Long-term monitoring data reveal pH dynamics, trends and variability in the western Gulf of Finland

Monitoring data on water pH are presented for the period between 1972 and 2009 from the sampling stations Längden and Storgadden, at the entrance to the Gulf of Finland, Baltic Sea. The overall pH in the area ranged from 9.2 to 7.4, on average 8.1, and showed a significant decreasing trend during the winter period corresponding to a median annual decrease of 0.006. The data corroborate previous findings about a seasonal effect, where pH is higher during summer than winter.