Factors influencing sea-ice algae abundance, community composition, and distribution in the marginal ice zone of the Southern Ocean during winter

Abstract

Microalgae and bacteria living in Antarctic sea-ice play a fundamental role in polar-ocean biogeochemistry, accounting for ∼25% of primary production in sea-ice-covered regions of the Southern Ocean. However, a paucity of measurements in the marginal ice zone (MIZ), particularly during winter, means that sea-ice algal dynamics are poorly understood, resulting in uncertainties in the drivers of Antarctic food-web and carbon-cycle variability. We investigated the relationships among biogeochemical parameters and sea-ice algae in the MIZ of the Indian Southern Ocean in winter 2017 through measurements of sea-ice algae functional groups, chlorophyll, and nutrient concentrations, in the sea-ice and the underlying surface seawater. We observed an abundant, active algal community within the sea-ice, which we attribute to passing cyclones that kept the ice permeable, along with biogeochemical and irradiance conditions that allowed for algal growth. The sea-ice conditions facilitated the proliferation of sea-ice diatoms, to chlorophyll concentrations >5 μg/L (versus 1.5 μg/L in the underlying seawater). The dominant taxa were of the genera Fragilariopsis, Pseudo-nitzschia, Coscinodiscus, and Chaetoceros spp., similar to those observed previously near east Antarctica. While sea-ice algae growth should have caused significant nutrient drawdown, nitrate concentrations were higher in the ice than in the underlying seawater. Since silicate was strongly drawn down, presumably by sympagic diatoms, we attribute the elevated nitrate concentrations to an active microbial loop (i.e., regeneration and nitrification). Our study highlights the ecological significance of the Southern Ocean MIZ in winter, providing a vital environment for overwintering sea-ice algae and bacteria that remain active despite the harsh conditions.