Qualitative and quantitative phytoplankton analysis was conducted with an upright Nikon E80i microscope and Nikon TMS inverted microscope according to HELCOM guidelines (2011). The size of the counting chambers (10, 20, or 50 cm3) and the sedimentation time (24 or 48 h) depended on the abundance of phytoplanktonic organisms. The counting units (N) were cells, coenobia, or trichomes 100 μm in length. The biovolume of phytoplankton was calculated using species-specific geometric formulas and standardized size classes (Olenina et al., 2006).