Phytopelagic Communities of the Powell Basin in the Summer of 2020

Abstract

Microphytic communities in the pelagic zone of the Powell Basin (Weddell Sea, Southern Ocean) were studied based on an extensive network of stations during two observation periods in 2020 (late January to early February and mid-to-late February). The surveys conducted in these periods showed a high diversity of cenoses and their significant spatial heterogeneity. In the first observation period, eight cenotic groups were identified (of which seven were in the open waters of the basin and one in the Antarctic Sound); in the second period, four groups were found, all in open waters. Substantial rearrangements in the structure of the communities by the late summer and a significant growth of their area were recorded. The complex structures that are formed on elements of tests of Corethron, Chaetoceros, and Thalassiothrix were described. They were based on the matrix of bacterial colonies that gradually colonize its surface and deeper layers with a variety of microphyte species, mainly from the ice and/or benthic flora, and also with planktonic species in some regions. These structures that we referred to as “aggregates,” having specific genesis and hierarchical structural patterns, proliferated most intensely by the late summer. In the first period of observations, Corethron pennatum dominated free-floating microphytes everywhere; in the second period, Thalassiothrix antarctica, Chaetoceros criophillum, and Ch. convolutus were among the dominant species over a vast part of the study site. Heterotrophic flagellates abundantly proliferated within aggregates in the southern regions; benthic diatoms from the genera Amphora, Halamphora, and Nitzschia were also abundant in the northern regions.