DIEL, SEASONAL AND ONTOGENIC VARIATIONS IN THE VERTICAL DISTRIBUTIONSOF 4 MARINE COPEPODS

Diel and seasonal variations in the vertical distributions of Calanus finmarchicus, Metridia longa, M. lucens and Chiridius armatus in Malan gen, northern Norway, were determined from February until December 199 2. The vertical behaviour differed among the species and among the dif ferent stages of a single species. The vertical distribution of C. fin marchicus was dominated by seasonal rather than diel vertical migratio n, and this species was found in surface waters during the spring phyt oplankton bloom (March to May), and at the bottom of the fjord in the fall. and winter. Metridia spp. were generally consistent diel vertica l migrators. However, M. longa usually stayed deeper and had a larger migration amplitude than M. lucens. M. longa was more deeply distribut ed in the winter than during the summer, while M. lucens showed no suc h seasonal trend. The vertical behaviour of C. armatus was related to the seasonal change in day length, and diel vertical migration was str ong in periods of large day/night contrast (spring and autumn), but ce ased during the periods of midnight sun and winter darkness. During th e summer C. armatus stayed deep both day and night. Diel vertical beha viour 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 obser ved in C. armatus. While seasonal variations in vertical behaviour are related to variations in food and light conditions, inter- and intras pecific differences may be due to life history, diet and susceptibilit y to predation.