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The Ross Sea is a highly productive system characterized by a seasonal succession of phytoplankton groups. However, most of the current understanding is based on observations on the continental shelf in spring and summer at relatively coarse taxonomic resolution. Here, we characterize community composition (class to species) using V4‐18S rRNA gene metabarcoding on transects to and across the Ross Sea (shelf‐slope and oceanic subregions) during two voyages covering the Austral summer–autumn progression in successive years. Phytoplankton composition shifted from low‐diversity diatom‐dominated (50%) communities during the summer to more diverse dinoflagellate‐dominated (48%) systems during the lower‐productivity autumn season. Prymnesiophyceae abundance was low on both voyages (10%), except on the southeast shelf‐slope, where Phaeocystis antarctica dominated a shallow mixed layer, contrary to its reported preference for deep‐mixing conditions. Amplicon sequence variant analysis identified distinct spatial patterns for two P. antarctica genotypes yet clustered certain species of Bacillariophyta and Prymnesiophyceae, indicating similar environmental preference for genotypes in these groups. Multivariate analysis of environmental drivers found a higher correlation of community composition variation with salinity and macronutrients, but less so with mixed layer depth, considered a primary determinant of taxonomic composition in the Ross Sea. Comparison between years established that community composition was temporally more stable in oceanic relative to shelf‐slope waters. This study of seasonal variation of phytoplankton community composition at finer taxonomic resolution provided insights into species‐ and strain‐specific distribution, ecological preferences, and relationships to environmental conditions in the wider Ross Sea to inform modeling and projection of future regional changes. [ABSTRACT FROM AUTHOR]

The combination of iron limitation and microzooplankton grazing controls phytoplankton productivity and taxonomic composition in high‐nutrient low‐chlorophyll (HNLC) regions. While increased productivity and diatom contribution triggered by iron enrichment support this view, direct measurements of underpinning group‐specific growth and grazing rates are scarce for the Southern Ocean. To assess these rates, we conducted dilution experiments coupled to high‐performance liquid chromatography and flow‐cytometry in sub‐Antarctic waters on and off Campbell Plateau, southeast of Aotearoa‐New Zealand. Off the plateau, growth and grazing were closely balanced for all groups despite a two‐fold difference between slow‐ and fast‐growing groups. On Campbell Plateau, where HNLC conditions were alleviated, the balance was disrupted, mainly by the preferential growth of diatoms and green algae, which was stimulated beyond grazing. Our results expand the recognized ability of diatoms to escape grazing control to picoplanktonic green algae that also avoid grazing and contribute significantly to phytoplankton productivity and biomass accumulation. [ABSTRACT FROM AUTHOR]