Kelp & Seaweeds

In 2021, Scotland celebrates the Year of Coasts and Waters, and COAST celebrates 25 years of the community of Arran’s efforts to protect our seas.

Here we bring you a little bit closer to what lies beneath our waves with a focus on the intriguing habitats of kelp and other seaweeds we find in Scotland’s seas.

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It’s spring, and as the days both warm and lengthen, vegetation is waking up and greenery blossoms forth. Just as on land, a similar transformation is happening underwater in the shallow, well-lit waters surrounding Arran, as our most common marine plants – seaweeds – start to flourish with clean, luxuriant new growth taking hold.


Seaweeds are relatively simple plants, although in some ways their biology is far from simple. They are often referred to as algae, organisms that have been on earth since before the first dinosaurs; fossils of the early relatives of our current seaweeds can be found which are 1.6 billion years old. Although they might be viewed by some as simply a slippery hazard on rocky shores, seaweeds play a vital role in our seas forming the basis of all food chains in the ocean.

While they lack the roots, stems and leaves of flowering plants, seaweeds have similar-looking structures: a holdfast (which anchors the seaweed to the seabed), a stipe (a stalk-like structure that holds the plants up) and fronds (the more leafy-like part of the plant). In contrast to seagrass which grows in sand, seaweeds require some sort of hard material for the holdfast to attach to, therefore they are most abundant in rocky areas. This said, even a small stone or piece of shell can be enough for them to hold on to, meaning seaweed can be found growing in more sandy and gravelly areas as well. 

The importance of Kelp & Seaweed

It is estimated that there are 9x more microscopic algae and seaweeds in the oceans than there are plants on land which in itself is amazing – but what else makes Kelp and other seaweeds so fascinating?

  • They have complex methods of reproduction and, depending on the structure, this can be used to help identify the specific species. Underwater currents act like the wind on land, as they carry gametes through the sea; although some have adapted to swim and others emit pheromones to increase fertilisation chances!

  • In the natural history boom of the Victorian era, there was a huge interest in collecting and pressing seaweeds, particular by women (as it was considered a suitably genteel outdoor activity), a few of whom published their findings providing early catalogues of species records. Samples from these historic seaweed collections have been used to provide an insight into environmental conditions in the sea in the 19th century.

  • Due to the dense nature of seawater, many seaweeds do not require the sturdy, rigid branches and stems like counterparts on land. This said, some – such as kelps and the fucoid species – have adapted to have air bladders to help them float in order to reach the shallower depths with greater light. These air bladders are the source of much childhood amusement as they ‘pop’ with the same sound and enjoyment of bursting plastic bubble wrap. Air bladders on some species can aid with identification, with bladder wrack having paired bladders whilst knotted wrack has single bladders along the midline of its fronds.

Special Species

The coastline and seas of the UK are home to over 600 species of seaweeds. They come in a huge variety which can broadly be grouped into red, brown and green varieties. All seaweeds need light to grow; all contain chlorophyll (the pigment in plants that gives them the green colour), but brown and red seaweeds contain other pigments which allows them to make use of different wavelengths of light, allowing them to survive in deeper water with lower light levels.

A large number of these red seaweeds are coralline algaes, meaning they deposit lime into their cell walls. We’ve introduced you to maerl before during #LivingReefMonth but there are other red seaweeds that are commonly found as encrusting layers over rocks and are therefore, like maerl, are not initially recognisable as a seaweed.

The large brown kelp plants that dominate parts of shallow rocky areas around Arran can create underwater forests which provide a home for many other plants and animals. The nooks and crannies of a kelp holdfast, which superficially resembles contorted roots, can shelter between 30-70 different species, including anemones, sea squirts, marine worms and crustaceans. Kelp stipes, on the other hand, can often be found covered in red seaweeds, sea mats, clusters of feather stars and other encrusting marine life. In contrast, kelp fronds produce a slippery mucous that makes it hard for animals to attach to, so they can often look devoid of attached life. However, by the end of summer they too have often become covered in growths of marine life such as sea firs and sea mats which in turn provide food for animals such as sea slugs and beautiful iridescent blue-rayed limpets. These kelp forests provide essential feeding grounds for some of Arran’s favourite megafauna – the Eurasian otter – and support commercially important lobster and crab.

One of the most common questions we get asked is “do you find sea horses in seagrass?” Well, you do, but not here off Arran. We have no records of sightings of seahorse around Arran, and there are currently no records north of SW Wales in the NBN Atlas for their genus. This said, we are lucky to have frequent sightings of their cousins – the pipefish. Most often we spot the Great pipefish (Syngnathus acus), but there is a reported sighting of a Nilsson’s pipefish (Syngnathus rostellatus) in the waters around Arran.

If you take a dive or snorkel over a seagrass bed, you have to focus your eye to marco setting to really see the wonderful life these habitats support. Many different species of nudibranchs and other sea slugs cling to the blades floating in the currents, and many also lay their eggs on them. Watch out though, you may get surprised by a spider crab or two, who perfectly blend in with the seagrass so as not to be spotted!

