The Baltic Sea: A Brackish Inland Sea, Not a Salty One

The Baltic Sea has exceptionally low salinity—among the world's lowest for a marine body. Average salinity: 7–8 PSU (practical salinity units) vs. global ocean average ~35 PSU. It is a brackish mediterranean sea, not a high-salinity one.

Why So Fresh?

• Restricted exchange: Narrow, shallow Danish Straits (Øresund, Great Belt, Little Belt) limit North Sea inflow. Deep water renewal occurs only during rare Major Baltic Inflows (MBIs).
• Massive freshwater input: 250+ rivers (Neva, Vistula, Oder, Daugava, Nemunas) discharge ~15,000 m³/s—equivalent to ~1% of volume annually.
• Positive water balance: Precipitation > evaporation.
• Stratification: Halocline at 40–80 m traps saline bottom water; surface layer is nearly fresh (2–4 PSU in Gulf of Bothnia/Finland).

Ecological Consequences

Salinity gradient drives unique species zonation: marine species (cod, herring) in southwest; freshwater (pike, perch) in north; brackish specialists (flounder, blue mussel, Fucus vesiculosus) in between. Low diversity but high endemism. Hypoxia in deep basins due to stratification + nutrient loading.

Climate Sensitivity

Warming reduces ice cover, alters stratification, may increase MBI frequency—but also increases runoff. The Baltic is a sentinel system for coastal ocean change.