Vertical migration and feeding behavior of Calanus pacificus females during a phytoplankton bloom in Dabob Bay, US

The abundance, vertical distribution, and feeding of Calanus pacificus females were measured at frequent intervals over 24 h during a March phytoplankton broom in a temperate fjord. Copepod guts contained high levels of phytoplankton pigment only during the nighttime when copepods were in the upper 50 m. Feeding, however, was not strictly a function of phytoplankton availability because copepods fed equally well in the 25-50-m layer where chlorophyll concentrations were 0.5-3 mg m(-3) and there was no phytoplankton production, and in the 0-25-m layer where chlorophyll concentrations reached 5-10 mg m(-3) and all phytoplankton production occurred. In five of six nighttime sampling cycles, >50% of the copepod population resided in the 25-50-m layer. Fifty-eight percent of the total phytoplankton ingestion occurred in the 25-50-m layer and was therefore derived from sinking algae. We conclude that a large fraction of the population of C. pacificus females was not migrating up to the surface layer because there was sufficient phytoplankton in subeuphotic zone waters to support high feeding rates. Although the early morning descent and the daytime depth of dielly migrating C. pacificus may be determined by risk from visually orienting predators, it appears that the nocturnal vertical distribution is modified by food availability.