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

The spatial distribution of the picophytoplankton (Pico) biomass and chlorophyll (Chl) a of Pico, as well as the contribution of Pico to the total Chl and primary production (PP) were investigated in the northwestern, southern, and western Kara Sea in the second ten days of July 2019. The integrated Pico biomass was 159.3 ± 103.5 mgC/m2 in the northwestern area and was significantly lower (p = 0.001), averaging 57.1 ± 15.9 mgC/m2, in the southern and western areas of the sea. The contribution of Pico to the total Chl was the largest in the subsurface layer of chlorophyll maximum (to 75%) in the northeastern region of the sea and in the surface layer in the other areas of the sea, averaging 28% in the southern region and 53% in the northern region. Total PP and PP of Pico ranged from 0.97 to 123.3 mgC m–3 day–1 and from 0.2 to 33.99 mgC m–3 day–1, respectively. The mean contribution of Pico to the total PP was 26% in the western area of the sea, 30% in the southern, and more than 40% in the northwestern. Pico was dominated by picoeukaryotes, and the contribution of cyanobacteria to the total Pico biomass did not exceed 6%. The spatial heterogeneity of the Pico distribution depended on the hydrological characteristics of the studied areas.