Stage-specific habitats of Calanus finmarchicus and Calanus helgolandicus in a stratified northern Norwegian fjord

Abstract

During 12 months from April 1997, Calanus spp. copepodites were sampled in the 380-m deep Saltfjord by means of a 200-μm WP-2 net at four depth intervals. Nauplii were sampled with a 100-μm bongo net. Chlorophyll a and phytoplankton were sampled with a hose in the 0–40 m depth range. Environmental variables were recorded by CTD and an in situ fluorimeter. C. helgolandicus occurred in low numbers as adults and CV during most of the year. During the spring bloom in April, it avoided the upper cold 0–50 m depth range, where C. finmarchicus reproduced. In August–September, adults of the two species were equally numerous in the 0–50 m depth range. The adult C. finmarchicus had probably developed from the local spring generation, whereas C. helgolandicus had probably been imported from shelf waters. Nauplii present in August may be attributed to both species, but they did not grow into CI locally. Therefore, C. finmarchicus CV that accumulated in September–October were more likely imported from the shelf, probably by surface advection. Predators possibly reduced numbers up to January 1998, when both species again increased, probably because of deep inflow of shelf water.