Seasonal vertical migrations of Calanus spp. in Oslofjorden

A mixed assemblage of Calanus helgolandicus and the less abundant C. finmarchicus was studied during 1995-1997. In the course of winter, Calanus became increasingly concentrated towards the bottom (120 m). Krill, which were preying on Calanus, were common among the overwintering copepods. However, the decrease rate of the Calanus assemblage was estimated to only ∼ 0.1-0.3 % per day, which was considerably lower than expected on basis of the krill abundance and their predation potential on Calanus. Therefore, we suggest that slowly sinking Calanus were continuously funnelled into the sampling location in the deeper part of the fjord during winter. A subsequent basin-water renewal appeared to wash a significant part of the overwintering Calanus away from the sampling station. No coupling between timing of seasonal vertical migrations of the Calanus assemblage and surface levels of chl.a was apparent. Most Calanus descended in September, during a period of surface chl.a increase. The spring diatom bloom did not trigger an immediate synchronous ascent of the Calanus assemblage, which took place on a time scale of months.