Under-Ice Mixed Layers and the Regulation of Early Spring Phytoplankton Growth in the Southern Ocean

Abstract

Under-ice phytoplankton “blooms” have been observed in the Southern Ocean, although irradiance is extremely low and vertical mixing is assumed to be deep. Most under-ice data have been collected using Argo floats, as research expeditions during austral fall and winter are limited. Hydrographic measurements under dense ice cover indicate that vertical mixing in weakly stratified systems may be less than previously suggested, and that the accepted determinations of mixed layer depths are inappropriate in regions with extremely weak stratification, such as those under ice. Vertical gradients in density suggest that mixed layers in the Ross Sea in early October are not extremely deep; furthermore, while phytoplankton biomass is low, it has begun to accumulate under ice. Growth rates indicate that phytoplankton growth in the Ross Sea begins in early September. Extending the period of growth may have substantial impacts on carbon biogeochemistry and food web energetics in ice-covered waters.