Maerl: a hidden and precious habitat

Diving over a maerl bed is best described as like flying over a shagpile carpet made up of purple twiglets! It is an incredible sight. A huge diversity of fascinating marine creatures live in and on the maerl.

Falmouth has arguably the largest and most healthy maerl beds in England. The shallow seabed of Falmouth harbour is actually the sides of a flooded river valley or ria, flooded after the last ice age. Maerl – a slow growing calcareous algae - thrives on these banks.

If you take a closer look at a single nodule of maerl they are extremely complex in shape and are created by slow growing red algae. They only grow 0.5mm per year and as they grow they branch in three dimensions creating a tiny branching nugget. Depending on environmental conditions the maerl nodules (or rhodoliths) vary in size from 1cm to 5cm wide.

Maerl beds are found in other part of the UK’s seas but the bed in the Fal harbour is incredibly special. Not only have they have been shown to live for thousands of years, but the beds inside Falmouth harbour have been dated at approximately 4000 years old! Researchers at the University of Exeter have also discovered that the maerl in the Fal estuary is genetically distinctive and significantly different to maerl from other areas.

Maerl beds are teeming with life, and the millions of tiny gaps between the nodules are perfect for juvenile crustaceans, worms, molluscs and anemones to live. In addition to this clear biodiversity value, maerl beds also store carbon, making them vital in our fight to minimize climate change. Sediment carried by passing water currents drops out of suspension as it gets trapped in the layer of maerl on the seabed. Over time this deposition builds and the living maerl layer rises above a deep layer of silt. The maerl itself preventing the silt from being removed by waves and tides.

The maerl beds of the Fal estuary are protected as a feature of the Fal and Helford Special Area of Conservation (SAC). Following campaigning by Seasearch divers, Cornwall Wildlife Trust and Marine Conservation Society, in 2009 a ban on scallop dredging and trawling within the SAC came into force.

This has helped the maerl beds both within the harbour and in Falmouth bay. Dive surveys subsidized by the EU LIFE ReMEDIES project and delivered by Cornwall Wildlife Trust’s Seasearch Cornwall team have revealed that the maerl beds in the Fal Harbour appear to be benefiting from this reduced pressure. On a recent dive survey, a large area of healthy maerl was surveyed to the southwest of Pendennis Point which previously had been damaged by scallop fishing. Not only did the maerl appear healthy and the nodules looked larger than previously observed, there were large numbers of mature scallops living on the maerl bed - a positive sign that the restriction on fishing is benefiting the entire ecosystem.

Diver over maerl bed Pendennis. Image by Matt Slater.

Maerl beds are found across the UK coast, but despite their clear importance for both biodiversity and carbon storge, they are still at risk from bottom towed gears. We hope that similar restrictions can be brought in to protect maerl habitats in other areas of Cornwall and the UK’s coastline, it seems common sense to protect such a valuable, productive and rare habitat.

Although the maerl beds of the Fal appear healthy at present, Seasearch divers are concerned that in some areas the seabed appears to be smothered in algae. It is very hard to accurately monitor levels of fouling algae as it is ephemeral and comes and goes, however water quality in the estuary is under threat due to increased sewage overflows and run off from agricultural land. For the long-term protection of maerl beds we not only have to prevent physical impact but we also have to work on land to reduce run off and nutrient increase. Nutrient levels are high in the Fal estuary and as well as causing concern for maerl there are also fears it could restrict and damage health of seagrass beds in the estuary – another vital blue carbon habitat that we need to protect.

Baby bull huss on maerl. Image Matt Slater.

The Fal is also home to a thriving population of native oysters (Ostrea edulis). The Fal is unique in that the oyster population has been helped by forward thinking fisheries management that banned the use of engines over a century ago. A traditional sail and oar powered dredge fishery has continued through the decades with oyster stocks fluctuating but not dying out – a stark contrast to the situation in other estuaries where overfishing with diesel powered vessels has resulted in fisheries collapses. The oyster fishers of the Fal are also keen to see the estuary’s water quality cleaned up so that they can continue to sell their oysters.

