<p>Iron is the primary limiting nutrient for phytoplankton growth, and consequently CO₂ drawdown, in the Southern Ocean. A recurring phytoplankton bloom above the Australian Antarctic Ridge was recently attributed to hydrothermally sourced iron. Here we examine satellite remote-sensing estimates of net primary production, earthquake location catalogues and Lagrangian plume modelling of particle trajectories in surface ocean currents to show that interannual variability in net primary production is related to seismicity and the advective spread of downstream surface waters. By spatially decomposing the relationship between seismicity, advective spread and net primary production, we demonstrate that net primary production at the surface, above the hydrothermal vents, can be predicted by elevated seismicity in the months before the growing season. Farther from the vents, greater advective spread reduces net primary production. We hypothesize that the connection between earthquakes and net primary production is mediated by the link between seismicity and hydrothermal emissions while advective spread controls the dilution of entrained iron; however, the physical mechanism behind the rapid surfacing of hydrothermal iron is still unknown. These findings challenge prevailing views on how geophysical processes influence ocean primary production.</p>