweed	Seagrass
What it is	An alga	A flowering plant
Roots	None. A holdfast that only anchors	Yes. Roots and rhizomes through the mud
Grows on	Rock and other hard surfaces	Sand and mud
Holds sediment in place	No	Yes
Buries carbon where it grows	No, it lives where sediment cannot build up	Yes, in the sediment below it

Two green things that look alike in a photograph and do different jobs under the water.

Frédéric Ducarme. A seagrass meadow. Flowering plants with roots: this is the vegetation that binds sediment and buries carbon where it grows. CC BY-SA 4.0, via Wikimedia Commons.

Shelter: The Claim That Holds Up Best

Where seaweed grows tall it makes a canopy. Kelp can rise from the seabed to the surface in a few months, and the space between the stipes fills with life.

A global study of six kelp genera found that these forests shelter important fisheries. Abalone, lobsters and reef fish feed and hide in them.

The same study put money on the services. Each genus was worth between $64,400 and $147,100 per hectare per year, adding up to $465 billion to $562 billion worldwide.

Those are potential values. They are what the services would fetch if they carried a price tag, not money anyone collects. The physical part behind the number is about 904 kilograms of fishery production per hectare per year.

Shelter is also the claim that needs no hedging. A fish hiding in a frond does not care how the carbon budget balances.

A canopy without wood

A kelp forest has no trees. The blades stream with the current instead of standing against it, which is how a plant with no wood reaches the surface.

That flexibility does two things at once. It lets the canopy reach the light, and it costs the canopy most of its power to stop a wave. The wave claim is taken up below.

oliver.dodd. Fish moving through a kelp forest. The canopy is shelter and hunting ground at the same time, which is the service kelp is best at and best documented for. CC BY 2.0, via Wikimedia Commons.

Carbon: Captured, Mostly Returned

Seaweed is very good at capturing carbon. Macroalgae cover about 3.4 million square kilometers of coast and fix roughly 1.5 billion tonnes of carbon a year.

The earlier version stopped at that point and called seaweed a natural carbon sink that helps slow global warming. The next number is the one that matters.

Of all that carbon, about 11 percent is estimated to end up stored in sea-floor sediment or deep water. The rest is eaten, breathed back as carbon dioxide, or washed ashore to rot.

Where seaweed grows, the carbon rarely stays. Most seaweeds live on rock, where no sediment builds up to bury anything. What gets stored is exported instead: currents carry detached fronds offshore until they settle.

Some of that export does arrive for good. Studies of seagrass sediment find that half of the carbon there came from other producers, seaweed among them. Seaweed is a donor to those stores rather than a store itself.

So is it worth anything for the climate?

Probably yes, and the honest answer is a range rather than a slogan. Researchers describe macroalgal carbon as the elephant in the blue carbon room: large, real, and still hard to account for.

A first-order global estimate puts the stored fraction at 173 million tonnes of carbon a year. That is comparable to what mangroves, salt marshes and seagrasses are credited with combined.

The accounting is unfinished. China and Korea already count farmed seaweed in their national blue carbon programs, and most other countries do not.

Eric T Gunther. Kelp canopy at the surface. Fronds that break loose drift away from stands like this one, and that export is where most of the carbon ends up. CC BY-SA 3.0, via Wikimedia Commons.

Nutrients: Cleaned, Then Taken Away by Harvest

Seaweed takes up nitrogen and phosphorus from the water fast. It works like a sponge with no preferences, which is why it is grown in farm ponds and wastewater channels on purpose.

At sea the numbers are measured in China, which grows about two thirds of the world's seaweed. One hectare of farm removes the nutrient load reaching 17.8 hectares of coastal water for nitrogen, and 126.7 hectares for phosphorus.

Inside those farms, dissolved nutrients measured about half the level in nearby open water. Across the country the farms strip roughly 75,000 tonnes of nitrogen and 9,500 tonnes of phosphorus from the sea each year.

Then comes the part the earlier version left out. The nutrients leave the water only when the seaweed is cut and used. Uptake holds them in the plant. Harvest takes the plant off the coast.

Cut seaweed left to decay gives the nitrogen and phosphorus back to the water. The cleaning finishes at the harvest, not at the absorption.

Scale has limits too. China's seaweed farms remove about 5.6 percent of the nitrogen entering its coastal waters each year, and close to 40 percent of the phosphorus. Useful, and not decisive.

The crop that leaves with the nitrogen

That harvest is an industry. The Food and Agriculture Organization counts 36.5 million tonnes of farmed algae in 2022, wet weight, worth about $17 billion at the farm gate. Production has more than tripled since 2000.

Seaweed goes to food, to thickeners like alginate and carrageenan, to animal feed and to fertilizer. The people who cut it and sell it are the same people removing that nitrogen.

NASA Goddard Space Flight Center from Greenbelt, MD, USA. Seaweed farms off South Korea seen from space. Farms at this scale are visible from orbit, and the nutrient they remove leaves the water only when the crop is harvested. Public domain, via Wikimedia Commons.

