Under-ice feeding and diel migration by the planktonic copepods calanus glacialis and Pseudocalanus minutus in relation to the ice algal production cycle in southeastern Hudson bay, Canada

The marine planktonic copepods Calanus glacialis and Pseudocalanus minutus typically dominate the copepod biomass in spring under the ice in southeastern Hudson Bay, Canada. Females of both species exhibited significant diel feeding cycles, as measured by gut pigment content, throughout a bloom of ice algae at the ice-water interface in 1986. Periods of grazing correlated well with a nighttime vertical migration by females to within 0.2 m of the ice-water interface, suggesting that feeding took place at or just below the thermohaline boundary between seawater and the interfacial layer containing the ice algae. Seasonal melting of the ice bottom in mid-May resulted in freshening of the surface layer and release of the ice algae into the water column. Female C. glacialis and P. minutus responded by ceasing migration to the interface.