Photic zone dynamics and environmental controls of pigment-derived phytoplankton size classes in the South-Eastern Black Sea.

Understanding the environmental controls on phytoplankton is essential for predicting ecosystem responses to changing ocean conditions. This study investigated pigment-derived phytoplankton size classes (micro-, nano- and picophytoplankton) along a river-coastal-offshore gradient in the south-eastern Black Sea using high-performance liquid chromatography (HPLC) and Canonical Correspondence Analysis (CCA). Monthly sampling was conducted between May 2015 and April 2016 at three stations (0.5, 5 and 20 nautical miles offshore). Water samples collected throughout the photic zone were analyzed for chlorophyll-a and diagnostic pigments in relation to hydrography, nutrient availability and light conditions. Microphytoplankton contributed 8-91% of total phytoplankton biomass and dominated nutrient-rich, low-salinity river-influenced waters, whereas nanophytoplankton prevailed in coastal transitional zones and picophytoplankton reached up to 70% in oligotrophic offshore environments. CCA showed that phytoplankton size structure was primarily governed by cross-shelf gradients in nutrient availability, salinity and depth, with chlorophyll-a associated with microphytoplankton dominance and temperature and stratification favoring smaller size classes. These findings demonstrate strong bottom-up control of phytoplankton size structure and emphasize the importance of cross-shelf environmental gradients in regulating ecosystem organization. The offshore shift toward smaller phytoplankton fractions suggests reduced trophic transfer efficiency and increasing reliance on microbial pathways for carbon cycling under enhanced water-column stratification.