Riverine discharge into a coastal bay: A stable isotope study in the Gulf of Gdańsk, Baltic Sea

The physicochemical parameters, nutrients, dissolved inorganic carbon (DIC), and the δ15N-NO3−, δ13C-DIC, and δ15N in particulate organic nitrogen (PON) were used to analyse the river spreading and thus the anthropogenic influence on the coastal system. Minor deviations from conservative mixing were found in 2001 in the eddy for δ15N-PON, and PON; they were attributed to elevated primary production rates due to slightly stronger stratification of the water column in the eddy. Nitrification of riverine NH4+ was assumed to lower the δ15N-NO3− values in 2002 along the coast. A significant removal of nutrients was also observed between a station 32 km upstream and the river mouth. This study overall suggests that half or more of the nitrate is lost downstream and that the other half is mainly transported eastward with the prevailing winds from the south west. The Vistula River load may thus mainly affect the coastal areas and less the open Baltic Sea.