These juvenile scallop resting on a kelp frond were captured by Howard Wood. It’s not surprising to see these juveniles on this large piece of seaweed, as research has shown that “juvenile scallops preferentially settle in structurally complex habitats, such as kelp stipes and macroalgal fronds”. It is therefore particularly important we protect these seaweed communities as their damage and removal has been shown to negatively impact scallop recruitment.

Human Benefits

As well as supporting a rich diversity of life and commercial fisheries, seaweeds provide humans with many other benefits. For a long time, seaweeds have been valued as a food and for providing thickening and gelling compounds used in a wide range of products such as toothpaste, cosmetics and ice cream.

Kelp forests provide a natural form of sea defence by absorbing and reducing the power of waves impacting on the coast and they are now recognised as an important blue carbon habitat, as they absorb carbon dioxide into the plant tissues which can ultimately be locked up as organic matter in marine sediments when the plant tissue breaks down.

Coralline red seaweeds have a number of economic uses thanks to their hard, calcified nature – they’ve even been uses in preparation of dental bone implants!

Threats to Kelp & Seaweed

As with many special habitats and species we find in our seas, kelp and seaweeds are under threat from things like ocean acidification and non-native invasive species, but what else?

Seaweeds are under threat from unsustainable extraction methods. Take kelp as an example; back in 2018 there were proposals to dredge kelp forests in Scottish waters on industrial levels, to extract biopolymers for food, cosmetics and textiles. The problem? The method proposed was mechanical dredging, which would involve heavy metal dredge heads, 3-4 metres in width, being dragged through these forests. Luckily, the Scottish Government didn’t approve the proposal, and recognised the evidence that the kelp would fail to regenerate.

While there is growing interest in farming kelp and other seaweeds for food, fuel and chemical production, we need to make sure that we look after our wild seaweed communities. Increasing seawater temperatures as a result of climate change are having a marked effect on the abundance and distribution of some seaweed species. It is in our own interest to help seaweeds thrive, but this requires urgent and effective climate change action on a national and global scale, as well as the sort of local spatial protection such as that provided by Arran’s Marine Protected Area and the No Take Zone.

In parts of Arran, the invasive non-native seaweed known as wireweed (Sargassum muticum), a large brown seaweed from the north-west Pacific, is another potential threat to seagrass. At Kildonan, the natural breakwaters of the rock dykes provide sufficient shelter for seagrass to grow in this otherwise exposed location. The seabed here is more stony, an unusual situation to find seagrass in. The stones allow wireweed to get a foothold and, where it does, its large, spreading growth can create a canopy that blocks out the sunlight and shades the seagrass affecting its growth.

The presence of commercial fish and shellfish make sand and gravel habitats a target for fisheries, some of which can have a significant and lasting impact on the marine life communities that live in them. Some of our burrowing bivalve (two shelled) animals are extremely vulnerable to bottom trawling which results in direct mortality of the animal when disturbed. Damaging fishing methods also destroy habitats to the extent they are no longer suitable as nursery grounds for juvenile fish.

Can we protect Kelp & Seaweed

To protect our marine environment it is vital we protect the kelp and seaweeds that support our marine ecosystems. Seaweeds mitigate the effects of climate change, coastal erosion and are immensely valuable species in our oceans.

Sadly the effects of climate change – ocean acidification, ocean warming and sea level rise – are threatening our seaweeds around Britain, altering their distribution on our coasts. Careful monitoring is required for us to gauge a better understanding of how these environmental impacts are influencing our seaweeds and, therefore, the wider ecosystem.

Harvesting of wild kelp – particularly by mechanical means – threatens this type of macroalgae. Seaweed aquaculture represents a potentially sustainable alternative and is currently carried out in 37 countries across the globe. At a local level, this type of aquaculture could offset carbon emissions of agriculture, and also improve water quality in areas with high run-off from land.

A number of experimental seaweed projects have been established in Britain, for example at the Scottish Association for Marine Science and the University of Highlands and Islands in Shetland. Sugar kelp is the main output from these facilities, with investigations focusing on the use of seaweeds as chemicals, biofuels and speciality products.

Get involved to make a difference!

As with many species we find in our seas, there are various ways to get involved to make a difference when it comes to Kelps and Seaweeds.

The Big Seaweed Search is a citizen science project, spearheaded by the Marine Conservation Society in partnership with the Natural History Museum. This project is all about monitoring the impacts of climate change on our marine environment and the unique species that live there, through recording what species of seaweed are found where and how their distribution is changing over time.

If you’re interested in learning more about foraging on the seashore whilst discovering the fascinating facts of different seaweeds then we’d highly recommend a course with Mark Williams of Galloway Wild Foods. Mark says “I take great satisfaction from helping people to unlock their edible landscape and get to grips with wild resources”. Summer is high season for plants along the coast so if you’re looking for something different to get involved with book a place now!  

Cover image Kelp & Seaweed © Paul Kay