Abundance and species composition of plankton in the Gulf of Gdańsk near the planned underwater outfall of the Gdańsk-Wschód (Gdańsk-East) sewage treatment plant

The aim of this paper is to determine the current biological state of life in the pelagic zone of the Gulf of Gdańsk in relation to the planned start-up of an underwater outfall which will discharge sewage from the Gdańsk–Wschód (Gdańsk–East) sewage treatment plant. The plankton material was collected during two research cruises in July and October 1998. The samples were taken at 15 stations in four profiles located near Wyspa Sobieszewska (Sobieszewo Island), perpendicular to the coastline. Both the taxonomic and numerical structure of phytoplankton and zooplankton were typical of the coastal area of the Gulf of Gdańsk. The species diversity depends on hydrological conditions, mainly input from the River Wisła (Vistula). The abundance and biomass of phytoplankton in 1998 were several times lower than in 1994 and 1995 in the area off Górki Wschodnie, the profile located closest to the planned construction site. This could have been caused by generally lower temperatures in 1998 in comparison to previous years. In the investigated area only traces of algal eutrophication indicator species were noted. However, potentially toxic species were confirmed and were most abundant near the Wisła mouth. The highest concentrations of pelagic fauna occur in the shallowest area closest to the shoreline. Long-term observations of the dynamics of the variations in abundance and species composition indicate the increasing significance of one particular species – Acartia bifilosa.