Coastal Environments

EXCLUDING offshore islands, the total length of Ireland's coastline exceeds 6,000 kilometres (3,700 miles). Its wide diversity of constituent rock types and sediments gives rise to an extensive variety of landscapes and important wildlife habitats.

The coast and its geology are exposed to different degrees of wave energy. Ireland's Atlantic seaboard is subject to high wave energy, which gives rise to characteristic cliffs, islands, rocky shores, storm beaches and wind-blown machair grasslands. On the south and east coasts the wave energy is lower and the coastal landscapes are softer, with many stretches of sand dunes, shingle and estuarine mud.

The coast is a complex interface — a zone of transition between the environments and habitats of land and sea. As a zone it contains many highly productive ecosystems in which species of fauna and flora have evolved to tolerate varying degrees of exposure to sea and brackish water or, conversely, air, salt spray and freshwater.

Intertidal zone
The intertidal, or littoral zone is between the high-water and low-water marks, i.e. that part of the shore that is covered by seawater twice a day. Beneath and beyond the low-water mark lies the sublittoral or subtidal zone. Above and beyond the high-water mark lies coastal land.

The most common intertidal habitat on the Irish coast is a low rocky shore covered with a blanket of brown macroalgae (seaweeds) such as wracks and kelps. The great mass of organic detrital material (mucus and decayed particles) produced by the seaweeds forms the basis of the web of intertidal life. Seaweeds are attached to rocks by their holdfasts and absorb nutrients directly from the surrounding seawater. In shallow, well-lit and constantly churned coastal water, seaweeds grow rapidly.

Large brown seaweeds such as serrated wrack and some Laminaria species (oarweed and kelps) dominate sheltered rocky lower-shores. On more exposed coasts, which are subject to greater wave action, these give way to shrubby red seaweeds such as carragheen.

Kelp forests, which may become exposed on low tides, are an important habitat below the low-water mark. Together with their under-storey of red seaweeds such as sea beech, dulse and Phycodrys rubens, which extends into deeper water beyond the kelps' sublittoral limits, kelp forests support a diverse and important faunal community that includes bryozoans, hydroids, sponges and colonial sea squirts. Kelp forests are also the favourite feeding ground of coastal otters.

The middle-shore is dominated by other seaweed species, including brown seaweeds such as bladder wrack, egg wrack and sea oak. Carragheen and the similar-looking Mastocarpus stellatus are red seaweeds that are more tolerant of wave exposure than the wracks. Other red seaweeds that are often abundant in middle- and lower-shore pools include Lomentaria articulata and L. clavellosa.

Green macroalgae occupy the upper and middle intertidal zone, often in pools. Sea lettuce is a green seaweed that grows over the whole shore, except for the extreme upper levels. Its growth, like that of other green algae, is kept in check by grazing species such as periwinkles and limpets.

The rocks and rock pools of rocky shores are host to an abundance of other seashore life: soft-bodied zoophytes such as anemones, sponges and colonial sea squirts; tube-footed, spiny-skinned echinoderms including starfish, brittlestars, sea urchins and sea-cucumbers; fishes including five-bearded rockling, Corkwing wrasse, common blenny, butterfish, goby, and lumpsucker; molluscs including barnacles, common mussel, common limpet, top shells, periwinkles, dog whelk, common whelk; cephalopods (a type of mollusc) including curled octopus and little cuttlefish; and crustaceans including prawns, lobster, squat lobster, hermit crabs, edible crab and green shore crab.

Rocky shore faunal communities may include some rarities such as the purple sea urchin (Paracentrotus lividus) which, apart from in the Channel Islands and a few locations in western Scotland and only extremely rarely in Devon and Cornwall, is only found in the British Isles in significant numbers on the west coast of Ireland from West Cork to Donegal. It is most abundant in County Clare, where it lives in large colonies.

The intertidal zone is an important habitat for several commercially exploited species such as lobster. During spring the adult lobsters move inshore to mate; the female returns offshore to lay her eggs in winter, where she carries them until they hatch as planktonic larvae. The common prawn likewise moves inshore in spring and early summer to breed, before moving offshore again from September to spend the winter in deepwater.

