Simulated partial pressure of carbon dioxide (pCO2) in subantarctic surface water. Temperature increases in summer (Dec–Feb) drive pCO2 up, while phytoplankton photosynthesis drives it down.
Increasing temperatures and light levels stimulate a phytoplankton bloom in November/ December which draws down pCO2 but is limited by nutrients. The atmospheric CO2 level used is shown by a dashed horizontal line. When the ocean pCO2 is below (above) this line the ocean acts as a sink (source).