Copepod diel migration, feeding, and the vertical flux of pheopigments.

Feeding and diel vertical migration in the copepods Calanus pacificus Brodsky and Metridia lucens Boeck were examined during dusk and dawn in Dabob Bay, Washington. Both species migrated from below 75 m during the day into the upper 25 m at night. Feeding on phytoplankton was confined to periods spent in the surface layer. Metridia lucens arrived in the surface 0.5 h after sunset, fed at high rates throughout the night until 1.0 h before sunrise, and then returned to depth. In contrast, C. pacificus appeared in the surface layer 0.5–1.5 h before sunset but fed at low rates until ~1 h after sunset, then increased its feeding rate. Feeding decreased before sunrise, although migratory descent was not completed until after sunrise. These patterns are not in agreement with models of diel migration that predict the onset of feeding 1–2 h before sunset. During the night in surface waters, these copepods defecated 2.3–2.9 mg pigment m−2. In comparison, active transport of pigment to deep waters via migratory descent was only 0.01–0.02 mg pigment m−2 d−1.