Rocky shores are vital habitats for a number of bird species. Turnstones and purple sandpipers are widespread on rocky shores along the Irish coast in winter, preferring the weed-covered boulder beaches that offer the greatest abundance and variety of invertebrate food. Rock pipits are common residents that frequent rocky coastlines. Wherever mussels grow on rocks, oystercatchers are common predators, while grey herons are commonly seen stalking for fish and crabs in weedy shallows and among rock pools at low tide.

Sandy shores also support rich and diverse faunal communities, particularly sheltered beaches with fine mixed sands, a good proportion of silt, and persistent wet areas. The lack of shelter, especially at low tide, and the absence of firm surfaces for attachment means that the animals must live within the sediment. As the tide goes out they simply retreat below the surface, to different depths depending on the species.

Characteristic species of the sandy shore include echinoderms such as the sand star, sand brittlestar and sea potato; bivalve molluscs including common cockle, razor shells, thin tellin, rayed trough shell and striped venus; gastropod molluscs including necklace shells; bristle worms (polychætes) including lugworms, ragworms and catworms; and crustaceans including masked crab, swimming crabs and common shrimps.

Sandy shores are vital habitats for several bird species. In summer, shingle shores attract a variety of breeding birds, including resident ringed plovers and oystercatchers, and summer-visiting little terns, which breed in small colonies scattered around the coast. In winter sanderlings are often found feeding along the tide line on sandy beaches.

Estuaries

Estuaries are complex environments comprising immensely important wildlife habitats that often support complex ecosystems and high species diversity. Well known Irish estuaries include Liffey Estuary (Dublin Bay), Lee Estuary (Cork Harbour), Slaney Estuary (Wexford Harbour), Nore/Barrow/Suir Estuary (Waterford Harbour), Shannon Estuary (incl. Fergus/Maigue/Deel Estuaries), Corrib Estuary (Galway Bay), Moy Estuary (Killala Bay), Erne Estuary, plus Lough Foyle and Belfast Lough, among others, in the North. As well as being rich wetland habitats, estuaries usually have a multiplicity of functions as shipping channels, fish nursery grounds, bait-digging sites, and areas for shellfish cultivation, waste disposal and recreation.

Within an estuarine environment tidal influence may extend several kilometres inland. Estuaries generally include a main waterway and conjoined river mouth, plus a number of creeks and other small inlets, rivers and streams that lead into the main waterway.

High-energy environments occur at the mouth at locations most exposed to wave and weather influences. Here, the seabed and intertidal areas consist predominantly of bedrock, coarse boulder or pebble deposits, with some finer sediment (sand) beaches. Progressively lower energy environments occur upstream, with extensive fringing areas of subtidal mud and intertidal mud banks, particularly in very sheltered environments. Consequently, estuaries comprise a wide range of aquatic habitats ranging from hard subtidal substrata through to rare saltmarsh and beds of marine grasses. Thus estuarine aquatic communities are generally very diverse and of great nature conservation value.

The soft mud of estuarine sedimentary deposition sites (sinks) may contain enormous quantities of invertebrates (worms, crustaceans, bivalve molluscs and gastropods such as Hydrobia), which comprise the staple diet of many species of wildfowl and wading birds. Beds of marine grasses are also important food resources for grazing wildfowl, especially during autumn and winter.

Estuaries are often important migration stop-off sites and wintering grounds for populations of wildfowl and waders. In late summer and early autumn, flocks of waders such as curlew leave their high moorland breeding grounds in Ireland, Scotland and Scandinavia and assemble in Irish estuaries to spend the autumn fattening-up on the abundant food resources before moving further south during winter. The chain of small estuaries along Ireland's south coast from Wexford to Cork fills with birds, including internationally important numbers of black-tailed godwit which breed in Iceland. Meanwhile, larger estuaries such as Lough Foyle and Strangford Lough in the North, and Tralee Bay and Castlemaine Harbour in the southwest, are the first landfall sites for flocks of brent geese, which fly in from staging grounds in Iceland after breeding in the Canadian high Arctic. Brent geese winter almost exclusively in Ireland, grazing on their preferred food, eelgrass — a marine flowering plant that grows in quantity in about 12 Irish estuaries.

Eelgrass is most abundant in September and October, after which it dies back to form deep organic detrital deposits on the estuarine strandline. As the geese and other eelgrass grazers, such as widgeon, exhaust the eelgrass beds they begin to disperse around the coast to find other less energy-rich foods, such as green seaweeds, during the rest of the winter.

