This paper describes the comprehensive studies of phytoplankton carried out in April 2018 in the northwestern Barents Sea, in the Polar Front region (74.8°–76.2° N), at a distance of 10–100 km from the ice edge at depths ranging from 0 to 50 m. The hydrological, hydrochemical, and microbiological parameters of the seawater are determined simultaneously. An early-spring species complex is observed on both sides of the front; 90% of the biomass of which is formed by the diatoms Pauliella taeniata, Fragilariopsis oceanica, and Thalassiosira antarctica var. borealis. The taxonomic composition of microalgae communities in water masses of different origin has an 83% similarity. The average abundance and biomass of microalgae and chlorophyll-a content per m² in Atlantic waters reaches 37 170 cells/L, 0.12 μg/L, and 0.5 mg/m³, respectively. In Arctic waters, these values are about 30 times higher, 0.87 million cells/L, 2.59 μg/L, and 3.13 mg/m³, respectively. The results obtained suggest that the Polar Front is not a boundary between various algal communities, but separates highly productive Arctic waters from Atlantic waters with low phytoplankton abundance.

This paper analyzes data on the state of the pelagic microalgae community of the Kara Sea for all hydrological seasons. The data were obtained during complex in situ observations in 1996–2006. Four phases in the annual succession cycle of the phytoplankton of the nearshore continental area of the Kara Sea were identified: a prevernal phase (cryoflora bloom), vernal phase (ice-edge bloom), summer-fall phase (mixed synthesis phase), and winter phase (dormant phase). These periods are clearly distinguished from each other in their composition of dominating species complexes and quantitative characteristics, i.e., the numbers and biomasses of microalgae. In the investigated region, which is completely covered by ice during most of the year, the primary production processes begin at the same time as in ice-free coastal areas. Growth and blooming of cryoflora occur under the ice cover long before it breaks down. The peak development of the pelagic microalgae community, which is comparable to the spring peak of microalgae in ice-free coastal areas, is observed in the summer, when freshwater runoff from large rivers increases sharply.

The paper describes the results of a complex study of phytoplankton carried out at the end of August to the first half of September 2020 in the southeastern part of the Barents Sea and the southwestern part of the Kara Sea simultaneously with the determination of hydrological and hydrochemical characteristics. The taxonomic list of microalgae found in the studied area included 35 representatives identified to species. Of these, 14 (40%) were found in both basins. In the Pechora Sea, the average number of phytoplankton in the water column varied from 10 650 to 41 840 cells/L, and the biomass varied from 71.04 to 300.55 μg/L; in the southwestern part of the Kara Sea, the values of these indicators were 3510–28 420 cells/L and 16.31–66.96 μg/L, respectively. Both communities were at the autumn stage of seasonal succession; in terms of biomass, forms of arcto-boreal origin, predominantly oceanic, prevailed, and in the list of dominants, large-celled centric diatoms and dinoflagellates accounted for equal proportions. Species not exhibiting high abundance values were distinguished by a great degree of patchiness in spatial distribution: they were present at a small number of stations and not in all seawater layers. Thus, the results of the comparative analysis allow us to assert that the pelagic algocenoses of the compared regions, despite the difference in the hydrological parameters, were characterized by a significant degree of similarity. In general, the situation described confirms the hypothesis of the floristic unity of the southeastern part of the Barents Sea and the southwestern part of the Kara Sea.

An autochtonous community of benthic diatoms has been discovered in upper bottom sediments in the central Barents Sea at depths down to 300 m. Their abundance and biomass analyzed suggest that the microalgae found represent a depleted fragment of the arctic littoral microphytobenthos. Compared to the coastal algal flora, the deep-water assemblage is characterized by low species diversity and demonstrates abundance of about 102– 103 cells per cm2 in the benthic. Until recently, the presence of benthic microalgae in marine ecosystems has been associated exclusively with the littoral and sublittoral coastal zones.