Environmental driving forces and phytoplankton diversity across the Ross Sea region during a summer-autumn transition

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.