POSTER Session 1

Shifts in summertime Antarctic phytoplankton assemblages raise concerns for Antarctic ecosystems

The biomass and taxonomic composition of phytoplankton assemblages are critical to zooplankton grazers and sustain the iconic Antarctic marine food web. Among phytoplankton, diatom abundance is particularly important as they are selectively grazed by krill, which are crucial prey of macrofauna including whales, penguins, and fish, and are also important in biogeochemical cycling and the export of carbon to depth. Recent analysis has shown that much of the Southern Ocean is getting greener, yet there has been no corresponding study of whether phytoplankton community structure is changing. Here we demonstrate that significant changes in Southern Ocean phytoplankton chlorophyll a (Chl-a) and taxonomic composition are likely to have occurred during the last 26 years (December 1997 to February 2023), based on random-forest models that estimate phytoplankton community composition from satellite observations and pigment data. Our model indicates that around Antarctica the proportion of diatoms declined by -3.6% per decade, whilst the proportion of other smaller, less nutritious phytoplankton groups, such as haptophytes and cryptophytes, increased by ~1 – 2% per decade. Of ~175,000 pixels in the Antarctic Zone, between 65 – 70% had statistically significant trends. These changes may have reduced the food availability for grazers, impacting Antarctica’s top predators, whilst also reducing carbon export to depth via the biological carbon pump, with potential positive feedback on climate change.

Alexander Hayward, Danish Meteorological Institute, Denmark, https://orcid.org/0000-0003-3500-9964

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