Intense Frazil Ice Production Promotes High Algal Biomass in Newly‐Formed Sea Ice

Abstract The coastal polynya formed off Cape Darnley, in the Southern Ocean, is a favorable site for a secondary phytoplankton bloom in the late summer and autumn. In late February of 2018, we conducted in‐situ observations onboard an icebreaker and measured surface water chlorophyll a concentrations reaching 5.5 μg/L. Concurrently, in turbulent conditions associated with wind speeds exceeding 20 m/s, ocher‐colored newly‐formed sea ice, in the form of grease and pancake ice, spread across the polynya. Chlorophyll a concentrations measured in the grease and pancake ice were 47 times (260.0 μg/L) and 11 times (61.8 μg/L) higher than in the surrounding seawater, respectively. The corresponding sea ice algal concentration was sufficiently high to discolor the ice. Moreover, water temperatures were at or below the freezing point at depths shallower than 30 m, suggesting that suspended frazil ice came into contact with phytoplankton, which were particularly abundant in the water column during the bloom, and scavenged them. The diatom Fragilariopsis curta represented more than 83% of both total diatom cell abundance and biovolume in sea ice. Combining known algal growth rates with our results of chlorophyll a concentration in newly‐formed ice, most of the ice algae originated from phytoplankton incorporated and accumulated by frazil ice. The considerable algal concentrations measured in new ice suggest that this accumulation process could contribute to the algal standing stock in Antarctic sea ice. The abundant bloom‐forming phytoplankton incorporated into sea ice suggests that they could be a seed population for subsequent ice‐algal or ice‐edge bloom formation.