A turning point in the development of phytoplankton in the Vistula Lagoon (Southern Baltic Sea) at the beginning of the 21st century

Phytoplankton community structure was studied from 2002 to 2016 in the Vistula Lagoon (southern Baltic Sea) in the context of the 2010 shift in its population, as well as the reason for this shift and its environmental impact. This evident shift was indicated by Multidimensional Scaling at the Bray Curtis similarity level of 31%. Before 2010, the primary components of phytoplankton were Cyanobacteria (up to 98% of the biomass, October 2007) and Chlorophyta (40%, July 2002). After 2010, the contribution of Cyanobacteria considerably decreased, and the proportions of other phyla increased. The total phytoplankton biomass positively correlated with phosphorus, and Cyanobacteria biomass with silica. Evident changes were also observed in the seasonal dynamics of phytoplankton. Before 2010, the highest values of biomass occurred in autumn, and were related to high biomass of Cyanobacteria. Higher biomass has been recently reached in spring, during the dominance of Ochrophyta associated with Chlorophyta, Charophyta, and Cryptophyta. Generalised additive models showed a significant decreasing trend of the total phytoplankton biomass, Cyanobacteria, Chlorophyta, and flagellates, suggesting a decrease in eutrophication. This trend is concurrent with a considerable increase in the ratio of zooplankton to phytoplankton biomass since 2010. The increased ratio, however, did not result from elevated zooplankton biomass, but from the drop in phytoplankton biomass. Therefore, the most probable reason for the decrease in phytoplankton biomass was the simultaneous decrease in the concentration of all nutrients. The potential additional impact of filtration by a new alien bivalve Rangia cuneata G. B. Sowerby I, 1832 is also discussed

Bacterial community structure influenced by Coscinodiscus sp in the Vistula river plume

The Gulf of Gdansk is influenced by freshwater inflow from the River Vistula and by a wind-driven current along the coast. Bacterial communities from five stations along a salinity gradient were sampled during one day and analysed by terminal restriction fragment length polymorphism (T-RFLP), catalysed reporter deposition-fluorescence in situ hybridisation (CARD-FISH) and 16S rRNA gene libraries. On the day of sampling, we observed a probable current-driven seawater influx into the inner part of the gulf that separated the gulf into distinct water bodies. Members of the diatom Coscinodisrus sp. dominated one of these water bodies and influenced the bacterial community. The coexistence of typically freshwater and marine bacterioplankton populations in the Vistula river plume suggested an integration of some freshwater populations into the Baltic Sea bacterioplankton