What is water salinity? Why is the salinity of the Baltic Sea less than that of the Kara Sea?

The salinity of seawater is the content in grams of all minerals dissolved in 1 liter of seawater. Salinity is expressed in g / l, that is, in thousands of parts-s (% o). The average salinity in the world is 35% o, which means that every liter of water contains 35 grams of minerals.
The greater the amount of evaporation, the greater the salinity of the seawater, because the salt remains during evaporation. Changes in salinity are strongly influenced by sea and coastal currents, freshwater outflow by a large river, and mixing of ocean and sea water. At a depth, salinity fluctuations occur only up to 1500 m, below the salinity changes little.