Marine Environments

Deep-sea

IT was once believed that in the open ocean beyond the continental shelf edge, the bathyal regions and deep ocean floor were deserts of sedimentary ooze that supported few life forms. However, surveys have shown that deep-sea ecosystems possess an unexpectedly high species abundance and diversity. The deep seabed off Ireland's Atlantic seaboard supports an amazing abundance and variety of life including cold-water corals, sponges, sea slugs, sea urchins, starfishes, deep-water fishes and many other benthic organisms.

Other research has gone some way to dispelling the notion that deep seabed habitats and communities are isolated from the water mass above — that there is in fact ecological continuity throughout the water column (Gage and Gordon, 1995). The existence of severe seabed currents, or "benthic storms" has been detected during research undertaken at 2,400m depth on the North Feni Ridge, west of Scotland. These submarine storms are capable of remobilising sediments and dispersing them vertically in the water column as well as horizontally across the seabed.

The bathyal regions of the Rockall Trough west of Scotland and north-west of Ireland, are thought to be among the world's most biologically rich environments, due mainly to a large number of species of microscopic mud-dwelling nematode worms and other invertebrates, most of which are new to science. Contrary to the conventional view that marine biodiversity declines with depth, maximum biodiversity occurs in these bathyal regions between 1,000-3,000m depths (Boucher and Lambshead, 1995).

In 1998, researchers from the University College Cork found cold-water coral reef structures some 350m high and several kilometres across at depths of up to 900m along the shelf edge west of Ireland.

Also in 1998, an undersea feature previously unknown to science was discovered during surveying of the deep seabed north of Shetland and west of the Hebrides (UK waters). Hundreds of seabed mounds about 5m high and 100m across with tails several hundred metres long were found. Both mounds and tails are characterised by a roughly 15-fold increase in the density of xenophyophores — giant deep ocean protozoa (single-celled animals), subsequently identified as Syringammina fragilissima, which grows up to 20cm across. The "Darwin Mounds" are at a depth of about 1,000m and are spread over 50 square kilometres of seabed composed of deep foram sand sediments (the microscopic shells from trillions of dead phytoplankton), in the northern Rockall Trough. Growing on the mounds are deep-water corals (Lophelia pertusa).

Such discoveries, and the questions they raise, emphasise that present knowledge of deep-sea ecology and physical processes is comparatively minimal.

The deep-sea comprises a habitat for many species of fish, including commercially important demersal species such as roundnose grenadier, Atlantic orange roughy; pelagic oceanic (i.e. living in water above the continental shelf edge and deep ocean) species such as albacore tuna and oceanic Atlantic redfishes (Sebastes sp.); and rays and sharks. Typically, little if anything is known about the behaviour and ecology of deep-living fish species, such as orange roughy, and their populations.

Shelf area

Ocean circulation forces cold, nutrient rich water up the continental shelf slopes to the edge where it mixes with warmer surface water. Ocean primary productivity is high at these areas known as "upwellings". The nutrients and available sunlight support abundant growth of planktonic plant organisms (phytoplankton), which in turn support both grazer and predatory planktonic animals (zooplankton), then fish, squid, and higher predators in the food chain, including tunas, sharks, marine turtles, seabirds, seals and cetaceans (porpoises, dolphins and whales).

Small shrimp-like crustaceans, such as euphausiids, copepods and amphipods, constitute the largest proportion of the zooplankton biomass; many spend their entire life-cycle floating at the surface. Other zooplankton organisms are the surface-dwelling (neustonic) eggs and larval stages of fish and bottom-dwelling worms, crustaceans and molluscs, which release millions of eggs to the water so that a few offspring may survive to disperse over a wide area before returning to the seabed as adults.

When meteorological conditions are appropriate, a thin lipid (fat) rich surface film or microlayer forms at the top of the water column. Marine surface films and microlayers, which may last for an extended period of time, are an important habitat for both phytoplankton and zooplankton.

Two dominant warm water ocean currents influence Ireland's marine environment. The first is the North Atlantic Drift (a continuation of the Gulf Stream that flows across open ocean from the Caribbean region), which warms the western coastal waters of Ireland. It brings with it an unusual faunal community including species such as the colonial hydrozoan Velella velella, also known as the "by-the-wind-sailor", and giants such as the sunfish and marine turtles, including the leatherback and loggerhead.

The second major influence is the warm water current that flows northwards along the European continental shelf edge carrying with it a planktonic community that originates in the Mediterranean Sea. This Lusitanian faunal community appears at the surface on the coasts of south, south-west and west Ireland, and is represented by adult stage animals such as the crawfish, red deadman's fingers and trumpet anemone.

Away from the warming influences of these two currents, the cooler waters to the north of Ireland support marine animals with an arctic-boreal distribution such as the stonecrab. While Ireland is at the northern limit of certain marine species with a sub-equatorial distribution, it is likewise at the southern limit of many sub-polar species.

To the east of Ireland, warm water enters the shallow semi-enclosed Irish Sea through the St George's Channel between County Wexford and south Wales, while a colder northern current flows south through the North Channel between Counties Antrim and Down and south-west Scotland. Where the two currents meet along a zone of "frontal mixing" across the north Irish Sea (near the Isle of Man), high densities of plankton are brought to the surface where they attract fish, including important numbers of filter-feeding basking sharks - the second largest fish in the world.

