Phytoplankton communities in the Southern Ocean are shifting as the region experiences rapid environmental change. Using 15 years of Southern Ocean Continuous Plankton Recorder (SO‑CPR) data (2007–2021), this study assesses trends in phytoplankton composition, with emphasis on the ecologically important diatom genus Chaetoceros. Overall diatom and dinoflagellate abundances increased significantly across the sampling domain, while the diatom:dinoflagellate ratio showed only a slight, but significant, increase, indicating broadly stable functional group structure despite higher total biomass. In contrast, Chaetoceros abundance declined per year, suggesting that this genus may be responding differently to recent environmental conditions than other diatom taxa. Twelve Chaetoceros taxa were recorded, with C. dichaeta, C. atlanticus, and C. criophilus dominating observations, and species richness peaking at higher latitudes. Spatial patterns show that Chaetoceros species were broadly distributed across the Indian sector, with elevated abundances near major Southern Ocean fronts. The divergence between increasing total phytoplankton abundance and declining Chaetoceros counts highlights potential shifts in community structure with implications for carbon export and the foraging ecology of key grazers such as Antarctic krill. These results demonstrate the value of long‑term CPR observations for detecting ecosystem change in a rapidly warming Southern Ocean.