Abstract
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.
Highlights
[1] 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
During the seasonal retreat of sea ice, relatively high chlorophyll concentrations are often observed within the marginal ice zone (MIZ), with blooms extending over thousands of kilometers in area [i.e. Smith and Nelson, 1986; Moore and Abbott, 2000]
[4] Iron control of phytoplankton production and growth rates is another very important dynamic within the MIZ because it lies within the general HNLC region of the Southern Ocean [Martin et al, 1990]
Summary
[2] Within the Southern Ocean (SO) lies a region where ecological dynamics vary from the usual High Nutrient, Low Chlorophyll (HNLC) regime. Phytoplankton blooms are most commonly found in coastal/shelf regions, in the Ross Sea, and in the southwest Pacific sector of the Southern Ocean, as well as the southwest Atlantic sector [Sullivan et al, 1993; Arrigo and McClain, 1994; Comiso et al, 1993; Arrigo et al, 1998a, 1998b; Moore and Abbott, 2000, 2002; Meguro et al, 2004; Arrigo and van Dijken, 2004] Outside these areas, generally low chlorophyll concentrations have been observed in open-ocean waters. Output from the biogeochemical elemental cycling (BEC) model [Moore et al, 2004] is used to aid our interpretation of the satellite data
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