Kelp and other seaweeds
Scotland’s Year of Coasts and Waters has officially been extended to 2021, given many events and promotions couldn’t go ahead last year as a result of the pandemic.
To recognise this, and to raise the profile of many exciting habitats and species we find in Arran’s seas, many protected by the South Arran Marine Protected Area, the Community of Arran Seabed Trust (COAST) is running a year-long media campaign to raise awareness.
Each month, they will focus on a particular habitat or collective group of inter-linked species and here is their instalment for May. For more information on COAST see their social media or visit www.arrancoast.com.
It’s spring and as the days warm and lengthen, vegetation is waking up and greenery is blossoming.
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.
Unlike seagrass, which featured in the Banner last month, 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.
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.
Variety
In contrast to seagrass which grows in sand, seaweeds require a 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 also be found growing in sandy areas and on gravel. There are a huge variety of seaweeds which are generally catagorised as being either brown, red or green.
The February Banner article about rocky reefs explained how seaweeds, like plants on land, need light to grow so are most abundant in shallow water. All seaweeds contain chlorophyll – the pigment in plant cells that gives them their green colour – but brown and red seaweeds also contain other pigments. The additional pigments in red seaweeds mean they can use different wavelengths of light, enabling them to survive in slightly deeper water where light levels are lower.
Microscope
The varied seashore and seabed habitats around Arran provide opportunities for a range of different seaweed species to thrive, but identifying these is not always straightforward. Some species can be quite easily identified upon sight, whilst others – such as different species of maerl – need to be examined under a microscope. The time of year can help with species ID, as some seaweed plants are present all year round, while others are annual or survive over winter as just small, insignificant crusts or stalks.
In spring, as light levels and seawater temperatusre increase, denser seaweed cover appears in rockpools and underwater. This new seaweed growth can have a clean, almost polished appearance which creates a vibrant underwater seascape, particularly in shallow, sunlit water. Seaweed growth can be very dense. 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 to 70 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 marine life such as sea firs and sea mats which in turn provide food for animals including sea slugs and 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.
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 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.
While there is growing interest in farming kelp and other seaweeds for food, fuel and chemical production, we need to make sure 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 spatial protection provided by Arran’s Marine Protected Area and the No