Trends in phytoplankton communities uncovered by the Southern Ocean Continuous Plankton Recorder

Context Phytoplankton communities in the Southern Ocean are shifting as the region experiences rapid environmental change. Aims To investigate long-term trends or changes in Southern Ocean phytoplankton communities, and model abundance patterns of key dominant genera against sampling year and sea-surface temperature (SST). Methods Using 10 years of Southern Ocean Continuous Plankton Recorder (SO-CPR) data within a 15-year period (2007–21), we examined long-term trends in phytoplankton composition, with emphasis on six ecologically important diatom genera (Chaetoceros, Thalassiothrix, Fragilariopsis, Pseudo-nitzschia, Rhizosolenia and Corethron). Key results Overall diatom and dinoflagellate abundances increased significantly across the sampling domain, whereas the diatom:dinoflagellate ratio remained stable, suggesting little change in the relative contributions of these major phytoplankton groups despite increasing overall abundance. Patterns among focal genera differed, with distinct temporal trends and contrasting relationships with sea-surface temperature, indicating ecological differentiation among dominant Southern Ocean diatoms. Conclusions No single genus accounted for the observed increase in abundance, suggesting a community-wide response to environmental change. Implications These results demonstrated the value of long-term CPR observations for detecting trends, natural variability and ecological change in a rapidly warming Southern Ocean.