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

Abstract

The outflow and spreading of the Vistula River plume in the Gulf of Gdańsk was observed in February/March 2001 and 2002, during the peak flow period with 1000–3000 m 3 s − 1 of fresh water and of 395 tons NO 3–N d − 1 . Time and space scales of the mixing patterns at two different wind situations were investigated; in 2001, southerly winds prevailed and the river plume front showed a strong meandering pattern and the formation of a detached eddy. In the eddy 293 tons of NO 3–N are transported up to 40 km offshore, while the eddy's lifetime did not exceed 1 week. In 2002, south-westerly winds predominated and that forced the plume along the shore towards the east. The plume along the coast was 5.5 times larger in volume and had 18 times more NO 3–N (5390 tons). The physicochemical parameters, nutrients, dissolved inorganic carbon (DIC), and the δ 15N–NO 3 −, δ 13C-DIC, and δ 15N 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 δ 15N-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 NH 4 + was assumed to lower the δ 15N–NO 3 − 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.