The Baltic Sea is connected to the North Sea through narrow and shallow sounds between Denmark and Sweden. The outlet consists of a series of basins separated by shallow sills which obstruct efficient water exchange. Consequently, it takes 25-35 years for all the water from the Baltic Sea to be replenished by water from the North Sea and beyond.
The environmental conditions of the Baltic Sea are defined by the fresh water input from rivers and precipitation, and by the limited inflow of more saline water from the North Sea. Without the constant, albeit small influx of saline water through the Danish straits, the Baltic Sea would have been transformed into a gigantic fresh water lake long ago. A clear salinity gradient exists from the almost oceanic conditions in the northern Kattegart to the almost fresh water conditions in the Northern Bothnian Bay.
A salinity barrier also exists between the surface to the seabed of the Baltic. Saline water, naturally heavier than fresh water, flows along the bottom of the sea. The fresh water on the surface of the sea does not mix appreciably with the saline water underneath. As a result, a marked stratification of salinity exists throughout the Baltic Sea at a depth of about 40-70 metres. The salinity barriers prevents the exchange of substances, i.e. oxygen, nutrients, and pollutants, between the two layers. The environmental conditions between the two layers are, thus, vastly different.
Due to the limited water exchange, oxygen poor water predominates near the bottom of many parts of the baltic Sea. Bacteria growing in this oxygen deficient water break down organic matter and release hydrogen sulfide, a toxic gas. Both the oxygen deficiency and the hydrogen sulfide production combine to make the bottom of the Baltic Sea virtually lifeless. The size of the seabed with impaired conditions varies from year to year and may reach 100,000 km2 (1/4 of Baltic Sea).
Only major deep-water inflows that bring large volumes of oxygen-rich water into the Baltic Sea can improve the living conditions in the deeper bottom layers. These inflows are, unfortunately, quite rare. Since 1976, only a few major inflows were observed and none were recorded between 1983 and 1992. Conditions in the deep water of the Baltic changed drastically and led to the most significant and serious stagnation period ever observed. In January 1993, a major deep water inflow occurred after 16 years of stagnation. But it was only an isolated event. No other major inflows have occurred since then, and since 1995 the conditions in the deep water have again started to stagnate.
Estonia is a mainly a lowland country. On average, the land reaches only 50 metres above sea-level. The highest point is Suur Munamägi (Big Egg Mountain) in the southeast. Most of Estonia's 420 rivers are short. The longest river is the Pärnu River, which is 144 km long. More than 1000 lakes cover the landscape. The largest lake, Lake Peipsi, on the eastern border, is the fifth largest lake in Europe. It covers an area of 3555 km2.
Twentyfive percent of the country is covered by agricultural lands (grasslands, meadows, and natural pastures). Forests account for 44% of the land mass. Mires (fens, bogs, swamps) cover an additional 20% of the territory.