Climate driven shifts in Antarctic phytoplankton groups: Implications of widespread diatom decline

The high biomass of diatom-dominated phytoplankton communities is critical for sustaining the iconic Antarctic marine food web. Among Southern Ocean phytoplankton, the abundance of diatoms is particularly important as this group is selectively grazed by krill — which are the crucial prey of whales, penguins, and fish. Diatoms also play a key role in biogeochemical cycling and carbon export. Here we show drastic changes in the taxonomic composition of Antarctic phytoplankton over the past 26 years, driven by factors such as declining sea ice concentration, reduced iron availability, and warming. Using a machine learning approach trained on 14,824 historic pigment samples, we show that the proportion of Antarctic diatoms has substantially declined by 18% (-7% per decade), whilst the proportion of other smaller, less-grazed phytoplankton groups, such as haptophytes and cryptophytes, increased by 6–10%. We find that 65–74% of the Antarctic continental shelf had statistically significant trends in the proportion of one or more phytoplankton taxonomic groups (p < 0.05). These recent shifts in phytoplankton assemblages will have reduced food availability for grazers and thus Antarctica’s top predators, whilst also diminishing the biological pump and export, likely increasing atmospheric carbon dioxide (CO ₂ ) levels.