Influence of Hydrodynamic Regime on Living Coccolithophores in the Cretan Sea and South Cretan Area (Eastern Mediterranean)

Coccolithophores are important components of marine phytoplankton and are found to be useful indicators of the environmental conditions of the upper water column. In this study, we investigate coccolithophore abundance and composition in the Cretan Sea and South Cretan area (Eastern Mediterranean), and their relation to prevailing hydrodynamic conditions during late February/early March 2019. Results showed that total coccolithophore abundance ranged from 26.3 × 102 to 258.8 × 102 coccospheres L−1, averaging at 135.8 × 102 coccospheres L−1. Among the 45 identified species, the opportunistic Emiliania huxleyi was the most dominant, representing 89% of the coccolithophore assemblage. In the Cretan Sea, this species showed relatively homogeneous abundances throughout the upper 100 m depth of the water column; however, towards the Rhodes Cyclone, where a weak stratification had started, and the mixed layer was relatively shallow, higher abundances were found at depths shallower than 50 m. Syracosphaera molischii co-occurred with Emiliania huxleyi, whereas Rhabdosphaera clavigera, Syracosphaera pulchra, and Syracosphaera mediterranea were also present but in lower abundances, reflecting the influence of warm, salty Levantine Surface Water. Based on the morphological analysis, Emiliania huxleyi was mostly represented by heavily calcified forms consistent with winter-spring patterns in the Aegean Sea. The observation of signs of dissolution with high relative abundances of etched/corroded coccospheres indicates the sensitivity of Emiliania huxleyi to the prevailing circulation pattern during the 2019 mixing event within the Rhodes gyre.