Seasonal development of Calanus finmarchicus in relation to phytoplankton bloom dynamics in the Norwegian Sea

Seasonal development of Calanus finmarchicus was studied in relation to the physical environment and phytoplankton bloom dynamics in the Norwegian Sea during eight basin-scale surveys from March to August 1995. Our main objective was to gain new knowledge about the life cycle of C. finmarchicus and its adaptation to the physical and biological environment of the Norwegian Sea. Time of spawning, estimated by temperature-dependent back-calculations from the occurrences of copepodite stage 1 (CIs), varied by water mass and occurred mainly during the phytoplankton pre-bloom and bloom periods. Recruitment to CI of the year's first generation (G1) generally occurred during the bloom and late bloom. The seasonal development of C. finmarchicus was progressively delayed from Coastal to Atlantic and to Arctic water, and from south to north within Atlantic and Arctic waters. This delay was partly linked to the phytoplankton bloom development that followed the same pattern, but development of C. finmarchicus also showed an increasing tendency to lag behind the phytoplankton development in colder waters. This may explain why C. finmarchicus are less successful in colder water. The consumption of nitrate was used as proxy for the seasonal history of phytoplankton development to aid interpretation of the lifecycle of C. finmarchicus. This approach allows us to align phytoplankton bloom and copepod development sequences despite temporal and geographical variation in bloom development, which otherwise tend to cause variability in quasi-synoptic and large-scale data. Two generations of C. finmarchicus were found in southern and northern regions of Coastal Water, and in southern Atlantic Water. In northern Atlantic Water and in Arctic Water, one generation was observed.