Abstract
Lagoon Notoro-ko is a large brackish lake on the Okhotsk Sea coast of Hokkaido. It is completely covered with sea ice during winter and hydrographic and biological conditions during this season are unknown. Previous investigations done in warm seasons indicate that the lagoon water is almost the same as the Okhotsk Sea water because there are few inflows from the land. We conducted a winter investigation into the temporal changes of microalgal chlorophyll a in the sea ice and water column beneath it during the period from February 6 to March 18, 2008. Of the surface PAR, 1% penetrated to 6–10 m depth in the water column beneath the ice, where
0.4–6.0 mg/m3 chlorophyll a was detected. Integrated chlorophyll a in 18 m ranged
8.7–119.1 mg/m2. In contrast, chlorophyll a levels in 20–31 cm thick ice ranged 2.5–11.6 mg/m2, relatively less than in the water column. However, because chlorophyll a was largely concentrated in the bottom-most part of the ice, it might be available to grazers in the underlying water. During
ice melting, a large proportion of the populations released from the ice likely sunk to the sea bottom rather than remaining in the water column.
