The Baltic - an introduction

The Baltic Sea is almost totally enclosed by land, and only connected to the North Sea by narrow and shallow straits around Denmark and Sweden. This limits the exchange of water with the open sea. It typically takes about 25-30 years for all the water in the Baltic Sea to be replaced. More than 200 large rivers bring fresh water into the Baltic, making it the world's biggest brackish sea. 14 countries lie within the catchment area of the Baltic Sea.

Natural conditions alone bring considerable stress to life in the Baltic. The present natural conditions have existed for only a few thousand years. A biological community has had just a little time to adapt and develop. Brackish bay water and surface water force the marine and fresh water species to live just on the very edge of their limits of survival.

Baltic ecosystems must cope with marked natural fluctuations - periods of stagnation broken up by sporadic inflows of saline water from the North Sea, which are the main source of oxygen for the deep waters of the Baltic Proper, and very significant for nutrient cycles throughout the Baltic Sea.

The biodiversity of Baltic marine ecosystems is largely shaped by physical factors - depth, the properties of the sea bed, light penetration, oxygen, and especially salinity.

The Baltic Sea has traditionally been divided into nine main areas. From narrow straits in the south-west, the Baltic Sea stretches east and north through a series of large basins separated by submarine ridges restricting the exchange of water.

	Extent: The Baltic Sea covers 415,266 square kilometres, while its catchment area extends over an area about four times as large as the sea itself. Depth: The average depth of the whole Baltic Sea is around 50 metres. The deepest waters are in the Landsort Deep in the Baltic Proper, where depths of 459 metres have been recorded. Catchment area: In Germany, Denmark and Poland as much as 60-70% of the Baltic's catchment area consists of farmland. Forests, wetlands and lakes make up between 65% and 90% of the catchment area in Finland, Russia, Sweden and Estonia. Population: Nearly 85 million people live in the Baltic catchment area - 26% of them in large metropolitan areas, 45% in smaller urban areas, and 29% in rural areas. Population densities vary from over 500 inhabitants per square kilometre in urbanised regions of Poland, Germany and Denmark to fewer than 10 inhabitants per square kilometre in northern parts of Finland and Sweden. Almost 15 million people live within 10 kilometres of the coast. Shipping: More than 500 million tonnes of cargo are transported across the Baltic Sea each year, along many busy shipping lanes. More than 50 passenger ferries also ply routes between Baltic ports.

Vital inflow from the North Sea

The inflow of water from the North Sea is the main source of oxygen for the deep waters of the Baltic Proper, and is very significant for nutrient cycles throughout the Baltic Sea. A stagnation period of more than sixteen years ended in January 1993 when masses of high-salinity water entered the Baltic, and for the first time since 1977 the Baltic Sea was free of hydrogen sulphide.

Unfortunately the effects of this clean-out did not last, and within a couple of years the depths of the Eastern Gotland Basin were again becoming anoxic. In 1996 hydrogen sulphide was once more recorded in the depths of the Bornholm and Eastern Gotland Basins.

Contaminants and nutrients enter the Baltic Sea in rivers, in runoff from coastal areas, through exchange of water with the North Sea, through atmospheric deposition, and due to human activities at sea. It takes about 25-30 years for all the water in the Baltic Sea to be renewed, so persistent pollutants can remain in the Baltic for a long time.

Read more about the special conditions in the Baltic Sea region