Hydrochemical variability of the ecosystem of the Gulf of Elbląg (north-eastern Poland)Abstract The Vistula Lagoon – one of the main recipients of the central and eastern parts of the Vistula delta – is not homogeneous in terms of its hydrodynamics and hydrochemistry. In the southern part a separate hydrographic object – the Gulf of Elbląg – can be delimited. This delimitation is due to different morphometric and hydrometeorological conditions that prevail in this part of the Vistula Lagoon. In order to determine the nature of the waters, measurements of the selected physico-chemical properties, including chlorides, as well as control hydrological measurements were performed in the years 1997–2007. The study area included the water of the Gulf of Elbląg, the estuary stretch of the Elbląg River, the watercourses flowing from the Elbląg Plateau and the polder areas surrounding the Gulf of Elbląg. One measurement point was located on the Vistula Lagoon. The chloride values in the Gulf of Elbląg ranged from 20 to 2015 mg·dm-3. The results may indicate that the Gulf of Elbląg is a reservoir under the hydrodynamic and hydrochemical influence of both the Vistula Lagoon and the watercourses in its mouth, as well as the Vistula delta and the Elbląg Plateau. They dictate the seasonal nature of the waters of the Gulf of Elbląg.