Grazing dynamics of Euphausia spinifera in the region of the Subtropical Convergence and the Agulhas Front

Abstract

The feeding ecophysiology of the subtropical euphausiid Euphausia spinifera was investigated in the Indian sector of the Southern Ocean during January–February 1999. Gut pigment levels varied from 1.7 to 40.6 ng chlorophyll a (Chl a) equiv.·individual–1 in adults and from 0.3 to 9.3 ng Chl a equiv.·individual–1 in juveniles. Highest levels were observed at the Subtropical Convergence (Chl a concentrations 0.4–0.6 µg·L–1) and minima in the area north of the Agulhas Front (Chl a concentrations 0.2–0.3 µg·L–1). Gut evacuation rates ranged between 0.59 and 0.96·h–1. Gut pigment destruction levels were among the highest ever recorded in euphausiids, with 94.2–98.5% of total pigments ingested converted to nonfluorescing end products. Size-selectivity experiments with natural phytoplankton showed that E. spinifera ingests mainly cells in the 0.7- to 20-µm range. The grazing dynamics of this important species are compared with those of subantarctic (Euphausia vallentini) and Antarctic krill (Euphausia superba). Individual ingestion rates, estimated from the autotrophic component of its diet (2.78–4.46 µg Chl a equiv.·individual–1·day–1), were just sufficient to account for respiratory requirements. While E. spinifera is clearly able to ingest large amounts of heterotrophic prey, it is not known to what extent these contribute to its total energy budget.