Oceans and seas


May 13, 2013, 7:53 pm
Content Cover Image

Kattegat along the Swedish coast near Molle. @ C.Michael Hogan

The Kattegat is a shallow sea situated east of Denmark's Jutland peninsula and west of the southwestern landmass of Sweden. This low salinity water body is a connecting link between the North Sea, via the Skagerrak, and the Baltic Sea.

Coastal zones of the Kattegat are presently eutrophic, by dent of large increases in nutrient levels that commenced as early as 1930. Corresponding benthic biomass decay has led to significant oxygen depletion, altering the historic occurrence of demersal fishes.

This narrow sea passage has played a key role in European naval history beginning with the Viking era.

Geography and limits

caption Regional setting of the Kattegat. On the south and west of the Kattegat lie the Danish Straits and Danish islands. the Swedish provinces of Västergötland, Scania, Halland and Bohuslän bound the Kattegat at the east.

The Baltic Sea drains into the Kattegat through the Øresund and the Danish Straits.

The limits of the Kattegat are defined by the International Hydrographic Organization as:

  • At the North: A line joining Skagen, northernmost point of Denmark and Paternoster Skær 57 degrees 54 minutes; 11 degress 27 minutes E, thence northeast through the shoals to Tjörn Island.
  • At the South: The Baltic Sea at the Belts and Sound: In the Little Belt a line joining Falshöft  54 degrees 47 minutes N; 9 degrees 57.5 minutes E and Vejsnæs Nakke 54 degrees 49 minutes N; 10 degrees 26 minutes E. In the Great Belt a line joining Gulstav (South extreme of Langeland Island) and Kappel Kirke 54 degrees 46 minutes N; 11 degrees 1 minute E on Laaland Island. In the Sound a line joining Stevns Light 55 degrees 17 minutes N; 12 degrees 27 minutes E and Falsterbo Point 55 degrees 23 minutes N; 12 degrees 49 minutes E.


The essential hydrography of the Kattegat is governed by lower salinity inflow from the Baltic Sea, with tidal influences driven from the North Sea.

caption Kattegat bottom. Hachure is sandy mud, dark is grey mud,
light blue is sand/gravel bottom. Source: Jorgensen

Like the Baltic Sea connected at the south, the Kattegut is a shallow sea. The Danish coastal shelf, for example, manifests depths no greater than 30 meters.

The Kattegat bottom is consists of almost equal portions of: (a) grey mud; (b) hard sand or gravel bottom; and (c) sandy mud.

Most of the influent sediment arrives via the Skagerrak from the North Sea, in turn originating from riverine discharge from some of the great rivers of northwestern Europe that discharge into the North Sea.

Benthic sediments are strongly seasonal, with maxima tracking the spring phytoplankton bloom.

Salinity of surface waters, influenced by inflow from the brackish Baltic Sea, are in the range of 15 to 25 percent; however, there is strong salinity stratification, such that the colder denser bottom currents from the North Sea are in the range of 32 to 34 percent.

Parts of the sandy bottom such as at Laholm Bay accumulate ephemeral organic rich benthic mud mats; during the resuspension cycle, these mats typically become transported to deeper parts of the Kattegat. Correspondingly some areas of mud mat formation may become transported by later advection into the deeper parts of Kiel Bight in the western Baltic Sea.

caption Kattegat sediment depth (March to February) measured as grams of nitrate in nmol per square meter. Source: Jorgensen

Marine ecology

caption Kattegat viewed from space. Source: NASA Since the early 1900s, parts of the Kattegat bottom were observed to suffer from hypoxia (e.g. oxygen concentrations below three milligrams per litre) in certain late summer conditions; by the 1970s large amounts of green algae (Cladophora) were observed in Laholm Bay, and since the early 1980s the problem has become more pervasive.

By 1980 about 65 million bivalves per annum were washed up on local shores in and near Laholm Bay, with mortality caused by benthic hypoxia. Correspondingly robust benthic population increases were noted in brittlestars (chiefly Amphiura filiformis).

The hypoxia driver was deduced to be increasing eutropication, which generates excessive biological oxygen demand from organic rich sedimentation, and ensuing benthic mineralization processes. In turn the eutropication is driven by excess nitrogen, mostly by excessive nutrient runoff from intensive terrestrial agriculture.

Phytoplankton primary productivity for the Kattegat is estimated to be 245 grams carbon (C) per square meter per year (gms C /sq.m/yr), after correcting for dark respiration.

The organic sedimentation of the water column is thought to be about 63 gms C /sq.m/yr.