Basking sharks are frequently seen in the Irish Sea and above the continental shelf off the south and west coasts of Ireland. Though little is known about basking shark behaviour, it is thought that during autumn and winter they move offshore to deeper water; in spring they move back inshore to feed on the seasonal abundance of plankton in coastal waters, especially at frontal mixing zones. Females give birth to live young; the litter size is unknown, except for one incident where a female gave birth to six young. Basking sharks, while being long-lived, are slow to reach sexual maturity and hence their reproduction rate is low. It is not known where around Ireland and Britain female basking sharks go to give birth.

The average water depth of the Irish Sea is about 100-150m. The faunal communities of the seabed reflect the range of bottom sediments, including widespread glacial deposits, and the degree of exposure to benthic currents.

In general, seabed communities around Ireland's coasts remain poorly understood. There are estimated to be more than 15,000 benthic species in the shelf seas around Britain, reflecting the wide range of environmental conditions around those coasts (DETR, 1998).

Several North-East Atlantic populations of commercially important fish species are present in Ireland's shelf waters, either as residents or migrants. For example:

• Atlantic cod is generally a demersal species (i.e. it lives and feeds at or near the seabed) that often migrates long distances to spawning and feeding grounds where it may become more pelagic in habit (i.e. living nearer to the sea surface). Though mostly found within continental shelf areas that are between 150-200m deep, Atlantic cod are widely distributed through a variety of habitats, from the near-shore to well down the continental shelf to depths of over 600m. Atlantic cod move inshore to spawn in depths generally less than 50m.
  
  
• Atlantic herring inhabit coastal pelagic and semi-pelagic waters (down to 200m) of the continental shelf. They have complex feeding and spawning migrations. Depending on race, spawning takes place in coastal waters down to 200m.
  
  
• Atlantic mackerel is a semi-pelagic and mesodemersal species (i.e. it lives in mid-water) that is most abundant in cold and temperate shelf areas. They over-winter in deeper waters but move inshore in spring to spawn in coastal waters.
  
  
• Blue whiting is a semi-pelagic species that feeds in the Norwegian Sea on plankton in the water column at a depth below pelagic feeders. They migrate south along the shelf edge to spawn west of Scotland and off the west and northwest coast of Ireland.

Other commercially important fish occurring in Ireland's shelf waters include demersal species allied to the cod such as haddock, white pollack and hake; demersal flatfish such as halibut, plaice, sole, megrim and turbot; demersal monkfish and sand eel; pelagic neritic (i.e. living in water above the continental shelf) species such as Atlantic salmon; and rays (e.g. skate) and sharks (e.g. spur dog and spotted dogfish).

Numerous other fish species such as wrasse, conger eel, flounder and sea bass are of importance to the sea angling industry.

Populations of shrimps (prawns) and other crustaceans such as Nephrops (also called Dublin Bay prawn, Norway lobster, or scampi), crawfish, lobster and crabs, as well as molluscs — squid and shellfish such as scallops, mussels and oysters — are also of commercial importance in Irish waters.

Near-shore waters

Ireland's near-shore[2] habitats support rich and diverse communities of marine fauna and flora, and often include unusual and rare species. For example, the region of Kilkieran Bay in County Galway is renowned for its unique white beaches such as Coral Strand at Carraroe. These are formed from the remains of an unattached calcareous algae (calcified seaweed) known as Lithothamnion, maërl, or simply "coral", which wash ashore during winter storms. Maërl grows in the clear, warm and shallow bay waters to form extensive beds on the sea floor.

The maërl beds, like tropical coral reefs, provide shelter for many other marine animals, including molluscs, sea cucumbers and sea urchins. Likewise, dense beds of horse mussels provide shelter for an assemblage of other marine animals such as tubeworms, sea squirts, variegated scallops and sea cucumbers.

Strangford Lough, a large sea inlet of great conservation value in County Down, is especially important as a spawning area for fish, including sand eels, which spawn in the sandbanks near the low-water mark. Sand eels are an essential component of the marine ecosystem in many coastal waters around Ireland. They are predated by a host of larger animals and are an important part of the diet of many seabirds such as puffins, especially during the breeding season.

Coastal waters in general are important habitats for many fish species, including migratory species such as Atlantic cod, Atlantic herring, Atlantic mackerel, blue whiting and Atlantic salmon. Likewise for a diverse array of crustaceans (shrimps, lobsters, crabs), cephalopod molluscs (octopus, squid, cuttlefish), molluscan shellfish (e.g. scallops, razor shells, mussels), marine worms, jellyfish, sponges, colonial bryozoans, and other marine animal groups.

Footnotes

2. We use the term "near-shore" to denote coastal waters from the intertidal zone down to a depth of about 50m. The term "inshore" refers to coastal waters out to the 12 nautical mile territorial seas limit; "offshore" refers to waters from the 12 nautical mile territorial sea limit out to the 200 nautical mile Exclusive Economic Zone limit and/or Continental Shelf designated area limit (up to 350 nautical miles).

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