Southern Ocean BGC-Argo Detect Significant Under Ice Phytoplankton Growth Before Ice Retreat

Mark Hague, ETH Zurich, Zurich, Switzerland and Marcello Vichi, University of Cape Town, Department of Oceanography, Marine and Antarctic Research centre for Innovation and Sustainability (MARIS), Cape Town, South Africa
The annual advance and retreat of sea ice in the Southern Ocean is the largest seasonal event on Earth (in terms areal coverage). Such considerable seasonal changes may have profound effects on the life cycle (phenology) of phytoplankton residing under the ice. However, the exact character of such effects is currently unknown, primarily because studies investigating phenology in these regions have relied on satellite data, which can contain missing data for half the year or more.

Nevertheless, based on the incomplete time series provided by satellites, several hypotheses have been proposed which claim to account for the timing of phytoplankton growth - often referred to as the bloom initiation. One prominent hypothesis holds that following ice retreat in spring, buoyant melt waters enhance irradiance levels, triggering a bloom which follows the ice edge. However, an analysis of BGC-Argo data sampling under Antarctic sea ice suggests that this is not necessarily the case. Rather than precipitating rapid accumulation, we show that melt waters enhance growth in an already highly active phytoplankton population. Indeed, we estimate that substantial growth occurs, on average, 4-5 weeks before ice retreat, typically starting in August and September. Novel techniques using on-board data (as opposed to highly uncertain satellite data) to detect the timing of melt were used.

Furthermore, such growth is shown to occur under conditions of substantial ice cover (>90%) and deep mixed layers (>100m), conditions previously thought to be inimical to growth. This lead to the development of several 0D model experiments in which we sought to investigate the mechanisms responsible for such early growth. The results of theses experiments suggest that a combination of ice permeability (with respect to light) and extreme low light adaptation by phytoplankton account for the observed phenology.