Food chain carbon flow is assessed as follows:

(a) Bacteria consumption 87 gms C /sq.m/yr;

(b) Mesozooplankton consumption 37 gms C /sq.m/yr; and,

(c) Microzooplankton consumption at 143 gms C /sq.m/yr.

The shallow waters are stongly stratified in the most productive spring and summer season; correspondingly the bottom waters have a much higher salinity than surface waters due to the influent surface flow from the Baltic.

Benthic sediments accumulate organic matter, which incur microbial metabolism producing methane. Some of this methane from the deeper pockets escapes as bubbles into the water column, sometimes forming carbonate chimneys, resulting from methane oxidation to carbon dioxide within the sediment. These benthic settings engender the growth of unusual pogonophore and bivalve species that hava a symbiotic methane or sufide oxidizing bacteria within their tissue; some of these species are classified as extremophiles due to their ability to thrive in a high methane or sulfide environment.

Marine mammals such as the Harbour porpoise (Phocoena phocoena) and European otter (Lutra lutra) occur in the Kattegat.


caption European plaice, dominant historic Kattegat fish. Source: Hans Hillewaert

Kyle's work provides an excellent historic snapshot of the Kattegat fisheries as of the year 1906. The European plaice (Pleuronectes platessa) fishery was dominant with a annual yield of 6.3 million kilograms (kg), with Atlantic herring (Clupea harengus) in second place with around 3.4 million kg/year, followed by Atlantic cod (Gadus morhua) with a catch of 1.2 million kg/year. Catches of European eel (Anguilla anguilla), garfish (Belone belone) and lumpsucker (Cyclopterus lumpus) followed these dominant three fish species. The fishing fleet of that era had five steamers of gross weight 95 to 189 tons, and about 800 sailing vessels employing drift nets, trawling or line-fishery equipment; this was the entirety of the fleet serving the North Sea, Skaggerak and Kattegat combined.

By 1980 the sea bottom was suffering from a more pronounced oxygen depletion, resulting in an alteration of demersal fish populations. Atlantic cod and Whiting (Merlangus merlangus) became appreciably reduced in numbers, whilst rough dab (Hippoglossoides platessoides) became dominant in these oxygen deficient zones.

Terrestrial ecology

See main article: Baltic mixed forests

The Kattegat is rimmed by the Baltic mixed forests ecoregion, which consist of temperate broadleaf and mixed forests within the Palearctic realm The Baltic mixed forests, comprised of both forests and wetlands, encompasses many different habitats, and accordingly a broad diversity of species. Migrating and breeding water-birds, shorebirds, and wading birds visit this habitat each year, such that 32 Important Bird Areas have been designated within the ecoregion as a whole.

A number of other threatened fauna are found in the ecoregion including the Near Threatened European otter (Lutra lutra), which is found on the Kattegat fringe as well as freshwater rivers tributary to the Kattegat. Other threatened mammals include the European rabbit (Oryctolagus cuniculus) and the Endangered European mink (Mustela lutreola), the latter occurring chiefly in riparian zones

There are a number of threatened avian species found in this ecoregion; some of the threatened bird species breeding here are the Vulnerable Aquatic warbler (Acrocephalus paludicola), Vulnerable Great bustard (Otis tarda), Near Threatened Ferruginous pochard (Aythya nyroca), Near Threatened Black-tailed godwit (Limosa limosa), Vulnerable Steller's eider (polysticta stelleri), Near Threatened Red kite (Milvus milvus) . Great bittern (Botaurus stellaris) and Little bittern (Ixobrychus minutus) are found among the cover of reed beds preying upon fish, frogs and other small fauna. Many of these avian species breed here and then migrate south for winter.


caption Viking stone ship burial ground, Gettlinge, Oland, Sweden. @ C.Michael Hogan The maritime history of the Kattegat almost certainly began with Viking voyages originating in the Kattegat circa 820 AD. Viking bases in both Norway and Sweden were most likely used at that early age, since Norse Vikings were noted to have made many expeditions to the west by the late 700s, and signs of early Viking activity are noted from the southeastern Swedish coast (e.g. such as Oland) since the ninth century. In particular, Hogan points out the existence of elaborate stone ship burial grounds on Oland dating to this approximate time.

In medieval times Dutch traders used the passage and gave the strait its present day name: The Dutch word kat (for cat) combined with gat (for gate), expressing the fact that only a cat-like creature could fit through this narrow sea passage, which at the narrowest is only several kilometres wide.

In subsequent times, including World War II, this narrow sea passage was a strategic channel for commercial cargo transport as well as naval military vessels. On April 8, 1940 the Nazis launched the sea invasion of Norway by passage of a naval fleet through the Kattegat.


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Hogan, C. (2013). Kattegat. Retrieved from http://www.eoearth.org/view/article/170842


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