Oceans and seas

Irish Sea

May 13, 2013, 9:43 pm
Content Cover Image

Irish sea coast along western Wales: site of an Iron Age fort. @ C.Michael Hogan

The Irish Sea is the body of saline water that divides the British Isles from the Republic of Ireland.

This sea is connected to the Atlantic Ocean via the Celtic Sea in the south by St George's Channel, and in the north by the North Channel. There are numerous islands within the Irish Sea, with Anglesey being the largest in areal extent, followed by the Isle of Man.

This sea basin has experienced a series of significant alterations over the previous 20,000 years as the most recent glacial period is subsiding and being replaced by warmer regional weather. At the peak of glaciation the central part of the present Irish Sea was likely a enlongated freshwater lake. Then, when, the ice retreated 10,000 years ago, the lake reconnected to the sea, becoming first brackish and ultimately completely saline.

caption Irish Sea. Source: NASA

Geography and Limits

The southern limit of the Irish Sea is generally defined by an imaginary line from Saint David's Head extending to Carnsore Point. The northern limit line is construed as the shortest line between Northern Ireland and the Mull of Kintyre. This definition includes both the North Channel and the Clyde Sea. Liverpool Bay, situated between northeast Wales and Lancashire, is an integral part of the Irish Sea; this bay is noted for its numerous historic shipwrecks, and the southern end as the first major windfarm in the United Kingdom.

Hydrology and Water Quality

The catchment areas that discharge to the Irish Sea are 11,100 square kilometres for Ireland and 34,500 square kilometres for the United Kingdom; surface runoff from these catchments involve an average of 123.5 cubic metres per second. The areal extent of the Irish Sea is approximately 54,000 square kilometres, with a volume of about 2800 cubic kilometers; these values include the North Channel and the Clyde Sea (Firth of Clyde). The average depth of the Irish Sea is a scant 52 metres, which fact explains why the sea did not exist at the height of the last glacial maximum, when sea levels were around 140 metres lower.

Water circulation in Liverpool Bay is a Region of Freshwater Influence, in which currents are driven by differential thermal and salinity densities between ocean shelf saline waters and freshwater discharges; residence times for saline-deficit riverine discharges are on the order of one year. Conversely, circulation in the western Irish Sea exhibits winter cooled water remaining stratified under summer heated water; in this western region, the summer circulation of the upper layer is a nearly closed cyclonic gyre.

There are a number of water quality issues in the Irish Sea, including discharge of nutrients, pesticides, herbicides and radioactive substances. Riverine discharge of the nutrients nitrogen and phosphorus significantly exceed atmosphere inputs. The chief radioactive discharge is at the site of the Sellafield nuclear recycling plant in West Cumbria.

There is no systematic de-oxygenation in the Irish Sea as compared to the Baltic Sea, in spite of massive sewage sludge dumping in Liverpool Bay; however, there are some localized areas of oxygen depletion in the Irish Sea, including Liverpool Bay and the mouth of the River Clyde. For example, historic dumping of sewage sludge in Liverpool Bay has amounted to as much as 100,000 to 250,000 tonnes per annum. High levels of PCBs have also been recorded, leading to mass die off of seabirds. Correspondingly there are some areas within the Irish Sea that have elevated levels of heavy metals in benthic sediment as well as fish and mollusk tissue. In addition, sediments at the mouth of the Clyde have been measured to exceed 300 parts per million of lead.


Primary productivity in the Irish Sea is promoted by its shallowness and by the heavy nutrient loading from terrestrial runoff; in fact, the Irish Sea contributes a net nutrient flux to the Scottish shelf to the north, enhancing primary production there.

The Irish Sea is not a highly productive set of fisheries, probably due to heavy historic overfishing and resulting low recruitment rates. Most catches in the Irish Sea generally peaked in the 1970s according to Caddy.

Significant commercial fisheries in the Irish Sea include Monkfish (Lophius piscatorius), Haddock (Melanogrammus aeglefinus), Albacore tuna (Thunnus alalunga). A recreational fishery exists within six miles of shore for Atlantic bluefin tuna (Thunnus thynnus). It is worth noting that seine fishing has reduced the stocks of Haddock, such that the species is listed as Vulnerable by the IUCN. The right-eyed flounder European plaice (Pleuronectes platessa) is considered fully exploited in the the Irish Sea. The Dover sole (Solea solea) and other sole species are considered to be fished within safe sustainable levels in the Irish Sea. The Common skate (Raia batis) has effectively vanished from the Irish Sea, chiefly due to bycatch consequences of heavy harvests of other species.

While the majority of the approximately 80,000 tonnes of marine species landed per annum in the Irish Sea are demersal species, the most valuable species taken is the Norway lobster (Nephrops norvegicus).

Human History

At the time that the ice sheet was effectively melting from the British mainland, Britain is thought to have been connected to the mainland of Europe by a combination of land and ice bridges, while Ireland quickly became a recognizable separate island, distinct from the European continent (by about 10,000 years BCE). The archaeological record exhibits the arrival of humans in Ireland in the far north at the lower Bann Valley near present-day Coleraine and in the southwest in the Shannon Estuary. Later they are thought to have diffused northeastward along the coast of Antrim and followed the Bann upstream to Lough Neagh.

These early settlers also entered into a productive life on the shore of Larne Lough immediately north of present-day Belfast, where they chipped flints for tool manufacture. These mesolithic hunter-gatherers arrived without domestic animals or farming skills, huddled for the most part along the coasts and waterways. As a consequence, these early arrivals had minimal impact on the environment. One of the earliest claimed radiocarbon dates for the human inhabitation of Ireland was about 7490 BC. The sample that documents this date was obtained from a primitive settlement that exhibited charcoal remains at Woodpark in County Sligo.

Perhaps the oldest recorded settlement within Ireland is Mount Sandel, County Derry, investigated by Peter Woodman in the 1970s. The excavations revealed hearths and postholes from early Mesolithic dwellings. Radiocarbon dates exhibit that this locale was occupied about 7000 to 6500 BC. Early Mesolithic people chiefly employed small flint blades termed microliths, many of which deriving from the immediate area of the setllement. For this era in Ireland the Mesolithic people enjoyed a diet of wild boar, berries, birds,marine fauna and hazelnuts.


  • Kon-Kee Liu and Larry Atkinson. 2009. Carbon and Nutrient Fluxes in Continental Margins: A Global Synthesis. Springer. 741 pages
  • J.F.Caddy. 1989. Marine invertebrate fisheries: their assessment and management. Wiley Interscience. 752 pages
  • A Perspective of Environmental Pollution Holdgate CUP Archive, 1981 290 pages
  • James F. Lydon. 1998. The making of Ireland: from ancient times to the present (Google eBook). Psychology Press. 425 pages
  • Ian Watson and Alister D.Burnett. 1993. Hydrology: an environmental approach. CRC Press. 702 pages
  • United Nations Marine Resources Service. 2005. Review of the state of world marine fishery resources. Food and Agriculture Organization of the United Nations. 235 pages


Hogan, C. (2013). Irish Sea. Retrieved from http://www.eoearth.org/view/article/153911


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