Temperature and salinity values indicated intensive water column stratification throughout the study. Halocline depth was generally at 2 m in all seasons, but the salinity difference between the layers varied depending on the freshwater discharge, as the surface salinity minimum ranged between 5.2 in spring and 17.4 in summer. The mean salinity of the upper layer varied between 14.6 and 28.0, while values below the halocline were > 34 m in all seasons. In addition,

during the summer, the thermocline contributed to the haline stratification due to the extensive heating of the surface layer. The mean temperature decreased from 27.9 to 20.1 °C between the upper and the bottom layers. In spring, the temperature distribution was uniform throughout the water column, and there was an inverse temperature gradient in the autumn and winter, when the surface layer was colder than the rest of CP-868596 concentration the water column. Nutrient find more concentrations were generally elevated above the halocline in all seasons with the highest mean values for total inorganic nitrogen (17.70 μmol L− 1) and silicate (22.86 μmol L− 1) recorded in the autumn and for phosphate (0.36 μmol L− 1) in the spring. The contribution of size-classes to the total phytoplankton carbon biomass indicated different distributions between the upper and lower

layers as well as between seasons (Figure 2). In the spring, microphytoplankton was dominant at all three stations in the layer above the halocline, accounting for > 70% of the total biomass, with the maximum total phytoplankton Thymidylate synthase biomass of 144.02 μg C L− 1 recorded at station BK1. Below the halocline, total biomasses were lower (< 40 μg C L− 1) and the pico size-class was predominant, accounting for > 80% of the total biomass. In the summer, picophytoplankton was dominant in both layers with a mean contribution of 73% in the whole water column. The total biomass

values were higher in the upper part of the water column and especially at station BK1, where they reached 173.02 μg C L− 1 owing to the contribution of both micro- and picophytoplankton size fractions. In the autumn, the total biomass was generally low, with values < 20 μg C L− 1 and the pico size-class predominated, accounting on average for 61% of the total biomass in the whole water column. The exception was at station BK1, where the micro size-class contributed to 60% of the total biomass. In the winter, microphytoplankton predominated throughout the water column at all stations, while the largest contribution of the pico size-class (40%) was recorded at station BK1 above the halocline, where it also contributed to the highest biomass values of 51.34 μg C L− 1. The highest contribution of the nanophytoplankton biomass (24%) was recorded in the winter at station BK1 below the halocline, but their contribution was generally < 20% in all seasons and layers.