The Baltic Sea is the youngest sea of our planet. It was formed approximately 10 000 – 15 000 years ago, after the latest ice age. The catchment area of the Baltic Sea is four times bigger than the sea itself, and it includes 14 countries and over 85 million habitants. Catchment area is the area from which surface water and groundwater flow into the sea. This is why operations done on land, such as farming and habitation, also impact the sea. The Baltic Sea has nine coastal states: Finland, Sweden, Denmark, Germany, Poland, Latvia, Lithuania, Estonia and Russia.
The Baltic Sea is shallow compared to the oceans, and almost entirely closed. Salt water occasionally flows in through the Danish straits, bringing salty ocean water to the sea. Due to massive freshwater runoff from the land and limited salt water inflows from the Danish straits, the salinity of the Baltic Sea is much lower than that of the oceans, and the water is considered to be brackish water instead of sea water. This makes the Baltic Sea a challenging environment for many species, since the water is too fresh for marine species but too salty for fresh water species. The Baltic Sea’s species have adapted to live in these peculiar conditions but even small changes in the sea can have dramatic effects on the sea’s habitants.
Although the Baltic Sea’s ecosystem is not as rich as some tropical aquatic ecosystems, recent research has revealed an abundance of species especially in the coast of Finland, where varied and diverse environment offers habitats for a diverse array of species. Biodiversity is important, as it improves the ecosystems’ resilience towards changing ecological conditions. This is especially crucial in the Baltic Sea, where low salinity already forces many species to live on the edge of their comfort zones.
So called keystone species, such as bladder wrack and eelgrass, are especially important. They ensure the wellbeing of many other species by providing them with e.g. shelter and food. Losing a keystone species will also lead to the loss of many other species that have depended on it. Protecting keystone species and their habitats is essential to conserve biodiversity.
Water in the Baltic Sea is stratified. Water near the surface is warmer, while colder water sinks to the bottom. These layers mix during spring and autumn, as the temperatures change. Water is also stratified as a result of different levels of salinity, due to saltier water being heavier and settling on the sea floor. Salty deep water doesn’t mix with fresher surface water, but sometimes salt water flows in from the Danish Straits and pushes the old deep water to the surface.
As a result of the large and heavily populated catchment area, the shallowness of the sea, and the stratification of water the Baltic Sea is very vulnerable. Nutrients and hazardous substances have accumulated in the sea for centuries and are still causing problems, such as eutrophication. Large areas in the sea bottom are dead due to poor oxygen conditions. Heavy marine traffic increases the risk for accidents, such as oil spills.