A Guide to Irish Seaweeds

Seaweeds have high value both ecologically and economically. Which ones can be found in Ireland?

What are Seaweeds?

The word ‘seaweed’ is a collective term for macroscopic marine algae that belongs to three groups: Rhodophyta (red seaweed), Chlorophyta (green seaweed) and Phaeophyta (brown seaweed), which live on seashores and in shallow seas throughout the world. 

Seaweeds have a high value both ecologically and economically. They are important primary producers, provide habitat for a wide range of intertidal and subtidal animals and protect shorelines from erosion by dampening wave energy. Seaweeds are also valuable commercially as they are exploited for biotechnology, biomedicine, agriculture and horticulture and seaweed aquaculture.

Seaweed in Ireland

Globally, there are approximately 7,000 red, 2,000 brown, and 1,700 green species of seaweed.

Approximately 501 species of seaweed occur in Ireland: 272 reds, 147 browns and 80 greens.

The Irish Atlantic coast has more diverse seaweed species compared to the Irish sea due to several factors making the area more suitable for large assemblages of seaweed.

The shallow seas and shores surrounding Ireland and the UK support around 6% of the world’s seaweed species, making the area highly important for seaweed biodiversity. This range of diversity may be because Ireland straddles the northern limit for some warm-water species and the southern limit for some cold-water species.

One reef in Finavarra, Co. Clare is known to harbour 336 species alone and is thought to represent the peak of seaweed diversity for the Irish and British coasts.Most of the seaweed biomass in the Northern Atlantic is provided by a small number of species: the kelps, the wracks, and the detached coralline red algae known as maërl.

Wracks Seaweed

Wracks are mainly found in the intertidal region,  the most common brown seaweed species in Ireland.

The majority of wrack biomass is made up of 5 Fucus species (F. ceranoides (Horned Wrack), 
F. guiryi (Guiry’s Wrack), F. serratus (Serrated Wrack) F. spiralis (Spiralled Wrack),
F. vesiculosus (Bladderwrack); and 1 Pelvetia species (P. canaliculata (Channeled Wrack) and 1 Ascophyllum species (A. nodosum).

Seaweed grows on different areas of rocky shores along the coast, which allows us to divide the shore into three zones, the upper, middle and lower zones. Different seaweed types define different zones, for example, Serrated Wrack defines the lower shore, Channelled Wrack defines the upper shore and Bladder Wrack defines the middle shore.

Kelp Seaweed

The dominant kelp species found in Irish waters is Cuvie (L. hyperborea) found on all Irish coasts. Cuvie (L. hyperborea) occurs in deeper water, while Laminaria digitata, sometimes called oarweed, is the kelp commonly seen in shallower water.

In general, kelps on the east coast occur to depths of a maximum of 8 metres, whereas along the western Atlantic coast, they may occur down to 25m and in some cases, 32m.

In the subtidal area, kelp biomass is mainly made up of Sugar Kelp (Saccharina latissima) and Furbelows (Saccorhiza polyschides).  Cuvie (L. hyperborea) forms dense forests along semi-exposed rocky coastlines, while Sugar Kelp (S. latissima) can form small forests on the hard substratum in non-exposed areas where the water is calmer.

In Ireland, kelp forests have been observed to be seasonal homes to commercial species like the edible crab (Cancer pagurus), European lobster (Homarus gammarus), and several species of juvenile fish that inhabit the kelp canopy.

Ireland has a tradition of kelp harvesting dating back to the 17th century. Kelp was gathered by coastal communities and burned in stone circles known as kelp kilns, with the remaining ash used for a number of uses including soap, pottery and glassmaking.

Red Algae

Red algae, Rhodophyta, are found in the intertidal and subtidal regions of Ireland at depths of up to 40m and in some cases, 250m.

One important group of red algae is the coralline algae, which secrete calcium carbonate onto the surface of their cells, which make up maërl deposits.

Maërl, a collective name for non-geniculate red coralline algae, are mainly found up to 25m, but occasionally occur at depths of 70m.Extensive subtidal Mearl beds, made up of mostly P. purpureum and Lithophane corallioides, can be found along the west coast of Ireland, particularly in the larger bays.  Roughly 8 million tons of Mearl can be found in Galway Bay, with one bed alone having more than 2 million tons.

Meal fragments at Trá an Dilyn (Coral Strand), Carraroe, County Galway, Ireland. Credit: Wikipedia Commons  

In several places along Galway Bay, “coral beaches” have formed from large amounts of broken up dead maërl fragments that have washed ashore. These coral beaches are sustained by live beds just below the water and wave action carrying the dead fragments to the shore.

Maërl extraction is currently being carried out in Bantry Bay on a subtidal bed made of completely dead fragments, as the seaweed type has a wide range of horticultural uses such as organic fertiliser.Dulse (Palmaria palmata) and Carrageen moss (Chondrus crispus and Mastocarpus stellatus) are two other types of red seaweed that have high economic value in Ireland.

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