Phytoplankton and light limitation in the Southern Ocean: Learning from high-nutrient, high-chlorophyll areas

Most of the Southern Ocean is a high-nutrient, low-chlorophyll (HNLC) area. There are exceptions to this situation downstream of some of the islands, where iron from the islands or surrounding shallow plateau fertilizes the mixed layer and causes a phytoplankton bloom in spring and summer. The main locations where this occurs are downstream of the South Georgia, Crozet, and Kerguelen islands. Data on mixed layer depths from Argo float profiles together with Sea-viewing Wide Field-of-view Sensor chlorophyll a (chl a) and photosynthetically available radiation from these high-nutrient, high-chlorophyll (HNHC) areas are combined to study the effects of mixed layer-averaged light availability on phytoplankton concentrations in areas where iron limitation has been lifted. The results of this analysis are then transferred to HNLC areas to assess the potential importance of light limitation through the year. We conclude that light limitation does not significantly constrain the annual integrated standing stock of chl a in the HNLC Southern Ocean.