The Baltic Sea is one of Earth’s largest brackish water environments. Salinity, the salt content dissolved in water, defines its unique characteristics. Its consistently low salt levels create an ecosystem distinct from fully marine or freshwater habitats.
Defining Baltic Brackish Water
The Baltic Sea’s average salinity is approximately 7 Practical Salinity Units (PSU) in its central basin, though this can vary. For comparison, the world’s oceans average around 35 PSU, making the Baltic Sea almost five times less salty. This blend of fresh and saltwater defines “brackish water,” which is saltier than freshwater but less saline than typical ocean water.
Causes of Low Salinity
The low salinity of the Baltic Sea results from a combination of significant freshwater inflow and limited exchange with the saltier North Sea. Over 200 rivers discharge into the Baltic Sea, contributing a substantial volume of freshwater. The total catchment area, the land from which rainwater flows into the sea, is vast, measuring over 1.6 million square kilometers, which is more than four times the sea’s surface area. This continuous influx of freshwater constantly dilutes the sea’s salt content.
The Baltic Sea’s connection to the North Sea is restricted by narrow and shallow passages known as the Danish Straits. These straits limit the amount of dense, salty water that can flow into the Baltic from the North Sea. While some saltier water does periodically enter during strong westerly winds, the overall exchange is slow, with water remaining in the Baltic Sea for an average of 20 to 30 years. Furthermore, relatively low evaporation rates in the cooler northern climate also contribute to the positive freshwater balance, preventing significant salt concentration through evaporation.
The Internal Salinity Gradient
Salinity within the Baltic Sea exhibits a distinct gradient. The highest salinity is found in the southwestern parts, closer to the Danish Straits, where it can reach around 20 PSU. Moving northward and eastward, particularly into areas like the Gulf of Bothnia and the Gulf of Finland, salinity progressively decreases, dropping to as low as 0-3 PSU. This horizontal variation is a direct consequence of the diminishing influence of North Sea inflows and the increasing impact of freshwater runoff in the inner basins.
A permanent vertical stratification also characterizes the Baltic Sea, where less dense surface water sits atop denser, saltier deep water. This separation is marked by a halocline, a layer where salinity rapidly increases with depth, typically found between 40 to 80 meters. The halocline acts as a barrier, limiting vertical mixing between the surface and deep-water layers. This stratification allows saltier, denser water from the North Sea to creep along the bottom into deeper basins, while the fresher surface water flows out.
Consequences for Marine Life
The Baltic Sea’s brackish environment poses significant physiological challenges for aquatic organisms. Both marine species, adapted to higher salinities, and freshwater species, accustomed to no salt, struggle to thrive. Marine organisms like Atlantic cod are often at the lower limits of their salt tolerance, impacting their reproduction and distribution. Conversely, freshwater species such as pike and perch live at the upper limits of their salinity tolerance.
This physiological stress means that only a limited number of species can successfully adapt to and inhabit the Baltic Sea. The sea exhibits lower species biodiversity compared to fully marine or freshwater ecosystems globally. Species that do survive often display specific adaptations, such as Baltic blue mussels, which grow smaller in lower salinities compared to their ocean counterparts. Its inhabitants are particularly sensitive to changes in salinity levels.