Diel vertical migration and feeding rhythm of copepods under sea ice at Saroma-ko Lagoon

Diel vertical migration and feeding rhythm of copepods were investigated in Saroma-ko Lagoon, Japan, one of the southernmost areas covered by seasonal sea ice in the Northern Hemisphere. Copepods were collected under sea ice every 4 h for 24 h at five depths (0, 1, 3, 6 and 9 m from the under-surface of the sea ice) to examine their density and ingestion rate. Distinct changes in the vertical distribution and ingestion rate of copepods were observed at dusk, when they migrated upward from the near-bottom layer to the food-abundant sub-ice layer. However, most copepods left the food-abundant sub-ice layer by midnight and reached near bottom again before sunrise. The ingestion rates of copepods increased after sunset throughout the water column as in areas without ice cover. The ingestion rates at the food-poor near-bottom layer were higher than those during the day in the food-abundant sub-ice layer. The estimated grazing rate by zooplankton, predominately copepods, was between 0.056 and 0.08% of the chlorophyll standing stock in the water column per day. This estimate is lower than that observed under Arctic sea ice, due to the lower biomass of copepods under sea ice at Saroma-ko Lagoon.