Maërl is one of those species that is indicative of clear, clean waters. Whilst it is known locally as Scottish coral, maërl is not a coral, but it does have a hard, bone-like structure. It is actually algae, and its beds can be quite common in a huge number of locations, from the Mediterranean to Scandinavia. Lawson Wood shares his insights and photography of this special species.
Painted gobies (Pomatoschistus pictus) on a bed of maërl
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Maërl beds are found along most of the Atlantic coastline, from Portugal to Norway, as well as the south coast of England, the Irish Sea (around the Isle of Man) and, in particular, Cornwall and Pembrokeshire. They live principally in the “photic” zone of high sunlight along the sublittoral coast down to around 30m (100ft); however, they can also be found in depths around 120m (400ft) in the Mediterranean.
Maërl is in fact a calcareous or calcified red seaweed, or coralline alga, which deposits lime (calcium carbonate) in its cell walls as it grows. Growing roughly “antler”-shaped in form, it can also form large, flattened discs in more exposed, wave-pounded locations.
Each alga is completely separate from its fellow cousins, forming three-dimensional structures, making for a loose collection of millions of individuals, with their lime skeletons interlacing with all the others around them. Unattached maërl deposits are referred to as “rhodoliths” (red rocks), which can be either dead or alive. Known to sequester carbon, they are of vital importance in our marine ecosystems, principally as hatcheries for a large number of species.
Maërl species and distribution
There are three fairly common types found around the UK coasts: Phymatolithon calcareum, which is the most common and widespread; Lithothamnion glaciale, which is more common in the more northern extent of its range; and Lithothamnion corallioides, which has a more southern range. There is a newly described species called Lithothamnion erinaceum, and this has been recorded amongst the Lithothamnion glaciale beds in Northern Ireland, Scotland, Iceland and Norway. Very difficult to tell apart to our untrained eyes, they all look very similar indeed.
Scotland is home to the most extensive maërl beds in Europe, and, as such, great emphasis is being lobbied for their protection. The alga is known locally as Scottish Coral. Natural England has also been very forthcoming in its efforts to raise awareness of the beds in the Fal Estuary, Helford Estuary and St Austell Bay.
Scientists have discovered around 550 different species of marine life that are fairly dependent on the maërl. When coupled with extensive seagrass and native oyster beds, the ecosystem is incredibly diverse.
Thousands of years old
The Falmouth Bay to St Austell Bay Special Protection Area (SPA) is headlining the importance of the beds on the south coast of England. The Carrick Roads Bed in the Fal Estuary is understood to be over 4,000 years old and is recognised as the best example in England, covering around 880 hectares and ranging from 6 to 10m (20 to 33ft) deep in some parts. (The Fal Harbour Commission banned the harvest of maërl in these locations in 2005.)
Carbon dating on some of the French beds has revealed that they are over 5,500 years old. One maërl bed off Iona, on the west coast of Scotland, was recorded as around 4,000 years old, and some of the most extensive Scottish beds are recorded as dating back around 8,000 years.
Growth rate and environmental threats
Growing at just 0.5 to 1.0 mm per year, it can literally take many hundreds of years for the living and dead maërl deposits to develop large, important beds, some of which can be over 9m (30ft) deep. Individual live rhodoliths or maërl “thalli” (shoots, twiglets or twigs) can grow to around 7cm (2.75 in) in size and can be over 100 years old. As such, these beds are irreplaceable habitats, and every effort is being made to protect them from threats such as offshore development, dredging, bottom trawling and the placement of ships’ anchors and heavy chains, as well as aquaculture facilities.
Disturbed sediment from any of these activities does impact the amount of available light, which maërl requires for photosynthesis. Once lost, it will be impossible to recreate the maërl beds again. Other problems, such as agricultural run-off, which can be very high in nutrients, and untreated sewage, also cause excessive growth of algae, which can smother maërl beds.
We are all very aware now of the immediate and long-term effects of climate change and ocean acidification. Maërl absorbs carbon dioxide, and its interlacing structures also hold organic matter, which can cement areas and further protect them from tidal surges and strong currents.
Weird fact: Maërl is often referred to as “The Trees of the Sea”. Like terrestrial trees, which have yearly growth rings, so do these algae, giving us a unique record of sea temperatures and chemistry dating back millennia.
Nursery for marine species
Maërl beds are home to a plethora of marine species, including juvenile scallops, burrowing molluscs and bivalves, sea cucumbers, sea urchins, anemones and various worms, as well as numerous species of fish. Some beds can be very dense, as the accumulation of the dead maërl builds up. So, it is vital that scallop dredging is completely banned in any maërl areas. Interestingly, those beautiful white sandy beaches on the west coast of Scotland are made up of the pulverised white maërl skeletons.
Harvesting and conservation
Unfortunately, maërl is still harvested in a number of locations off the coast of Brittany in France and Bantry Bay in Ireland, where it is crushed to a powder and is a popular fertiliser for organic gardening. Scientists are currently studying the extensive maërl beds on the west coast of Ireland, particularly in County Galway, at Trá an Dóilín near An Cheathrú Rua (Coral Beach or Coral Strand), and at Ballyconneely (Mannin Bay), also in County Galway. A recognised tourist attraction, there are huge amounts of dead maërl cast up onto those beaches, creating the coral beaches. The deposits off Bantry Bay are dead (subfossil) beds covered in a layer of mud, with extraction being done by pump-dredging.
There are 11 marine protected areas (MPAs) in Scotland that have maërl beds, and they are recognised as UK BSAP habitat and OSPAR threatened and declining habitat (under the Convention for the Protection of the Marine Environment of the North-East Atlantic).
The beds of the Fal Estuary are in a Special Area of Conservation. Following extensive campaigning by Seasearch scuba divers, the Cornwall Wildlife Trust and the Marine Conservation Society, a total ban on scallop dredging in the Estuary came into force in 2009.
The IUCN Red List has the species listed as a Priority Marine Feature in Scotland, as the country hosts 30 percent of all the maërl found in Northern Europe.
Where to see it
The best places to enjoy this natural and ancient wonder are the Fal and Helford Estuaries, St Austell Bay, Gribben Head, Veryan Bay, Gerrans Bay, St Mawes Bank in Falmouth and Milford Haven in Wales. In Scotland, the best locations are South Arran, Loch nam Madadh, Wester Ross, Sound of Barra (which has the largest maërl bed in Scotland), Sound of Arisaig (Loch Ailort to Loch Ceann Tigh), Loch Sween, Loch Laxford, Loch Carron, Wyre and Rousay Sounds in Orkney, Fetlar to Haroldswick in Shetland, southern Skye and the Isle of Rona. The coast from Broadford to Plockton in Skye has some of the best and most biodiverse beds.
Whilst I was aware of the species, my first encounter with maërl was whilst researching my book on the West Coast of Scotland back in 2000, when I was staying on the Isle of Skye. I had managed to get a lift with the local vet, who had to inspect the loss of a dairy cow on the Isle of Rona, to the north.
Not knowing what I would encounter, the boat dropped me off as we entered the bay, and, to my delight, I landed on a maërl bed at around 12m (40ft). This gave me around 45 minutes to do my thing before I had to be picked up for the return journey. Not only did this first encounter open my eyes to these amazing algae, but it was also an incredible opportunity to witness the vast amount of different species of marine life living very happily in this remarkable blue carbon ecosystem. ■
