The Kattegat (; ; ) is a sea of roughly bounded by the Skagerrak in the north, Jutland in the west, the Danish Straits and Sjaelland in the south and the Swedish provinces of Bohuslän, Västergötland, Halland and Scania in the east. The Baltic Sea drains into the Kattegat through the Danish Straits.
The Kattegat is rather shallow and can be dangerous to navigate due to many sandy, stony reefs and the tricky shifting currents. In modern times, artificial seabed channels have been dug, many reefs have been dredged either by sand pumping or boulder clearance, and a well-developed light signaling network has been installed to protect the heavy international traffic on the small sea.
There are several large cities and major ports on the Kattegat, including, in descending size, Gothenburg, Aarhus, Aalborg, Halmstad, Varberg and Frederikshavn.
Etymology
According to Den Store Danske Encyklopædi and ', the name is from the Dutch words ' 'cat's' and ' 'gate, passage'. It derives from late medieval navigation, in which captains of the Hanseatic trading fleets compared the Danish Straits to a passage so tight that even a cat would have difficulty squeezing its way through, owing to the many reefs and shoals. At one point, the passable waters were wide. (The name of the Copenhagen street Kattesundet also means a narrow passage, .)
Boundaries
thumb|Different definitions of the boundary between the Kattegat and the Skagerrak.
On 31 December 1932 Denmark, Norway and Sweden signed a convention regulating the fishing of European plaice in the Skagerrak, the Kattegat and the Sound. There the Kattegat's boundary with the Skagerrak was defined a line from Skagen to Vinga Lighthouse and a line from there to the nearest point of Hisingen, while the boundary with the Danish Straits was defined by two lines, one from Hasenøre to Gniben and one from Gilbjerg hoved to Kullen Lighthouse. In the 3rd edition of the International Hydrographic Organization's (IHO) Limits of Oceans and Seas, published in 1953, the Kattegat is bundled together with the Danish Straits and this areas northern boundary was defined as a line from Skagen to the Pater Noster Skerries (around north of Vinga) and a line from there to Tjörn. HELCOM's definition has the northern boundary line running slightly further south than the IHO's does, stretching roughly from Skagen to the western tip of Klåverön (around south of Tjörn, and around south of Marstrand), The International Council for the Exploration of the Sea's definition of the Kattegat aligns with the 1932 convention in defining the southern boundaries, but has the northern boundary going from Skagen Lighthouse to Tistlarna Lighthouse (around south of Vinga) this results in Gothenburg being on the Skagerrak instead of the Kattegat.
Bathymetry
thumb|Bathymetric map of the Kattegat.
The majority of the Kattegat has a depth of less than , with the average being , making the Kattegat deeper than the Danish Straits but shallower than the Baltic Sea and much shallower than the Skagerrak with its Norwegian Trench that reaches depths of over . An extension of the Norwegian Trench, known as (), stretches along the Swedish coast into the Kattegat, with waters being around deep near the boundary with the Skagerrak before becoming shallower as the trench stetches southwards, reaching around deep off the coast of Gothenburg, and eventually ending at a sill.
Beginning in 1429 in the Middle Ages, the Danish royal familyand later the state of Denmarkprofited greatly from the Sound dues, a toll charged for passage through the Øresund, while Copenhagen sheltered and repaired ships and provided trade opportunities and protection from piracy. The dues were lifted in 1857.
Biology
The salinity in the Kattegat has a pronounced two-layer structure. The upper layer's salinity is between 18‰ and 26‰, while that of the lower layer (separated by a strong halocline at around ) has a salinity between 32‰ and 34‰. The lower layer consists of inflowing seawater from the Skagerrak, with a salinity on par with most other coastal seawaters, while the upper layer, consisting of inflowing seawater from the Baltic Sea, has a much lower salinity, comparable to brackish water, but still a great deal higher than the rest of the Baltic. These two opposing flows transport a net surplus of of seawater from the Baltic to the Skagerrak every year. During strong winds, the Kattegat's layers are completely mixed in places such as the Great Belt, making the overall salinity highly variable. This leads to some distinctive conditions for sealife in the area.
Cold seeps, known locally as bubbling reefs (), can be found in the northern Kattegat. Unlike those in most other places (including the North Sea and Skagerrak), the Kattegat's cold seeps are at relatively shallow depths, generally from below the surface. The seeps rely on methane deposited during the Eemian period and during calm weather the bubbles can sometimes be seen on the surface of the water.
Ecological collapse
The Kattegat, characterised by widespread anoxia, was one of the first marine dead zones to be noted in the 1970s, when scientists began to study the effects of heavy industry on the natural world. Since then, research has lent much insight into processes like eutrophication, and how to deal with it. Since the first Action Plan for the Aquatic Environment in 1985, Denmark and the EU have begun costly, far-reaching domestic projects to stop, repair and prevent these environmentally destructive and economically damaging processes and are now busy implementing the fourth Action Plan. The action plans sum up a broad range of initiatives and include the so-called Nitrate Directives. The action plans have generally been viewed as a success, although the work is not finished and not all goals have been met yet.
Protections and regulation
thumb|Grenen in Denmark is important for bird migrations and is a protected area.
thumb|Bjärekusten Nature Reserve with Hovs Hallar in Sweden.
Due to its very heavy sea traffic and many large coastal settlements, the Kattegat has been designated as a Sulphur Emission Control Area as part of the Baltic Sea since 2006. As from 1 January 2016 the benchmark for sulphur in fuels was lowered to 0.1%.
Several large areas of the Kattegat are designated under Natura 2000 and various bird protections such as the Ramsar Convention. The remaining larger shallow reefs are among the protections, as they are important spawning and feeding grounds for fish and marine mammals and support a thriving but threatened biodiversity. Protected areas include:
Denmark
- Grenen
- The Bay of Aalborg, comprising a shallow sea area.
- Beach meadows on Læsø and the stony reefs south of the island.
- Anholt and the sea north of the island.
Sweden
- The Nordre älv estuary north of Gothenburg. An important spot for migratory birds and fish.
- The Vrångö archipelago (Swedish: Vrångöskärgården), part of the Archipelago of Gothenburg. An important reproduction area for seabirds and seals.
- Kungsbacka Fjord. A shallow water fjord between Gothenburg and Varberg, including important salt marshes.
- Hovs Hallar
- Kullaberg Nature Reserve
Gallery
<gallery>
Anholt Havmøllepark.jpg|There are several offshore windfarms in the Kattegat.
Bianca Rambow (North of Funen).2.ajb.jpg|Some of the world's busiest shipping lanes pass through the Kattegat.
Fladen Light.PNG|The larger shallow sand and stony reefs have been equipped with light signaling in modern times.
Kungälv Municipality from the air.jpg|There are several small archipelagos near the mainland in the Swedish part of the Kattegat.
Barakullen.jpg|The Swedish coasts in the Kattegat are rocky shores, like this one in Kullaberg, or sandy or gravel beaches.
Grenå Strand.JPG|All the Danish coasts in the Kattegat are sandy or gravel beaches with no exposed bedrock.
</gallery>
See also
- Øresund Bridge