One of the essential parameters when planning employment for a vessel is the salinity level of the water at the destination.

Salinity is usually measured in PSU (Practical Salinity Unit), which is based on seawater’s conductivity properties. It is equivalent to a unit in thousand or g/kg. So 1 PSU = 1.001%.

Based on the history of the formation of the Baltic Sea, we know that this basin has been a lake several times (the last time about 7,000 years ago), and if it were to be cut off from the ocean today, it would become a freshwater lake again. The reason for this is the positive water balance of the Baltic Sea, resulting from the excess of freshwater inflow over evaporation. Numerous rivers and rainfalls provide a constant supply of fresh water. In total, about 250 rivers drain into the Baltic Sea.

The Baltic Sea is the largest brackish water basin in the world. The average salinity of the oceans is about 35 PSU, while the average salinity of the Baltic Sea is almost five times lower at 7.5 PSU. Salinity varies throughout the Baltic Sea and gradually decreases northwards, further away from the Danish Straits.

The main large Polish ports are located at the mouths of rivers. Gdansk is at the mouth of the Vistula River, which lowers the average salinity at port basins to 3-4 PSU. Nearby Gdynia records 6-7 PSU. The harbor basins of Swinoujscie Port, located at the mouth of the Swina River, have a salinity of around 3-4 PSU. Szczecin Port, which is strictly a river port, has a salinity that can fall below 1 PSU.

Source: https://satbaltyk.iopan.gda.pl/  https://fundacjamare.pl/en/hydrology-of-the-baltic-sea/