The seas around Britain and Ireland are home to around 600 species of seaweed. These plant-like organisms are marine macroalgae and are widely found in coastal areas, often attached to rocks and other substrates.

Hundreds of species around UK shores make up more than 6% of the world's total known varieties. Ecologically vital, they provide shelter and food for numerous other marine creatures.


© Kuttelvaserova Stuchelova /

Seaweed is divided into three taxonomic groups: red, brown and green. As well as the different colours, the groups also differ in more complex biochemical and structural features, including their cell structure and photosynthetic pigments.

Related to algae, protozoans and diatoms, brown seaweeds are classified as part of the Kingdom Chromista. Red and green seaweeds are classified in the Kingdom Plantae, a group that also includes all land plants.


Identifying types of seaweed

Identifying the types of seaweed in terms of their classified group by colour is simple. However, within the three groups, there are many different individual species around the UK.

An olive-brown seaweed, Bladderwrack is recognised by its smooth-edged, branching fronds, which can be anything from 15cm to 100cm long. It is commonly found on rocky shores.

Knotted wrack, also known as egg wrack, is a yellowish-brown seaweed that can grow up to two metres long. It can live up to 15 years and consequently can become dominant, mainly on sheltered coastlines.

Oarweed has distinctive dark brownish fronds up to two metres long. They divide into long, finger-like blades and attach themselves to rocks with a claw-like grip. They grow in dense beds at a depth of up to 20 metres. The tips of the fronds can often be seen on the surface at low tide.

As one of the biggest seaweeds, sugar kelp, also known as sea belt, has long, yellowish-brown blades with ruffled sides that can grow up to five metres long.

At around 25cm long and 30cm wide, sea lettuce is a translucent, delicate seaweed that looks like a large lettuce with floppy leaves. It is often found floating in rock pools or attached to rocks.

Gutweed, also called grass kelp, has inflated hollow fronds with bubbles of air trapped inside. A bright green, it grows up to 40cm long in many different inter-tidal habitats.

Appearing iridescent when submerged, Carrageen is a dark reddish-purple seaweed that branches out. It is prevalent in estuaries, on rocky shores, in pools in inter-tidal zones and on rocks.


Can humans eat seaweed?

Seaweed is most prevalent in Asian cuisines, where it's a staple of salads, sushi, soups and stews. Historians believe eating seaweed dates back to prehistoric times. With many health benefits, it is usually considered safe for most people.

There are some 145 varieties of edible seaweed across the world. As a source of the mineral iodine that supports thyroid health, it can improve gut health and reduce blood pressure and cholesterol.

Prepared dried seaweed can be bought in many organic and health food shops. Edible species found growing around British shores include kelp and gutweed - there are no poisonous species in the UK.

After collecting seaweed, usually in the spring and summer, it can be eaten raw or cooked. Wash well in cold water before eating or cooking. Many people cut up and deep-fry fresh seaweed for up to 20 seconds to make a crispy accompaniment.

Delicate species like sea lettuce can be eaten raw, or gently cooked. To save for a later date, spread it out on a baking tray and dry it slowly at the bottom of the oven. Once dry, add salt and sesame seeds and pop it in a blender to break it up into small pieces. To add a second flavour, you can also wrap fish in sea lettuce before cooking. Its bright green colour looks attractive on the plate too.

As well as eating seaweed, it is also used in natural medicines, skin products and even fertiliser.


Has climate change impacted seaweed?

In recent years, more seaweed is being found on beaches. Data compiled during the past decade reveals the likely causes include warmer water temperatures caused by global warming.

Ironically, some scientists believe the increase in kelp can help combat climate change. Seaweed absorbs carbon emissions, in the same way as trees. It also regenerates marine ecosystems, creates biofuel and produces marine protein.

While there's no need for people to avoid seaweed on UK beaches, you must stop your dog from eating it raw. It will absorb water and expand in the dog's digestive system. This can cause potentially fatal blockages. In addition, if your dog eats unwashed raw seaweed, it could contain pollutants or bacteria.


Does seaweed affect surfers?

Scientists believe seaweed thrives at surfing spots because the extra turbulence produced by breaking waves stirs up nutrients in the water, which feeds the algae.It has created a few problems for surfers in some regions.

Surfing the Outer Kom, South Africa's famous big-wave spot, surfers in recent years have found themselves trapped in a large blanket of kelp. Jumping off the rocks and paddling towards the waves, they meet resistance from an "impenetrable carpet" of thick, brown, seaweed leaves, from the Ecklonia Maxima species. Widespread at reef breaks in South Africa, kelp has evolved to live in the surf zone. It can reduce the energy of the waves, but is a nuisance, rather than hazardous.

Off the coast of northern Scotland, surfers can encounter another type of kelp. Laminaria Hyperborean sits just below the water level at mid-tide. While it can cushion wipeouts, it is also slippery, making walking out to the break treacherous.