Small Photosynthetic Flagellates of the White Sea: Seasonal Dynamics and Their Role in Plankton and Ice Communities

Abundance and biomass of small photosynthetic flagellates (SPF; 3–10 µm), chlorophyll a concentration , and the contribution of SPF to total phytoplankton biomass at different stages of seasonal succession in ice and in surface water were studied in Kandalaksha Bay of the White Sea in September 2016 and February and July 2017. In summer, SPF biomass in the photic layer averaged 38.36 ± 9.77 mg C/m 3 , 2.22 ± 1.43 mg C/m 3 in autumn, 2.6 ± 1.72 mg C/m 3 in under-ice water, and 14.79 ± 11.25 mg C/m 3 in ice. The contribution of SPF to total phytoplankton biomass depended on the season and ranged from 29 to 95%; the contribution of SPF to sympagic communities averaged 66%. The size structure of photosynthetic flagellates varied by seasons. Flagellates with a cell size of 6–10 µm dominated in summer plankton and in the ice. Flagellates with a cell size of 3–6 µm prevailed in autumn and in the under-ice water. The obtained data of SPF abundance showed higher values of the total phytoplankton biomass of the White Sea in the autumn–winter period compared with the estimates obtained previously. Applying the method of epifluorescence microscopy confirmed the idea that photosynthetic flagellates were the main producers in the winter period as well as in the summer when the biomass of planktonic algae with cell size exceeding 10 µm is low.