Vertical distribution and seasonal dynamics of mesozooplankton in the Iceland Basin

The abundance and seasonal vertical distribution of dominant zooplankters in the Iceland Basin was studied from data collected on four cruises between November 1996 and June 1997. Copepods were by far the most abundant group, making up ∼70% or more of the mesozooplankton. Two species constituted >90% of the copepod biomass: Calanus finmarchicus and Pareuchaeta norvegica. A seasonal migration pattern was evident in C. finmarchicus, P. norvegica, Oithona spp., and Metridia lucens: from December to February they inhabited the deeper layers, whereas from April to June they were most abundant in the upper layers. The vertical distribution of Pleuromamma robusta was rather similar year round. Oncaea spp. stayed deep during winter, but only a limited part of the population rose to upper waters during the summer. The reproduction of C. finmarchicus took place in May in the surface layers and it was linked to the phytoplankton spring bloom. In contrast, the reproduction of P. norvegica occurred at depth in February and was uncoupled to the spring bloom. Calanus hyperboreus is an expatriate in the Iceland Basin that does not reproduce there. The data on Oithona spp. and Oncaea spp. indicate that the former group reproduces between April and June in the upper layers, whereas the latter appears to reproduce year round at depth. As judged by the incidence of adults of both sexes the reproduction of M. lucens took place in spring and summer. The breeding of P. robusta was probably not confined to a particular season of the year.