The approach of winter brings even greater numbers of birds to Ireland's estuaries as large flocks of migrant waders, including dunlin, knot and godwit, join residents such as oystercatchers and redshank. Oystercatchers feed on dense beds of shallow-burrowing common cockles and mats of common mussels. Many Irish estuaries have substantial mussel beds, which grow attached to rocks and gravel exposed to moderate, food-laden currents. Mussel beds harbour numerous other small animals and — as on the east side of Lough Foyle — may also act as a sediment trap forcing the tide to deposit fine silt along the shore.

In higher energy estuarine environments, where the shore is sandier, other waders such as sanderling and grey plover feed on tiny molluscs, crustaceans and worms at the tide's edge.

Estuaries also support important numbers of wildfowl winter visitors, including widgeon, pintail, mallard, tufted duck, goldeneye, shelduck, barnacle goose, white-fronted goose, Bewick's swan and whooper swan. Several species of gulls also over-winter in estuaries.

During prolonged periods of severe cold in Britain or continental Europe many migrant birds move west to Ireland to take advantage of its milder, damper oceanic climate. In some years Ireland's estuaries, especially along the east and south coasts, are important to the survival of large continental and British flocks of curlew, lapwing and golden plover.

Eighteen Irish estuaries regularly hold internationally important numbers of wildfowl or waders. These are sites of the utmost conservation value as they are hugely important links in the East Atlantic Flyway for migratory wildfowl and waders.

Many of Ireland's estuaries are also highly sensitive fish spawning and nursery areas. In some cases estuaries are vital breeding grounds for commercially important fish species such as halibut, herring and sea bass.

Under certain conditions (generally involving periods of high river discharge, elevated sediment loadings and warm summer temperatures), environmental conditions within estuary waters sometimes give rise to both thermal and haline stratification events (i.e. physical and chemical layering of the estuarine water column), which may inhibit natural dilution and dispersion processes within the estuary.

Bay environment

A bay is an arm or inlet of the sea extending inland, with a wide mouth. The general characteristics of bay environments are a relatively wide sea area with a shallow near-shore zone (which may extend up to several kilometres offshore), and maybe also a shallow offshore zone. Some of the best known of Ireland's numerous bays include Dundalk Bay, Dublin Bay, Wexford Bay, Youghal Bay, Roaringwater Bay, Dunmanus Bay, Bantry Bay, Kenmare River, Dingle Bay, Tralee Bay, Galway Bay, Kilkieran Bay, Killary Harbour, Clew Bay, Blacksod Bay, Killala Bay, Sligo Bay, Donegal Bay and Gweebarra Bay.

Rivers, estuaries, streams and other watercourses drain into the bay. Seasonal fluvial inputs of freshwater, with accompanying loads of terrigenous sediments as well as natural and anthropogenic nutrients (derived from of agriculture and sewage), may be high. High sediment inputs give rise to extensive intertidal sedimentary mud flats and saltmarshes in near-shore regions adjacent to estuaries, and subtidal mud and silt deposits further offshore.

Further out in the offshore zone, where fluvial influences are less, the seabed may consist predominantly of well sorted muddy-sand and gravel. During periods of elevated fluvial input and/or prolonged strong winds, the water column of both near-shore and offshore zones can become enriched with suspended or re-suspended sedimentary material.

Bays — especially shallow ones — are often recognised as having high conservation value in respect of their marine and coastal environments. Bays often hold important spawning and nursery sites for fish, including offshore demersal and pelagic fish, and shellfish species. Within the bay there may be important commercial harvests from populations of flatfish such as halibut, plaice, sole and turbot, and other fish such as sea bass and sandeel, as well as shellfish such as mussel, cockle and edible periwinkle, and bait species such as ragworm and lugworm.

Compared to estuaries and other more enclosed water bodies, the study of bay environmental fluxes and dynamics — factors such as tidal and current flushing, fluvial influence, sedimentation, and seasonal thermal and haline stratification events — is often poor (Sherwin et al., 1994). When a seasonal thermal and/or haline stratification event does occur, such as when normal current-driven mixing processes collapse, the bay water column is divided into a warm, brackish surface-water component and a cold, more saline (more dense) bottom-water component. Reduced exchange and mixing across the interface between the two components limits, or prevents, vertical mixing and dilution.

