Impact of the Słupia River waters on microbial communities in the port of Ustka and adjacent Baltic Sea waters

The distribution of bacterial and ciliate abundance, ciliate community composition and other parameters were studied during summer along a transect from the mouth of the Słupia River to offshore waters (southern Baltic Sea). Bacteria were examined under an epifluorescence microscope and ciliates were observed under an inverted microscope. Two water masses were identified along the transect. Less saline waters in the river mouth and in the surface layer in the port of Ustka were characterized by high bacterial abundance (5.51–6.16 × 10 6 ml −1 ) and low ciliate abundance (0.34–0.90 cells ml −1 ). More saline waters in the near-bottom zone in the port of Ustka and in the surface layer outside the port contained smaller numbers of bacteria (0.99–2.14 × 10 6 ml −1 ) and larger numbers of ciliates (2.65–5.40 cells ml −1 ). The differences were statistically significant. The separation of the two water masses indicated that the Słupia River exerted a minor impact on the marine waters. The ciliate community composition changed along the transect studied. The main statistically significant difference observed was the low contribution of oligotrichs and choreotrichs to ciliate biomass (3–4%) in less saline waters and their dominance (45–80% of ciliate biomass) in more saline waters.