Diel patterns in stage-specific vertical migration of Calanus finmarchicus in habitats with midnight sun

Diel vertical migration (DVM) and feeding of Calanus finmarchicus were studied in relation to phytoplankton biomass and predator abundance along a latitudinal gradient in the Norwegian and Greenland Seas, including areas with midnight sun. Young stages (CI-CIII) were generally confined to the upper 30 m both day and night, but weak, significant DVM took place in Atlantic water masses. Older individuals were distributed progressively deeper with increasing stage. Stages CIV and males were non-migratory. Combining all stations, CV and females displayed significant DVM, but migrations were within a narrow depth range. Separating stations according to water mass, significant DVM was only observed within the Arctic domain. Older developmental stages consistently occurred in low numbers in the upper 10 m, both day and night. Contrary to expectations, the amplitude of the DVM tended to be largest in the northwestern part of the study area, representing habitats with midnight sun. Over the Norwegian Shelf and in the east of the Norwegian and Greenland Seas, herring and mackerel were regionally abundant in the upper 50 m, but their abundance decreased strongly towards the northwest. Calanus spp. were the most frequent prey items in their stomachs. The amplitudes of DVM were generally strongest in habitats with very low predator abundance. Neither concentrations of chlorophyll a nor its vertical distribution could account For the observed variations in DVM. Diel feeding rhythms (from gut fluorescence) were found throughout the ocean, except for areas with very low biomass of chlorophyll a. However, feeding patterns varied, and both enhanced and reduced nocturnal gut fluorescence were recorded. (C) 2000 International Council for the Exploration of the Sea.