Diel Feeding, Laminarinase Activity, and Phytoplankton Consumption by Euphausiids

AbstractThree corroborative techniques—stomach fullness, gut fluorescence, and digestive enzyme activity (laminarinase)—were used to examine the feeding ecology of two common euphausiid species off the California coast. Euphausia pacifica showed a diel feeding periodicity: stomach fullness, gut pigment levels, and laminarinase activity increased at night when this species entered the surface waters (< 80 m). In situ phytoplankton consumption by E. pacifica was quite variable and was positively correlated with surface chlorophyll a concentration at the time of collection. E. pacifica collected during March 1981 in Monterey Bay had the highest ingestion rates, and those collected in the Santa Barbara Channel during October 1980 had the lowest. Diel changes in digestive enzyme activity of E. pacifica were influenced by phytoplankton ingestion during a single feeding period. However, overall digestive enzyme levels (average day or night enzyme activity) were probably influenced by longer-term feeding conditio...