Yann Macherez. Seaweed cut by hand at Jambiani, Tanzania. The harvest is the step that completes the cleaning: cut the crop and the nitrogen goes with it. CC BY-SA 4.0, via Wikimedia Commons.

Waves: A Dampener, Not a Barrier

The last claim is the one that failed hardest when it was tested. The earlier version described thick seaweed beds as natural wave barriers that prevent erosion and storm damage.

The researchers who measured this properly were more careful. They placed wave loggers at four sites, each with a kelp bed and a bare patch of the same reef side by side.

At three of the four sites, waves lost more height over the bare reef than over the kelp. At the fourth there was no difference, except under one wind direction where the kelp did about 10 percent better.

Their conclusion was that evidence for kelp as coastal defense is thin, and that the rock under the kelp and the kelp itself must be measured separately.

The reason is the same flexibility that lifts the canopy. A blade that bends with a wave takes less energy out of it than a reef that stands still.

So seaweed helps a little at the edge. It is not a sea wall. The shoreline claims that hold belong to rooted vegetation: seagrass offshore, salt marsh and mangrove at the line.

Key Terms in Plain English

These are the words in this article that would send you to a dictionary. Each one is given here the way it is actually used above.

Term	What it means
Macroalgae	Large seaweeds: the red, brown and green algae big enough to see without a microscope.
Thallus	A body with no true tissues and no true organs, which is what every seaweed is built as.
Holdfast	The structure that attaches seaweed to rock. It anchors only, and absorbs nothing.
Kelp	The large brown algae of the order Laminariales, which build the underwater forests.
Canopy	The top layer of a kelp forest, near the surface where the light is.
Blue carbon	Carbon captured and stored by coastal ecosystems such as marshes, seagrass and mangroves.
Eutrophication	Too much nutrient in water, and the algal blooms that follow from it.
Rhizome	An underground stem. Seagrass uses it to creep through mud and to hold on.
Detritus	Dead organic matter. What most of a seaweed crop becomes if nobody harvests it.
Farm gate	The moment a product leaves the farm. Prices quoted at the farm gate ignore transport and processing.

Terms used above, in roughly the order they first appear.

Key Takeaways

• Seaweed is an alga with no roots: the holdfast only anchors, so seaweed cannot take nutrients from sediment or bind sand in place.
• Shelter is the strongest service: kelp forests house abalone, lobster and reef fisheries, and the six main genera carry a potential value of $64,400 to $147,100 per hectare a year.
• Carbon capture is real but mostly temporary: about 11 percent of the carbon seaweed fixes is estimated to reach lasting storage.
• Nutrient cleaning works through harvest: China's farms strip about 75,000 tonnes of nitrogen a year, but only because the crop is cut and removed.
• Wave protection is the weak claim: at three of four measured sites, kelp beds sheltered the shore no better than bare reef did.
• Farming is large and growing: 36.5 million tonnes of algae in 2022, worth about $17 billion at the farm gate.

Frequently Asked Questions

Is seaweed a plant?

No. It is an alga. Plants have roots, stems and vessels that move water inside them, and seaweed has none of the three.

The similarity is real but shallow. Both photosynthesize, and that is where the shared design ends.

Does seaweed hold the shore together?

Not really. Its holdfast grips rock and absorbs nothing, so it never binds sand or mud.

Measured against bare reef, kelp beds took a little more energy out of waves at one site in four and less at three. Seagrass, salt marsh and mangrove are the shoreline holders.

Does seaweed fight climate change?

It captures a great deal of carbon and stores a small share of it. The stored share is estimated at about 11 percent.

That is still a large amount in absolute terms, and it is why two countries count farmed seaweed in their blue carbon programs. The word sink, on its own, hides the rest of the story.

Does seaweed clean polluted water?

It strips nitrogen and phosphorus out of the water very well. Inside Chinese farms, dissolved nutrients measured about half the levels outside.

The cleaning only counts when the seaweed is harvested. Cut and removed, the nutrients leave with it. Left in the water to decay, they come back.

Key references
• Seaweed: the macroalgal group and its uses
• Kelp forest: the habitat and its global distribution
• Seagrass: flowering plants with roots, not to be confused with seaweed
• Holdfast: an anchor with no absorbent function
• Krause-Jensen et al. 2018, macroalgal carbon in the Blue Carbon framework, Biol Lett 14:20180236 -- the source for the 11 percent stored fraction
• Morris et al. 2019, kelp beds as coastal protection, Ann Bot 125:235-246 -- kelp beds measured against bare reef at four sites
• Xiao et al. 2017, nutrient removal by Chinese seaweed aquaculture, Sci Rep 7:46613
• Eger et al. 2023, ecosystem services of global kelp forests, Nat Commun 14:1894
• FAO 2024, State of World Fisheries and Aquaculture: farmed algae output and value

Figures are from Wikimedia Commons, with the author and license named in each caption. Measurements are from the cited papers and the FAO report.