Zooplankton and phytoplankton abundance, and stomach content of polar cod in the southern Arctic Ocean and Rijpfjorden

The diel vertical migration (DVM) of zooplankton and pelagic fish in the southern Arctic Ocean (82°N) and in Rijpfjorden (Svalbard, Nordaustlandet, 80°N) was investigated in autumn 2004, when there were distinct differences in light conditions between day and night. We collected data on zooplankton and fish distribution using plankton nets, trawls, and an echo sounder together with data on environmental parameters including sea-ice distribution, hydrography, and in situ fluorescence in the water column. In Rijpfjorden, the Arctic phytoplankton bloom had culminated several weeks prior to the present study and all three Calanus species had migrated down to depths for over wintering and did not show any changes in vertical distribution. However, in the Arctic Ocean pack ice at 82°N, Calanus hyperboreus, was found in high abundances (up to 65 ind/m**3) actively feeding on the Arctic bloom in September. They performed a classic DVM, feeding in the upper 50 m during the night and descending to depths below 50 m during the day. In Rijpfjorden, distinct DVM patterns were observed among the amphipod Themisto libellula, adult and juvenile stages of the pteropod Limacina helicina, and polar cod. Polar cod in Rijpfjorden fed on Calanus spp. and on L. helicina. We conclude that animals display DVM in the Arctic during autumn if they need to feed in food-rich surface waters.