Samples of water and phytoplankton were taken in the coastal zone of the Gulf of Gdansk in 2011 and 2012 in spring, summer and autumn. The stations were located near to the Tricity agglomeration (Gdansk, Sopot, Gdynia) and the mouth of the largest Polish river – the Vistula. The deepest station UW (40 m) was located 7 km north-east of the mouth of the Vistula. Station GN (37 m) was located in the middle of the Gulf of Gdansk, 19 km north-west of the Vistula estuary. Three other stations were located in shallower parts: SP (17 m) 7 km north of the Kacza river mouth, GDY (12 m) 4 km from the port of Gdynia and ME (4 m) in the vicinity of the wastewater treatment plant at Dębogórze, about 1 km from the coast