Photic zone dynamics of phytoplankton size classes derived from pigments in the South-Eastern Black Sea

Spatial and temporal changes of phytoplankton size classes (PSC) derived from pigment analyses between May 2015 and April 2016 in the South-Eastern Black Sea were investigated. Within the scope of the study, monthly samples from surface to 50 m depth were collected from 3 different stations (0.5, 5 and 20 nautical miles). Samples were obtained by using SBE32 Carousel rosette sampler. Samples were filtered throughout a 47-mm diameter GF/F, and stored in liquid nitrogen (-196C) until analyses. The dominant pigment in the area was chlorophyll-a (mean 2,50 μg/L) revealed a consistency with phytoplankton cell numbers. The second prominent pigment was fucoxanthin (mean 0,63 μg/L) and followed by peridinin (mean 0,54 μg/L) and 19-Hexanoyloxifucxanthin (mean 0,24 μg/L). HPLC analyses revealed that the contribution of phytoplankton size classes to total phytoplankton community composition ranged from 8% to 91%, from 1% to 68%, and from 2% to 70% for microplankton, nanoplankton, and picoplankton respectively. Coastal stations were generally dominated by microplankton, however, offshore stations were mainly represented by nanoplankton or picoplankton. In terms of temporal changes, contribution of microplankton was generally high in spring, and their contribution decreased throughout summer, which picoplankton were characterised with high contribution.