The freshwater station in the River Vistula at Kiezmark (KIE) differed from the station in the vicinity of the river mouth – ZN2 and the seawater stations E53, MK-8776 order E54 and E62 in that salinities and silicate concentrations were both lower (Table 1). The water temperature (17.3–18.9°C) was relatively constant at all stations. The large differences in salinity (between KIE and ZN2), together with the linear vertical salinity and temperature profiles (down to 20 m depth, data not shown), indicated a mixing of freshwater with the seawater in the river mouth or upstream of station ZN2. The nutrient concentrations were in the micromolar range, but generally 2–25 times higher (except silicates) at the Kiezmark

station (Table 1). At the same station, the concentration of dissolved organic carbon was the highest (5.6 mgC dm−3), but simultaneously less labile. Allochthonous organic matter, as determined by the specific ultraviolet absorbance measurements (SUVA) (the higher the SUVA, the higher the ratio of molecules with aromatic rings and the less labile DOC), had its maximum at the river station KIE, with 18.8 dm−3 gC−1 cm−1 (Table 1). SUVA values (11.6–12.6 dm3 gC−1 cm−1) were the lowest at stations E53, E54 and E62, which potentially indicated DOC of phytoplankton origin. Interestingly, station E54 differed from the neighbouring stations E53 and E62 in terms of its organic nitrogen and silica concentrations.

We suggest that the slightly higher organic nitrogen content and the reduced silica content indicated a local water body. According to the ecohydrodynamic model of the University SCH727965 of Gdańsk (http://model.ocean.ug.edu.pl/, Jędrasik et al. 2008, Kowalewski & Kowalewska-Kalkowska 2011), three days before sampling, a strong south-easterly current along the Hel Peninsula had pushed water masses from the open sea into the inner parts of the Gulf of Gdańsk (Figure 1). The more saline waters at stations ZN2 and E53 may have originated from the open sea, whereas the water around station E54 was a separate ‘aged gulf’ water body. The freshwater Kiezmark station had the most productive phytoplankton community. The concentration

of either chlorophyll a ( Table 1) coincided with the biomass of phytoplankton ( Figure 2) and the highest primary production ( Table 1). Our microscopic inspection detected 67 taxa, of which 32 belonged to green-algae, 10 to cyanobacteria and 8 to diatoms. Quantitatively, 85% of the phytoplankton biomass were diatoms. The dominant species was diatom Cyclotella meneghiniana (77% of the total phytoplankton biomass). Freshwater species were represented by Skeletonema subsalsum (2%) and the green-algae Pediastrum duplex (2%) and Chlamydomonadales (2%). The highest growth efficiency of phytoplankton (assimilation number, AN) was found at the river mouth station ZN2 (Figure 3). This location reflects the direct influence of the River Vistula, where nutrient concentrations were higher compared to the other seawater stations.