Ingestion, gut passage, and egestion by the copepod Neocalanus plumchrus in the laboratory and in the subarctic Pacific Ocean 1 2

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 µ g Chl liter −1 of Thalassiosira weissflogii and constant at higher concentrations. Gut passage time was constant at food concentrations >4.0 µ g Chl liter −1 , but passage slowed dramatically at lower concentrations. Levels of gut pigments in N. plumchrus collected from the central subarctic Pacific Ocean during June 1983 and May 1984 were very low. There was no diel periodicity in gut contents, and no vertical spatial patterns in gut fullness were observed in copepods collected from the upper 80 m. The gut pigment levels were converted to in situ ingestion rates using a gut passage time derived from laboratory and field experiments. Resultant ingestion rates, although extremely low, were approximately what laboratory experiments predicted for the low food concentrations available in this oceanic region. Budgetary considerations suggest that N. plumchrus in the subarctic Pacific Ocean must be supplementing its ingestion of phytoplankton with microzooplankton or that metabolic expenditures must be less than reported for the same species in the Bering Sea.