North Sea

May 13, 2013, 10:37 pm
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Southwestern Norwegian fjord, arm of the North Sea. @ C.Michael Hogan

The North Sea is a shallow saline water body that is part of the northeast Atlantic Ocean. It is generally bounded by England and Scotland on the west, and by Norway, Germany, Denmark, Netherlands and Belgium at the east. In oceanographic terms it is classifed as a marginal sea of the Atlantic Ocean, and as an epieric sea due to lack of depth and its location on the European continental shelf.

The North Sea basin has produced enormous amounts of hydrocarbon resources due to the geologic history of biotic decay and deposition and presence of appropriate shale source rock below the present day sea floor. There are several environmental threats to the North Sea biota, chiefly agricultural runoff from terrestrial agriculture, overfishing and man-caused sea floor disturbances.

caption North Sea basin. Source. U.S.Department of Energy


caption Igneous cliffs of Fowlsheugh, Aberdeenshire. @ C.Michael Hogan In the Precambrian, basement of the North Sea basin was created via intraplate setting. Rigid blocks were overlaid with diverse deposition material including sands and salts. These blocks were transformed to a metamorphic base via tectonic processes such as continental collisions inducing horizontal pressure, friction and distortion in the Caledonian and Variscan plate cycles. The blocks were also were subjected to metamorphic evolution during both the Jurassic and Triassic when the rock was heated up by intrustion of hot molten rock from the interior of the Earth.

During the Jurassic algae and bacteria became buried under the depositional muds of the sea floor; in this period North Sea oil and natural gas began formation. Considerable quantities of these hydrpcarbons were trapped in depositional sandstone, when the sea level fell, producing swamps and saline lagoons inhabited by dinosaurs.

In the middle Jurassic volcanism manifested in the central North Sea Basin, with mantle warping and a domal uplift amid the Viking Graben, Central Graben and Moray Firth Basin. During the subsequent Cretaceous Period dark anoxic shales were laid down in the North Sea basin floor; these formations are key as source rock for the oil and gas formations that modern man has inherited in the basin.

During the Pleistocene the present coastlines of the North Sea took shape, with major forces of glaciation and erosion. During the last glacial maximum during the fifth ice age, sea levels were approximately 120 metres below present, such that an effective land bridge existed between parts of the European continent and Great Britain. 


caption Schematic diagram of general circulation in the North Sea. After Turrell et al. (1992). Source: OSPAR Commission for the Protection of the Marine Environment of the North-East Atlantic The main circulation pattern within the North Sea is a counter-clockwise flow, which is most pronounced near the coastlines. The chief external hydrologic connection is to the open Atlantic between the Orkney Islands and Norway. A secondary flow is from the English Channel at the south.  Water exchange between the North Sea and the Baltic Sea is controlled by stratified countercurrent fluxes formed by the restrictive Skagerrak Strait that  flows between southern Norway and the northern Jutland Peninsula of Denmark. 

The mean depth of the North Sea is only 90 metres, with the only exceptional depths being the Norwegian Trench extending to a depth of 725 metres, and the Devils Hole off of Dundee, Scotland, which is a series of trenches reaching over 200 metres deep.

An interesting element of the eastern margin of the North Sea is a very large lens of freshwater under the Frisian Islands; this aquifer has been estimated to be as large as 100 million cubic metres. caption Mudflats at the Leyhörn Peninsula, Germany. Source: Bermicourt

The average air temperature in summer is 17°C in the winter. Basin air temperatures in January on average are 0 to 4°C. Often gales and storms accompany the winter months. The sea provides a warming influence to coastal areas of Scotland and Norway during the winter, where inland terrestrial temperatureres are much colder than that of the sea.

Environmental Factors

The chief ongoing threats to the North Sea biota are water pollution, overfishing and sea floor disturbance. Water pollution sources are non-point runoff of nitrates and phosphates from terrestrial agriculture; wastewater discharge (even though most of the wastewater has at least secondary treatment) and industrial point source discharges. Overfishing has been noted in the North Sea basin for over a century, particularly for key commercial species such as cod and plaice.


