The relationships between ingestion rate, gut content, gut passage time, and egestion rate in the copepod Neocalanus plumchrus were determined in laboratory experiments over a wide range of food concentrations. Ingestion, gut content, and egestion were related to food concentration in a rectilinear manner; increases were linear up to about 4.0 pg Chl liter-' of Thalassiosira weissflogii and constant at higher concentrations. Gut passage time was constant at food concentrations >4.0 pg Chl liter-l, but passage slowed dramatically at lower concentrations.

Feeding and diel vertical migration in the copepods Calanus pacijicus Brodsky and Metridia hens 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 Iucens 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. paczjhs 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 l-2 h before sunset.During the night in surface waters, these copepods defecated 2.3-2.9 mg pigment m-*. In comparison, active transport of pigment to deep waters via migratory descent was only 0.01-0.02 mg pigment m-* d-l.-

Three species of copepods, Temora turbinata, Eucalanus pileatus, and Neocalanus plumchrus, were fed cultures of the diatom, Thalassiosira weissflgii, at concentrations ranging from 0.3 to 14.5 fig Chl liter-' to determine whether fecal pellet size was related to food concentration. In all three copepods, pellet size increased with food concentration up to about 3 pg Chl liter-'. At higher concentrations pellet size was constant. Pellets were less compact and appeared to be more fragile at low food concentrations.The level of gut contents in these copepods was also related to food concentration up to about 3 pg Chl liter-l. Below this concentration, ingestion and defecation were balanced in such a way that the gut did not fill, and therefore pellet size was smaller. Food concentrations sufficient to allow a copepod to fill its gut result in the production of fecal pellets of maximum size.It is concluded that fecal pellets produced by large copepods under conditions of low food availability are less likely to sink out of the euphotic zone than pellets produced by the same copepods under conditions of higher food availability.