Vertical segregation of Calanus finmarchicus copepodites during the spring bloom

Factors influencing the vertical distribution of the most abundant grazer in the Norwegian Sea were studied in the open ocean. During May 5–14 2003 the upper 60 m of an oceanic area (10,000 km2) in the northern Norwegian Sea were covered with a towed instrument platform collecting simultaneously in situ data on hydrography, chlorophyll a concentration and zooplankton abundance with high resolution in space and time. A consistent vertical distribution pattern of Calanus finmarchicus differing markedly from previously reported data was observed: CII–CIII copepodites were located below moderate to high chlorophyll a concentration, at depths below 30 m. CIV copepodites were located slightly higher in the water column and CV copepodites and adults were consistently found in the upper chlorophyll-rich layer. Partial constrained correspondence analysis revealed that the effect of physical factors and diel vertical migration was clearly secondary to chlorophyll a concentration in explaining the observed vertical segregation. The effects of toxic algae and predation are discussed in evolutionary context. These findings underline the importance of understanding stage-specific behaviour to build realistic models.