Overfishing has included practices of overharvesting, but also include bycatch issues, where unintended species become entrained in nets. In the case of cetaceans, the Norwegians are still taking Minke whales, which cetacean species is threatened according to CITES. The Faroe Islanders are likewise taking considerable numbers of small cetaceans. Furthermore, significant numbers of porpoises and dolphins are killed as bycatch in net entrainment.Overall intensive purse seining by Norwegians is the main cause of overfishing; unfortunately the European Union has been inefffective in correcting the overfishing issues, in spite of its recognition of these effects. From an historical perspective, many small fishing villages along the east coast of Scotland have had their livelihoods diminished by unrelenting overfishing of the last century.

Sea Floor Disturbance

Chief forms of sea floor disturbance include intensive trawling for harvesting of marine biota and sand/gravel extraction. In the case of trawling, conseequences have included excessive reduction of certain shellfish species. With regard to sand and gravel extraction on the sea floor, there has been considerable loss of benthic habitat, but there has also been aggravation of turbidity through the mechanical disturbance of the sandy and silty parts of the sea floor.

Coastal Ecology

Coastal Marshes of the European Continent

There are significant expanses of coastal marsh, particularly in the vicinity of major river discharge into the North Sea at the continental margin. At these tidally dominated effluxes are the largest freshwater tidal areas for all of Europe. Some of the extensive tidal mudflats are dominated by Phragmites in areas where water coverage is not present during the whole day, so that worms and other air breathing animals are resident. Marsh marigolds attain their greatest height of any place in their range, and the bright yellow Celandine covers considerable expanses. The resultant plant community is termed a Marsh marigold reed-swamp. In the Elbe estuary there is an endemic grass Dechampsia wibeliana; slightly further up the Elbe estuary is the Shore rush, Bolboschoenetum maritimi. Somewhat upland are such characteristic flora as Bidentetum tripartitae, Salix alba, Carex arenaria and Thymus species.

Skerries and Cliffs of the United Kingdom

caption Staple Island birdlife, Farne Islands, Northumberland.
@ C.Michael Hogan
The dolerite igneous coastal rocks of England and southeastern Scotland (south of Stonehaven) offer stunning cliffs and skerries, which provide habitat to a wide array of birdlife. Notable locations include the rocky Farne islands which are home to Lesser black-billed gull, Sandwich tern, Roseate tern, Herring gull Fulmar, Common eider, European shag, Rock pipit, Atlantic puffin, Razorbill, Guillemot, Black-legged kittiwake and Arctic tern.Grey seals can also be seen hauling out on the coastal shoreline from Northumberland to Aberdeenshire. Notable northern cliff and skerry bird habitats include Fowlsheugh near Catterline and the cliffs of eastern Shapinsay in the Orkney Islands.

Scottish Dune and Estuary Systems

caption North Sea coast viewed from grassy dune, Ythan Estuary.
@ C.Michael Hogan
There are a number of coastal dune syand estuary ecosystems in the east of Scotland and northeast of England along the North Sea. These dune systems are often stabilised by Marram grass and other hardy dune grasses. They support a diverse plant community of herbaceaous species. The estuaries that penetrate these dunes include those at the mouths of the River Ythan, Lunan Water, River Don, River North Esk and the Dunes of Menie. At the latter site there is a large proposed golf course proposed that would undermine much of the dunes ecosystem integrity.

At the River North Esk estuary there is a surrounding dunes system that features plants such as the Water speedwell, Purple-flowering searocket, Yellow-flowered kidney vetch, Common kidney vetch  and Hairy St.Johnswort. Other rare taxa are found in the River North Esk dunes system such as the moth Lobesia abscisana, the Cinnabar moth and the Small Blue butterfly.


See Seas of the world



Hogan, C. (2013). North Sea. Retrieved from


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