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

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 Subtropi cal Convergence (Chl a concentrations 0.4-0.6 mug.L-1) and minima in the ar ea north of the Agulhas Front (Chl a concentrations 0.2-0.3 mug.L-1). Gut e vacuation 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. spinifer a ingests mainly cells in the 0.7- to 20-mum range. The grazing dynamics of this important species are compared with those of subantarctic (Euphausia vallentini) and Antarctic krill (Euphausia superba). Individual ingestion r ates, estimated from the autotrophic component of its diet (2.78-4.46 mug C hl a equiv..individual(-1).day(-1)), were just sufficient to account for re spiratory requirements. While E. spinifera is clearly able to ingest large amounts of heterotrophic prey, it is not known to what extent these contrib ute to its total energy budget.