Algal enrichment process for newly formed sea ice in the Dalton polynya off East Antarctica during the late summer–early autumn

Newly formed sea ice can incorporate considerable amounts of organic matter including phytoplankton during its freezing and growth. This initial incorporation influences the standing stocks of ice algae, a fundamental component of high-latitude marine ecosystems. Physical incorporation via the accretion of frazil ice, instead of congelation growth, is considered to incorporate particles efficiently. However, for thin ice (<0.20 m thickness), information on the relationship between ice texture and particle incorporation is scarce. Here, we assessed sea-ice texture (proportion of frazil ice-origin layer), macronutrient concentrations, fraction of snow, and algal composition in Antarctic sea ice and their effects on the enrichment of algae into thin ice in terms of chlorophyll a (Chl a) and biogenic silica (BSi). Chl a and BSi concentrations of thin ice ranged over 0.2–11.6 µg L−1 and 0.5–11.1 µmol L−1, respectively, being significantly concentrated compared to seawater. BSi enrichment in thin ice correlated negatively with the proportion of frazil ice-origin layer, while no relationship was observed with Chl a enrichment. These results suggest that frazil ice could play a minor role in entraining algal cells under certain circumstances, possibly due to the low turbulence and wind speeds of the late summer–early autumn season dampening the interaction between frazil ice and phytoplankton. Algal composition data could suggest that ice algae from the previous winter’s sea ice were released and re-incorporated into the surrounding thin ice on its formation, given the high similarity in species composition observed between older ice and several thin ice samples. This mechanism potentially promotes algal enrichment without the accretion of frazil ice, as indicated by the maximum enrichment in thin ice which developed via congelation growth. Our results show that algal concentration in thin ice could be affected by the presence of old sea ice and hydrographic and meteorological conditions.