An atmosphere-sea ice-ocean interaction study was conducted at the Saroma-ko Lagoon, Hokkaido, Japan from 27 February to 9 March 2021. Air-sea ice CO 2 flux measurements were conducted by eddy covariance and chamber methods together with meteorological observations. Sea ice cores were collected to understand the interaction between the atmosphere and under-ice water with respect to fluxes of heat and biogeochemical components. Physical and biogeochemical parameters such as temperature, salinity, ice structure, environmental DNA, and concentrations of gases, nutrients, and trace metals were measured. Under-ice water properties were monitored to quantify heat budgets and interactions with sea ice biogeochemical properties. Incubation experiments with ice algae were conducted. Equipment such as airsea ice CO 2 /CH 4 flux chambers, an eddy covariance system, a trace metal analyzer, and a pump and sampler for environmental DNA were tested and compared to prepare for future Arctic and Antarctic expeditions. In addition, movies and pictures were taken under the auspices of the educational program of Hokkaido University (LASBOS) to enhance educational programs in ocean science. The field campaign activities provided useful information for inter-comparison research and future studies of sea ice in polar oceans.

We elucidated the effects of snow and remineralization processes on nutrient distributions in multi‐year landfast sea ice (fast ice) in Lützow‐Holm Bay, East Antarctica. Based on sea‐ice salinity, oxygen isotopic ratios, and thin section analyses, we found that the multi‐year fast ice grew upward due to the year‐by‐year accumulation of snow. Compared to ice of seawater origin, nutrient concentrations in shallow fast ice were low due to replacement by clean and fresh snow. In deeper ice of seawater origin (the lower half of the multi‐year fast ice column), remineralization was dominated by the degradation of organic matter. By comparison between first‐ and muti‐year ice, the biological uptake and the remineralization were dominated in relatively young ice and older ice, respectively, under the physical process of brine drainage.

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Antarctic multi-year landfast ice grew upward due to the year-by-year accumulation of snow • Nutrient concentrations decreased in the upper sea ice due to the replacement by clean snow • In deeper sea ice, remineralization by degradation of organic matter drove nutrient concentrations