Phytoplankton balance in the oceanic subarctic Pacific: grazing impact of Metridia pacifica

Abstract

Ingeshon and respiration by Metridia pacifica, the dominant large copepod during autumn and winter in the subarctic Pacific, were investigated by shipboard and laboratory experiments. Die1 vanation in the rate of grazing on phytoplankton by M. pacifica was determined from measurements of gut pigment content and gastric evacuation rate. Both adult females and CS copepodites exhibited marked die1 variation in gut contents, and thus feeding intensity. Night gut pigment values were 10 times greater than daytime values. Ingestion rates during May 1984 were 51.7 and 9.8 ng chl a copepod-' d-' for adult females and C , copepodltes, respectively. Estimated filtering rates were 76 m1 female-' 6-' and 15 m1 C,-' d-l. Feedlng rates at low food concentrations in incubation bottles were similar to eshmates obtained from in situ studies. Adult females consumed approximately 7.5 % of body carbon d-', and C , copepodltes only 2.5 'Al d.'. Respiration was 4 to 10 '10 of body carbon d" for both C, and adults, lndicatlng approximate energy balance for females but h g h e r daily energy expenditwe than gain for C5 Based on the respiration measurements, a model was developed to evaluate the seasonal grazing impact of M. pacifica on the phytoplankton standing crop in the subarchc Pacific. Dunng peak phytoplankton production in summer, the low densities of M. paciflca requlre less than 10 % of dally primary production to satisfy metabolic, growth and reproduction requirements. In contrast, dunng autumn and winter, the A4. pacifica population requires 36 to 57 % and exceptionally 175 % of dally primary production to satlsfy its energy requirements. M. pacifica contributes sigrnlflcantlv to the total qrazinq potential responsible for maintaining low stocks of phytoplankton dunng the unproducbve, fall-winter season in the eastern subarctlc Pacific.