Department of Earth System Science

Analysis of satellite ocean color and wind speed data within the seasonal ice zone (SIZ) of the Southern Ocean sheds light on the physical processes that influence phytoplankton biomass distributions. A compilation of monthly averaged chlorophyll and percent sea ice cover data within the SIZ from 1997 to 2005 has been compared with monthly average wind speed data from 1999 to 2005. The size of the marginal ice zone (MIZ, areas of recent ice retreat) was fairly consistent from year to year, always peaking in December, with a mean area of 6.0 million km². The mean area within the MIZ with phytoplankton blooms (chlorophyll exceeding 0.8 mg/m³) was 0.36 million km². While the bloom areal extent seems small compared to the MIZ, in reality, because of gaps in the chlorophyll data, blooming regions comprise a much larger fraction of the MIZ. Considering only areas with valid chlorophyll data, the percentage of the MIZ with blooms was 17%, 21%, and 24% for the months of December, January, and February, respectively. December always has the largest MIZ area, but MIZ mean chlorophyll concentrations sometimes do not peak until February. Wind speed strongly impacts phytoplankton bloom dynamics within the MIZ. There is an inverse relation between wind speed and bloom occurrence, with blooms largely suppressed at high wind speeds. At low wind speeds (∼5 m/s), blooms are observed over about one third of the MIZ. Blooms are also much more frequent near the continent than in offshore waters, likely due to increased iron availability. Open ocean phytoplankton blooms in the Southern Ocean are likely to become iron-light co-limited except in regions where the mixed layer depth is relatively shallow.