Phytoplankton size classes (PSCs) estimated by using high-performance liquid chromatography (HPLC) pigment data and pigment indices in the south-eastern Black Sea were used to characterise seasonal phytoplankton structure from November 2015 to August 2016. Results revealed significant seasonal variation in phytoplankton size classes hence community composition and pigment indices along the south-eastern Black Sea, varied from 2% to 71%; from 3% to 77%; from 5% and 91% for picophytoplankton, nanophytoplankton and microphytoplankton, respectively. Phytoplankton community composition was dominated by microphytoplankton with a high content of photosynthetic carotenoids. In-situ Chl-a concentrations changed during the study period, the highest concentration was detected in autumn (2.60 mu g.l(-1)) and spring (2.48 mu g.l(-1)). Pigment indices indicated that microphytoplankton carrying majority of photosynthetic carotenoids (PSCTP) dominated the community along the south-eastern Black Sea. PSCTP positively correlated with MicroDP, indicated that microphytoplankton seasonally optimise their light-harvesting capability along the south-eastern Black Sea. However, concomitant increase in PPCTPwith PicoDP and dominancy of PPCTP in summer indicated enhancing photoprotective carotenoid content during warm and nutrient depleted periods along the study area. Canonical correspondence analysis (CCA) explained general pattern of PSCs along the study area are controlled seasonally by physicochemical drivers (i.e. nutrient dynamics and pigment content). The differences detected phytoplankton community composition also indicated phytoplankton adaptive strategy to changing environmental factors. (C) 2020 Elsevier B.V. All rights reserved.