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

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  • Ingestion and respiration by Metridia pacifica, the dominant large copepod during autumn and winter in the subarctic Pacific, were investigated by shipboard and laboratory experiments. Diel variation 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 C₅ copepodites exhibited marked diel 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⁻¹ d⁻¹ for adult females and C₅ copepodites, respectively. Estimated filtering rates were 76 ml female⁻¹ d⁻¹ and 15 ml C₅⁻¹ d⁻¹. Feeding rates at low food concentrations in incubation bottles were similar to estimates obtained from in situ studies. Adult females consumed approximately 7.5 % of body carbon d⁻¹, and C₅ copepodites only 2.5 % d⁻¹. Respiration was 4 to 10 % of body carbon d⁻¹ for both C₅ and adults, indicating approximate energy balance for females but higher daily energy expenditure 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 subarctic Pacific. During peak phytoplankton production in summer, the low densities of M. pacifica require less than 10 % of daily primary production to satisfy metabolic, growth and reproduction requirements. In contrast, during autumn and winter, the M. pacifica population requires 36 to 57 % and exceptionally 175 % of daily primary production to satisfy energy requirements. M. pacifica contributes significantly to the total grazing potential responsible for maintaining low stocks of phytoplankton during the unproductive, fall-winter season in the eastern subarctic Pacific.
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  • Harold, B. P. (1986). Phytoplankton balance in the oceanic subarctic Pacific: Grazing impact of Metridia pacifica [Electronic version]. Marine Ecology Progress Series, 34, 213-225.
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  • 34
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