Extreme temperature events are becoming widespread with global warming, impacting phytoplankton, the foundation of the marine ecosystem. In the Southern Ocean, these impacts are not well understood, despite the key role of phytoplankton in global carbon cycling and climate. Here, we use 26 years of satellite observations and confirm previously identified impacts of marine heatwaves (MHWs) on phytoplankton in the Southern Ocean, while systematically comparing the opposite impacts of marine cold spells (MCSs). MHWs decrease phytoplankton chlorophyll-a (Chl-a) in subtropical regions (-21.11%) but less so in polar regions, with Chl-a even increasing in the Sub-Antarctic Zone (+22.26%). MCSs exhibit opposite patterns, enhancing Chl-a in subtropical regions (+32.37%) while inhibiting it in southern regions (-21.19%). These regional differences in Chl-a anomalies are mediated by distinct responses in phytoplankton size composition to MHWs and MCSs.  As extreme events intensify with global warming, Southern Ocean's phytoplankton will be disrupted, with implications for global biogeochemical cycles. These findings highlight the importance of simultaneously considering both MHWs and MCSs when assessing the ecological impacts of climate extremes.

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