Diel, seasonal and ontogenetic variations in the vertical distributions of four marine copepods

Diel and seasonal variations in the vertical distributions of Calanus finmarchicus, Metridia longa, M. lucens and Chiridius armatus in Malangen, northern Norway, were determined from February until December 1992. The vertical behaviour differed among the species and among the different stages of a single species. The vertical distribution of C. finmarchicus was dominated by seasonal rather than diel vertical migration, and this species was found in surface waters during the spring phytoplankton bloom (March to May), and at the bottom of the fjord in the fall. and winter. Metridia spp. were generally consistent diel vertical migrators. However, M. longa usually stayed deeper and had a larger migration amplitude than M. lucens. M. longa was more deeply distributed in the winter than during the summer, while M. lucens showed no such seasonal trend. The vertical behaviour of C. armatus was related to the seasonal change in day length, and diel vertical migration was strong in periods of large day/night contrast (spring and autumn), but ceased during the periods of midnight sun and winter darkness. During the summer C. armatus stayed deep both day and night. Diel vertical behaviour in young stages of all 4 species was weak or undetectable by the sampling method used. Younger stages of C. finmarchicus and Metridia spp. remained higher in the water column, while the opposite was observed in C. armatus. While seasonal variations in vertical behaviour are related to variations in food and light conditions, inter- and intraspecific differences may be due to life history, diet and susceptibility to predation.