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

Abstract

Microalgae and bacteria living within Antarctic sea-ice play a fundamental role in polar ocean biogeochemistry and are thought to be responsible for approximately 25 % of the primary production in sea ice covered regions of the Southern Ocean. Yet, a paucity in temporal and spatial measurements in the marginal ice zone (MIZ) during winter means that the understanding of sea-ice algal dynamics are poorly constrained, leaving large uncertainties in parameters driving Southern Ocean food-web variability. Therefore, this work aims to clarify the understanding of relationships between biochemical parameters and overwintering sea-ice algae within the MIZ during two austral winter campaigns in the Southern-Indian and Southern-Atlantic MIZ within 2017 and 2019 respectively. Data was collected for phytoplankton abundance, biomass, species description and nutrients concentrations. Two methods of sea-ice algae extraction were used to analyse and thus highlight any effect sea-ice processing may have on species community composition and abundance. Results revealed the presence of an abundant functional sea-ice algae community residing within sea-ice microhabitats during 2017, as opposed to much lower concentrations in 2019. Disparities between the two winter campaigns were likely owed to the passing cyclonic storms in 2017, facilitating the proliferation of sea-ice algae, to biomass’ values above 8 µg/L in the sea-ice interior. Comparison between methods of sea-ice algae extraction suggest the possible underestimation in dinoflagellate abundance during the 2017 winter campaign owed to direct melting of sea-ice samples. Yet, similar dominant taxa (genera: Fragilariopsis spp., Pseudo-nitzschia spp., Coscinodiscus spp. and Chaetoceros spp.) and size classes (>2.7 µm) were observed in both cruises. Additionally, a significantly higher sea-ice algae biomass (<0.5 µg/L) was observed when compared to surrounding surface water (>4µg/L). This highlights the ecological significance of the austral sympagic MIZ during winter, providing a vital environment for overwintering sea-ice algae communities, independent from pelagic phytoplankton species.