Salinity variations of the surface water at the southern coast of the Baltic Sea in years 1950–2010

• Decrease in surface water salinity along the southern Baltic coast was observed.
• The strongest water salinity trends were in winter at Władysławowo, Hel and Gdynia.
• Clear decrease in salinity in Hel, 1990–1999 corresponded to Vistula outflow increase.
• In Międzyzdroje the decreasing sea water salinity trends were not significant.

This work aims to examine trends in surface water salinity along the southern Baltic Sea coast over the period between 1950 and 2010. Major trends in hydrological and meteorological factors that potentially influenced variations in salinity, and their relationships with salinity are examined as well. The study is based on monthly surface water salinity values from Międzyzdroje, Władysławowo, Hel and Gdynia (1950–2010), monthly atmospheric precipitation totals from Świnoujście, Hel and Gdynia (1951–2010), annual values of the North Atlantic Oscillation (NAO) index (1951–2010), monthly number of days with particular atmospheric circulation types over Poland according to Lityński classification (1951–2005), and monthly discharge values for Vistula and Oder rivers (1951–2010). Pearson correlation analysis and linear regression analysis were applied in this study.A decrease in surface water salinity along the southern Baltic Sea coast was observed over the study period, especially pronounced in the eastern part of the coast. Winter salinity trends at Władysławowo, Hel and Gdynia were considerably statistically significant even at α=0.001 level. For the remaining seasons, salinity trends were weaker, but still significant, at least at α=0.05 level. For Międzyzdroje, however, salinity trends are not significant.Even though increasing tendency prevailed over the study period, no statistically significant trends were detected in atmospheric precipitation sums, nor in river discharge. This probably results from a high annual variability in these parameters. An increasing trend in Vistula river discharge was observed in the last decade of the 20th century, i.e. a period of pronounced salinity drop.