Shallow sills turn the Baltic into Europe’s great brackish bowl
Nine northern European states wrap an Atlantic arm that barely flushes—a shelf sea so exchange-poor it behaves like a vast inland water. Outflow squeezes through Danish straits toward the Kattegat, while dug canals steal shortcuts to the White Sea and the German Bight.
Nine countries ring the Baltic: Sweden, Finland, Russia, Estonia, Latvia, Lithuania, Poland, Germany and Denmark. It spans latitudes 53° to 66° north and longitudes 10° to 30° east, and it is the largest body of brackish water on earth. Its arms include the Gulf of Bothnia, split into the Bothnian Bay and Bothnian Sea, the Gulfs of Finland and Riga and the Bay of Gdańsk, while the central basin, the Baltic Proper, stops at about 60° north near Åland. Water leaves through the Danish Straits, the Øresund and the Great and Little Belts, into the Kattegat, and canals link it to the White Sea and, via the Kiel Canal, to the North Sea.
The shallow entrance explains much of its character. The Drogden Sill between Dragør and Malmö, used by the Øresund Bridge and tunnel, is only 7 metres deep, and the Darss Sill reaches just 18 metres; southern Øresund is typically 5 to 10 metres. These thresholds block heavy salty water from the Kattegat from reaching the deep basins near Bornholm and Gotland. As a result the Kattegat and south-western Baltic are oxygen-rich and full of life, while the rest is brackish, oxygen-poor and species-poor.
Where the sea begins is therefore more than pedantry. Most geographers draw the line at the Langelandsbælt and the Drogden Sill, making the Danish Straits an entrance while the bays of Kiel and Mecklenburg count as Baltic; another common line runs from Falsterbo to Stevns Klint. The Helsinki Convention covers the Kattegat without calling it Baltic. Include more of the entrance and the sea looks healthier; define it narrowly and it looks more endangered. Denmark long profited from the chokepoints by levying Sound Dues at Kronborg near Helsingør, at Nyborg and at the Little Belt.
Tacitus called it the Suebic Sea after the Suebi and Ptolemy the Sarmatian Ocean. The name Mare Balticum first appears with the eleventh-century chronicler Adam of Bremen, who said it stretched through the land like a belt. It may relate to the Germanic word belt, as in the Danish Belts, or to Latin balteus.
Source: Baltic Sea
