Substantial advective iron loss diminishes phytoplankton production in the Antarctic Zone

Abstract

After 1 decade of research it is a well‐established fact that iron limits photosynthetic CO2 fixation and phytoplankton growth in the Southern Ocean; intense blooms are scarce. However, the input of iron to the Southern Ocean is considerable. An important factor for diminished phytoplankton production refers to the meridional circulation of the Southern Ocean. Intense, spatially relatively homogeneous upwelling of Upper Circumpolar Deep Water (UCDW) causes a large iron flux into the surface layer. However, the main entrainment of upwelled UCDW into the surface layer occurs in autumn and winter, which strongly restricts the usefullness of iron supply for phytoplankton due to unfavorable light conditions. Moreover, the meridional transport within the Ekman layer is intense enough to export at least 25% of the iron input away from the Antarctic Zone before it can be used by phytoplankton. This also depresses the potential phytoplankton primary production by at least 25%. Most iron that crosses the Polar Front unused probably leaves the surface ocean north of the Polar Front because the surface water participates in Antarctic Intermediate Water/Mode Water formation.