The dynamics of freshwater discharge from river systems into coastal waters, especially bays, are also significant, and in particular the close relationship between river-borne nutrients, algal blooming and high oxygen demand by bacteria (eutrophication), which can give rise to hypoxia (oxygen depletion) and anoxia (oxygen absence) in well-stratified coastal waters. It is evident that in such circumstances high anthropogenic nutrient loading from agriculture and sewage can easily overwhelm the natural equilibrium of coastal marine ecosystems, leading to high levels of net productivity and seasonally significant mortalities of bottom-dwelling marine species such as common whelk, dogfish, razor shells and starfish (Deere-Jones, 1995 & Justic et al., 1993).

If currents are weak, together with seasonal onset of thermal stratification and elevated saline stratification of the bay water column, it may be that the exchange and flushing of bay water to the open sea are also seasonally weak. There is, therefore, growing concern that many bay marine environments around the Irish coast are already under significant stress.

Sea cliffs and coastal land

Coastal land is that which lies above the high tide mark. This encompasses a variety of geomorphologic features and habitat types.

Sea cliffs that provide suitable ledges for nesting are vital habitats for seabirds such as gannets, razorbills, guillemots, kittiwakes and fulmars, and for other birds such as peregrine falcons and rock doves. Cliff tops and scree slopes provide habitat for puffins, storm petrels and Manx shearwaters, which nest in burrows. Cliffs also support assemblages of salt- and wind-exposure tolerant plants such as sea campion, scurvy grass, rock samphire and thrift. Sometimes, as in the cliffs of Slieve League in County Donegal, the sheer height of the cliffs and exposure also make them an important habitat for a range of unusual and rare arctic-alpine plants.

Cliff bases, including the rocky shore intertidal zone, also support birds such as rock pipits and turnstones, and provide resting and nesting sites for species such as black guillemot and eider.

Maritime grassland that is grazed provides ideal feeding conditions for choughs and other birds of the coast, such as rock dove, which require open ground to forage for seeds and invertebrates.

Ireland has the largest population of choughs in Western Europe, making them of special conservation importance. They are distributed around the coast from the Great Saltee island off County Wexford, around the west coast to Rathlin Island, County Antrim. Choughs nest on both sea and inland cliffs, as well as in quarries and ruins.

Mature sand dune systems are rich and diverse habitats that support a large number of species. The ecological succession of plant and animal communities progresses with dune maturity (usually represented by distance inland), from the strandline/high-water mark of the sandy or sand-shingle shore, through unstable fore-dunes, dunes stabilised first by marram grass then other colonising plants, to mature back-dunes with a developing soil structure that eventually supports old-growth plant assemblages that include mature trees. The hollows in-between various mid-dunes and mature back-dunes may be waterlogged, either seasonally or permanently. These "dune slacks" add a further important wetland habitat type. The relatively deep water in dune slacks allows the rare natterjack toad — found in only a few coastal locations in County Kerry — to forage and spawn. Dune slacks also provide an important source of fresh drinking water for birds and mammals, including badgers and foxes.

There are many important sand dune systems around Ireland's coasts. Some of the geomorphologic features associated with dune systems include spits such as those at Inch and Castlegregory on the Dingle Peninsula in County Kerry, and coastal lagoons such as those on the south coast of County Wexford where a number of lagoons are linked together by barrier beaches. At one of these, Lady's Island Lake, the unstable dune habitat — where the shingle barrier is frequently overwashed by the sea on high tides backed by an onshore wind — is the only known location of one of Ireland's rarest coastal plants, the cotton weed, which is at the northern limit of its European range.

Another feature is the 18-kilometre long shingle beach known as the Murrough in County Wicklow. In July its distinctive shingle beach floral community includes some rare plants such as the oyster plant, which was once widespread on the east coast but is now restricted to just a few more northerly shingle beaches. The sand and shingle of the Murrough provide habitats for birds such as ringed plovers that nest there in early summer.

The northwest coasts from Inishowen Peninsula in County Donegal to Galway Bay have a unique kind of sandy plain or machair, best developed in the northwest corner of County Mayo around the Mullet Peninsula. This habitat is unique to the Atlantic fringes of Ireland and Scotland, where the shelly sands have been blasted into a flat plain by high winds. These machair grasslands are rich in plant species and are of special importance to nesting waders such as lapwing, ringed plover and dunlin.

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