494 / 2024-09-18 08:13:29

Changes in carbon cycling in the Weddell Sea seasonal ice zone revealed by biogeochemical-Argo floats

BGC-Argo,Antarctic sea ice,Carbon fluxes,organic carbon export

In recent years, the Southern Ocean seasonal sea ice zone (SIZ) has experienced extremely low sea ice cover, but how these changes affect ocean carbon cycling is still unclear. Assessing sea ice dynamics and ocean carbon cycling in the SIZ has become more feasible with the growing observational platform of BGC‐Argo floats. Through multiple years of fleet observations in the Weddell Sea, we relate the timing of ice break to bloom phenology, community production and carbon uptake and export. The climax (maximum change rate) of the phytoplankton bloom strictly follows the sea ice break-out, and its apex (maximum biomass) occurs ~ 2 months later, which indicates the earlier ice break (also longer open water periods) boosts seasonal community production. The early ice retreat is also positively correlated with larger seasonal ocean uptake of CO₂ (FCO₂) and particulate organic carbon export (E_BGP). The time of the apex of FCO₂ was delayed with the later ice break, but the apex of E_BGP was earlier under these same conditions. Grazing losses were quantified by nitrate depletion in the mixed layer, after subtracting conspicuous particulate organic carbon pulses at depth. The peak of the grazing rate usually occurred around March no matter the ice break date, which helps to explain the mismatch of the apex timing of the FCO₂ and E_BGP. That is, the relatively consistent grazing pressure is more likely to limit the carbon export for the blooms in the later ice break cases. These results indicate that in a warmer and more ice-free Weddell Sea, the annual community primary production is enhanced, but the grazing pressure and micronutrient availability might become the key limiting factors.