Heterogeneity of Spatial Distribution and Functional Role of Phototrophic Picoplankton in the Kara Sea in Mid-Summer

The spatial distribution of picophytoplankton (Pico) biomass and chlorophyll “a” (Chl) Pico, as well as the contribution of Pico to the total Chl and primary production (PP) were investigated in the north-western, southern, and western areas of the Kara Sea in the second decade of July 2019. The integrated Pico biomass was 159.3 ± 103.5 mg C/m in the north-western area. The integrated Pico biomass was significantly lower (p = 0.001), averaging 57.1 ± 15.9 mg C/m in the southern and western parts of the sea. The largest contribution of Pico to the total Chl was found in the subsurface chlorophyll maximum (up to 75%) in the north-eastern region of the sea; in the other areas of the sea, the maximum Pico contribution was found in the surface layer, averaging 28% for the southern region and 53% for the northern region. Total PP and Pico PP ranged from 0.97 to 123.3 mg C/m per day and 0.2 to 33.99 mg C/m per day, respectively. The average contribution of Pico to total PP was 26% in the western part of the sea, 30% in the southern part, and more than 40% in the north-western parts of the sea. Pico was dominated by picoeukaryotes; the contribution of cyanobacteria to the total Pico biomass did not exceed 6%. The spatial heterogeneity of Pico distribution was determined by the hydrological characteristics of the studied areas.