From white to green to blue? Trajectories of phytoplankton communities in a warming Southern Ocean

The Southern Ocean is experiencing warming, melting of ice, and increasing wind. Predictions of increasing phytoplankton biomass associated with warming and declining glacial and sea ice are supported by observations. As wind intensity continues to increase and glacial and sea ice decline, however, the long-term projections indicate that phytoplankton biomass will decline. The transition will vary geographically around the Southern Ocean depending on the base state of the system and rate of climate change. The initial increase in phytoplankton biomass will be driven by diatoms and Phaeocystis. The stratification-driven increases in phytoplankton biomass will plateau, accompanied by shifts in community composition with the increased prominence of cryptophytes and decreased phytoplankton size distributions. Over the next several decades, climate projections indicate continued declines in ice coupled with increasing winds, with the resulting deeper mixed layers leading to declines in diatoms and cryptophytes and the dominance of mixed flagellates. Over the long term (decades to a century), winds will continually deepen the upper-ocean mixed-layer depth, the importance of picoplankton will increase, and biomass will decline. These changes will profoundly impact Southern Ocean ecology and biogeochemistry.