Picocyanobacteria in the Ob Estuary and Adjacent Kara Sea Shelf in Late Autumn: Composition, Distribution and Functional Role

The spatial distribution of picophytoplankton (Pico) abundance and biomass, as well as the distribution of PE-rich and PC-rich picocyanobacteria (PCB), was investigated in the waters of the outer Ob estuary and the adjacent shelf of the Kara Sea in late autumn during the 92nd cruise of the R/V Akademik Mstislav Keldysh. The Pico chlorophyll “a” and its contribution to total chlorophyll “a”, total phytoplankton primary production (PP) and Pico PP were estimated. Pico abundance and biomass varied from 8.18 to 46.3 × 10⁶ cells/L and from 3.74 to 20.48 µg C/L, respectively. Pico was dominated by PCB of the genus Synechococcus, its contribution to the total Pico biomass ranged from 81 to 94%. The abundance of PC-rich Synechococcus varied from 4.02 to 29.21 × 10⁶ cells/L, whereas the abundance of PE-rich Synechococcus was significantly lower (p < 0.001) and varied from 0.98 to 8.0 × 10⁶ cells/L. The contribution of Pico to total chlorophyll “a” did not exceed 11% on average. The values of total PP and PP Pico were low, averaging 3.99 ± 1.53 and 0.77 ± 0.36 mg C/m³ per day, respectively. The contribution of PP Pico to total PP ranged from 14 to 33%. Factors determining the spatial distribution of PE- and PC-rich Synechococcus and production parameters of Pico were identified.