Kattegat

The Kattegat (Danish: [ˈkʰætəkæt]; Swedish: Kattegatt [ˈkâtːɛˌɡat]) is a 30,000 km² body of water bordered to the west by the Jutland Peninsula, to the south by the Danish islands in the Danish Strait and the Baltic Sea, and to the east by the Swedish provinces of Västergötland, Skåne, Halland, and Bohuslän. The Baltic Sea flows into the Kattegat through the Danish Strait. This sea area is a continuation of the Skagerrak and can be considered a bay of the North Sea, although this is not the case in traditional Scandinavian terminology.

The Kattegat is a relatively shallow sea and can be very difficult and dangerous to navigate due to numerous sandy and rocky reefs and the treacherous, often shifting currents. In modern times, artificial sea bed channels have been dug, many reefs have been dredged through sand pumping or stone fishing, and a well-developed light signal network has been established to secure the very heavy international traffic in this small sea.

Several large cities and important ports are located in the Kattegat, including, in descending order, Gothenburg, Aarhus, Aalborg, Halmstad, and Frederikshavn.

Geography of the Kattegat Region According to a 1932 agreement signed by Denmark, Norway, and Sweden (registered in the League of Nations treaty works in 1933-1934), the northern boundary between the Kattegat and the Skagerrak lies at the northernmost point of Skagen on Jutland, while the southern boundary to the Øresund runs at the tip of the Kullen Peninsula in Skåne.

The major rivers draining into the Kattegat are the Göta älv near Gothenburg, the Lagan, the Nissan, the Ätran, and the Viskan in the province of Halland on the Swedish side, as well as the Gudenå in Jutland, Denmark.

The key islands in the Kattegat include Samsø, Læsø, and Anholt; the latter two are located in the so-called "Danish Desert Belt" due to their relatively dry climate.

The Kattegat is surrounded by several notable coastal areas, including the Kullaberg Nature Reserve in Skåne (Sweden) with rare species and a picturesque rocky coastline, the town of Mölle with its scenic harbour and views of Kullaberg, and Skagen at the northern tip of Denmark.

Since the 1950s, a bridge project, commonly known as the Kattegat Bridge (Kattegatbroen), has been considered to connect Jutland and Zealand across the Kattegat. Since the late 2000s, several influential politicians in Denmark have shown interest in the project. The bridge is expected to connect Hov (a village south of Odder in the Aarhus region) with Samsø and Kalundborg.

Etymology According to Den Store Danske Encyklopædi and Nudansk Ordbog, the name derives from the Dutch words "katte" ("cat") and "gat" ("gate, passage"). It originates from late medieval maritime jargon, where the captains of the Hanseatic trading fleets compared the Danish Strait to a passage so narrow, due to many reefs and shallows, that even a cat would struggle to squeeze through. At one point, the passable waters were only 3.84 km wide. The name of the Copenhagen strait, Kattesundet, shares a similar etymological meaning, "narrow passage."

An archaic name for the Skagerrak and Kattegat was the Norwegian Sea or Jutland Sea (in the Knytlinga Saga, the name Jótlandshaf is mentioned). Its old Latin name was Sinus Codanus.

History Control over the Kattegat and access to it was of great importance in the history of international shipping. Until the completion of the Eider Canal in 1784, the Kattegat was the only sea route to and from the Baltic region.

From 1429 onwards, during the Middle Ages, the Danish monarchy—and later the Danish state—profited greatly from the Sound tolls levied for passage through the Øresund, while Copenhagen provided refuge, trade, repair facilities, and protection from piracy. The tolls were eventually abolished in 1857.

Biology The Kattegat has a distinct two-layer structure in its salinity. The upper layer has a salinity between 18‰ and 26‰, and the lower layer—separated by a strong halocline at about 15 meters (49 feet)—has a salinity between 32‰ and 34‰. The lower layer consists of inflowing seawater from the Skagerrak, whose salinity is comparable to most other coastal waters, while the upper layer consists of inflowing seawater from the Baltic Sea, which has a much lower salinity, similar to brackish water, but still much higher than that of the rest of the Baltic. These two opposing currents carry a net surplus of 475 km³ of seawater from the Baltic into the Skagerrak each year. Stronger winds can mix the layers at certain points, such as the Great Belt, causing the overall salinity to fluctuate significantly in this small sea, creating unique conditions for marine life.

In the northern Kattegat, there are cold water lakes locally known as bubble reefs (Danish: boblerev). Unlike most other cold-water sources (including the North Sea and the Skagerrak), these bubble reefs in the Kattegat are located at relatively shallow depths, typically between 0 and 30 meters below the surface. The sources rely on methane deposited during the Eemian period, and in calm weather, bubbles can sometimes be seen at the surface. Through carbonate cementation and lithification, cliffs or columns up to 4 meters high form, hosting a rich biological diversity. Due to their uniqueness, the bubble reefs in the Kattegat are protected and recognized by the European Union (EU) as a Natura 2000 habitat (Type 1180).

Ecological Collapse The Kattegat was one of the first dead zones discovered in the 1970s when scientists began investigating the impact of intensive industrial activities on nature. Since then, studies and research have provided many insights into processes such as eutrophication and how to manage it. Denmark and the EU have initiated costly and extensive national projects since the first action plan for aquatic environments in 1985 to stop, repair, and prevent these environmentally destructive and economically harmful processes. They are currently implementing the fourth action plan. The action plans include a broad range of initiatives, also covering the so-called Nitrate Directive. The action plans are generally seen as successful, although the work is not yet completed, and not all goals have been fully achieved.