Impact of the inflow of Vistula river waters on the pelagic zone in the Gulf of Gdańsk*

The biomass, production, composition of autotrophic phytoplankton and heterotrophic bacteria were studied along with environmental and biological parameters. Samples were taken from Vistula river water (at Kiezmark) and from the river plume to the outer stations in the Gulf of Gdańsk (Baltic Sea) in June 2005. The phytoplankton biomass gradient appeared to be simply the result of dilution of the river water in the sea water, whereas the bacterial abundance and biomass dropped between the river station and the first sea water stations, a decrease that cannot be explained by the dilution effect. The Vistula water stimulated the production mainly of bacterioplankton but also of phytoplankton in the river plume as compared to rates measured in Vistula waters and at the open sea stations. However, this stimulation did not result in a measurable increase in biomasses, probably because of the short retention time of water in the river plume. Phytoplankton production was correlated with phytoplankton biomass (Chl a), while bacterial production was correlated with phytoplankton production and phytoplankton biomass (Chl a).