The Celtic Sea is that part of the Atlantic Ocean bordered on the north by the southern coast of Ireland and the Irish Sea at St. Georges Channel; moreover it is bounded to the east by the English Channel and Brittany in France.
There are no land elements to denote as southern or western boundaries of the Celtic Sea; however, the 366 metre marine benthic contour line along with Ushant Island along the tip of Brittany are taken as the formal bounds.
Regional setting of the Celtic Sea in Europe.
Hydrology and Depths
The seabed beneath the Celtic Sea is termed the Celtic Shelf, an element of the European continental shelf. The northeast portion has a depth of ninety to one hundred metres, increasing towards Saint George's Channel. In the opposing direction, sand ridges pointing southwest have a similar depth, separated by troughs approximately 50 m deeper. These ridges were created by tidal effects at a time when the sea level was lower, approximately 40,000 years before present. South of 50th parallel the benthic topography is more irregular.
The Celtic Sea consists of a considerable portion of epipelagic zone depths (less than 200m), but also has elements within the mesopelagic zone (200 to 366m). Note that the entirety of the mesopelagic zone extends to depths of 1000m; however, the deepest part of the Celtic Sea is only 366m.
Temperatures at the bottom of the Celtic Sea vary between nine and twelve degrees Celsius, considerably cooler, for example, than the Adriatic Sea. In current times there is seasonal stratification of the Celtic Sea, in which summer months have a clear vertical temperature profile. Correspondingly, present times exhibit a well vertically mixed thermal environment when winter storms appear. Benthic core data indicate that the mixing regime changed approximately 6200 years before present; before that time the Celtic Sea was vertically mixed year around. Such a historic change is deduced from the abundance of Quinqueloculina seminulum, a species that is an indicator of year around vertical mixing.
Ecology and Fisheries
Ecologically the Celtic Sea is classified as a major element of the Celtic-Biscay Shelf large marine ecosystem. Within the Celtic Sea (combined with the adjacent Bay of Biscay) the marine resources include a wide range of organisms. In terms of biogeography, the Celtic Sea is a region of ecological transition with considerable species richness in marine flora and fauna. The Celtic-Biscay Shelf is considered a Class I, highly productive ecosystem, exceeding 300 grams of carbon per square metre per year.
The benthic zone of the Celtic Sea midshelf is dominated by the foramifera Stainforthia fusiformis and Nonionella turgida; Eggerelloides scaber and Adercotryma wrighti. In deeper zones of this sea Hyalinea balthica are dominant organisms.
Many of the fisheries of the Celtic Sea are considered overexploited. For example, cod stocks are not being fished sustainably, as is also the case for cod in the Irish Sea and the whole west of Scotland. Correspondingly Celtic Sea plaice is overfished, and reductions in the fishing pressure for that species has been recommended. Blue whiting, Northern hake and Western horse mackerel are also systematically caught in the Celtic Sea.
Based upon both target catches as well as bycatch data two of the most prominent cephalopods in the Celtic Sea are Inshore squid (Loligo forbesi) and Flying squid (Todaropsis eblanae). The cephalods here (as well as many other Atlantic fisheries) suffer from high levels of heavy metal concentrations,especially mercury and cadmium. While the highest metal concentrations are typically found in the viscera of cephalopods, those levels pose a severe threat to human health for food consumption, especially since it is customary in some countries (like Spain and Italy) to consume the species whole; furthermore, peoples of other countries such as Japan view the cephalopod digestive tract as a delicacy to consume.
The history of the Celtic Sea is closely coupled with that of the Irish Sea. 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 BC). 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.
Early Irish 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. 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.
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