The Fal is evidence that nature conservation and fishing can work together to protect and restore marine habitats.

What is maerl?

Maerl beds are intricate habitats created on shallow seabeds by a very special group of red algae species.

These unique seaweeds grow as twiglet shaped hard nodules called rhodoliths - literally meaning, red stones! These pinkish-purple nodules range in size from half a centimeter to lumps up to five centimeters wide. Just like all plants they grow using sunlight and photosynthesizing but they also extract minerals from the water around them as they grow, thus building up a hard coral-like structure.

As the individual nodules of maerl grow they form intricate branching twiglet like shapes and millions of these three-D shaped nodules of maerl building up over long periods of time create a deep bed of living coralline algae.

Why are maerl beds important?

Maerl beds are incredibly rare in the UK and amazingly rich in marine life, the lattice structure is perfect for juvenile crustaceans and molluscs, worms, sponges and seaweeds, to grow on and within, and a huge variety of microscopic organisms and bacteria also live on and around the maerl nodules.

Maerl beds are a UK biodiversity action plan habitat, and a protected feature of the Fal and Helford SAC. They occur in moderately sheltered shallow seabeds and are found on west coasts of Scotland, Ireland and in England they are only found in a handful of locations and the maerl beds in the Carrick roads, in the Fal estuary are the best example in England.

Why are maerl beds important in our fight against climate change?

Maerl beds are a blue carbon ecosystem as they actively draw down carbon dioxide and store it in the form of calcium carbonate as they grow, additionally the structure of maerl beds result in trapping of organic matter and that organic matter is stored between the maerl nodules. As this rich organic matter builds up it is protected from being eroded or swept away by currents due to the protection the maerl creates as a living layer on top of the sediment.

How many species use them?

Maerl beds are important for biodiversity and in northwest Europe 349 species of seaweed were found to live in and around maerl beds, 500 different invertebrate species have been identified that live in maerl beds (Birkett et al. 1998).

Where are maerl beds found in Cornwall?

The largest single maerl bed in England is in the Fal estuary, just off St Mawes castle, there are also large beds in the Helford estuary and in Falmouth bay. Ancient deposits of dead maerl which have built up over thousands of years are found across Falmouth bay and east along the coast as far as St Austell bay.

Seasearch divers from Cornwall Wildlife Trust led by Angie Gall discovered live maerl off Gribben head back in 2012 and since then research by Natural England has shown that there are extensive maerl beds across St Austell bay and in Veryan bay and Gerrans bay too. Seasearch divers from Cornwall Wildlife Trust surveyed maerl beds in St Austell bay in 2022 and found high biodiversity including large numbers of juvenile scallops, and rare species such as the sea mouse and the lancelet.

Maerl facts

Maerl beds are found in shallow subtidal seabeds from a depth of 5 to 25 meters.
Maerl is incredibly rare in the UK and maerl beds are a Biodiversity Action Plan habitat
Nodules of maerl are very slow growing. A maerl nodule grows from 0.5 to 1mm per year.
An individual maerl nodule can live for over 100 years.
Maerl beds build up very slowly, the maerl beds at Falmouth are estimated to be 4000 years old (Birkett et al. 1998).
There are thought to be 3 main species of maerl in NW Atlantic waters. The species we find in Cornwall are Phymatolithon calcareum and Lithothamnion corallioides.
Researchers from University of Exeter who analysed maerl DNA from samples taken from Portugal to Norway, discovered that the maerl found in the Fal estuary is genetically distinct from maerl from other areas (Jenkins et al. 2021).
Maerl is fragile and particularly vulnerable to damage by bottom towed fishing gear such as trawls and scallop dredges.
Good water quality is also very important for maerl and sewage pollution, agricultural run off and urban run off all present threats as increased nutrient levels are likely to increase the risk of maerl beds being over-run by algae.