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

• Published in: Geophysical Research Letters
• Main Authors: Walker O. Smith Jr., Yisen Zhong
• Format: Article in Journal/Newspaper
• Language: English
• Published: Wiley 2024
• Subjects: ice; vertical mixing; phytoplankton; stratification; Geophysics. Cosmic physics; QC801-809; Ross Sea; Southern Ocean; ice covered waters
• Online Access: https://doi.org/10.1029/2023GL106796; https://doaj.org/article/38d10407d4d046519050ea10860f403e

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.