Linking phytoplankton phenology and carbon cycling to sea ice retreat timing in the Weddell Sea

The Southern Ocean seasonal sea ice zone (SIZ) is a critical region for primary production and ocean carbon uptake, heavily influenced by seasonal variation in sea ice extent, which has changed dramatically in recent years. Here we show that the ice retreat timing (IRT) is linked to phytoplankton phenology, air-sea CO2 exchange, and organic carbon export from the surface layer. Specifically, early IRT extends the productive season, resulting in greater biomass accumulation and CO2 uptake by the ocean, while late IRT shortens the productive period and limits air-sea CO2 exchange. Additionally, early IRT enhances the export of organic carbon, while late IRT is associated with less export, likely due to the impact on the life cycle mismatch between primary production and grazing. This study highlights the sensitivity of the SIZ carbon sink to ice variability, emphasizing the urgency for sustained observations and advanced modelling to understand the causes and consequences of recent sea ice retreat.