An increase in the proportion of thin first-year ice due to the recent global warming alters the dynamics of arctic ecosystem. In these thin ice, massive blooms of micro algal communities are often found, and may play an important role as primary producers. In February-March 2020, ice algal communities blooming at Saroma-ko lagoon were analyzed. Quantum-yield of photosystem II of the community showed higher activities of photosynthesis and responses to the light conditions in the ice. Cell sizes of the community tended to decrease in the upper layers. Especially the dominant species of Detonula confervacea (Cleve) Gran decreased its cell size in the upper layers of sea ice. The volume of the cells inhabiting upper layer was 44% lower than those inhabiting bottom layers. These characteristic responses of ice algal species suggested the differences from the planktonic algae.