Life cycle of Calanus finmarchicus south of Iceland in relation to hydrography and chlorophyll a

Abstract

The life history of Calanus finmarchicus was studied in relation to hydrography and chlorophyll a dynamics south of Iceland between February 1997 and March 1998. Concentrations of chlorophyll a on the shelf were low through February and March (<0.5 mg m-3), started to increase in early April and reached a peak in mid-May (∼ 5 mg m-3). Another peak was observed in mid-June (∼ 5-7 mg m-3), and a small increase in August (∼ 2.5 mg m-3). During winter, C. finmarchicus was virtually absent from the bank, and the population resided mainly in deep (>400 m) water beyond the shelf. Overwintering animals emerged from diapause in the oceanic area and moulted to adults during February, March, and April, during which time some were advected onto the shelf. The number of C. finmarchicus on the shelf started to increase in April and showed two main peaks during summer, in May/June (∼ 105 000 individuals m-2) and June/July (∼ 95 000 individuals m-2) and a minor one in autumn (September/October. ∼ 10 000 individuals m-2). The peaks reflected three recruitment events which, by back-calculation, may be linked to spawning events in April, June, and August/September. All three estimated peaks of reproduction were in close association with periods of relatively high phytoplankton biomass. As judged by the abundance of young developmental stages (C1-C3), the spawnings in April and June were most important and produced similar numbers of offspring, whereas that in August/September contributed insignificantly to the overwintering stock. The results indicate mixing of generations during summer. © 2000 International Council for the Exploration of the Sea.