Feeding ecology of the Antarctic krill, Euphausia superba: The role of phytoplankton community composition in the krill's diet

Phytoplankton blooms in the austral spring and summer are important food resources for the Antarctic krill. Major taxa comprising these blooms include diatoms, prymnesiophytes and cryptophytes. The relative contributions of these phytoplankton groups in the krill's diet were examined, with an emphasis on the prymnesiophyte Phaeocystis. Studies of grazing on Phaeocystis have produced conflicting results, and little is known about the trophic link between Phaeocystis and euphausiids.;Several approaches were included in this study. First, clearance and ingestion rates by E. superba on unialgal cultures of Phaeocystis antarctica and Thalassiosira antarctica were compared in the laboratory (Chapter 1). Egestion rates were also measured, and carbon and nitrogen assimilation efficiencies were calculated (Chapter 2). Next, selectivity by krill for particular phytoplankton taxa. was determined from grazing by krill on phytoplankton mixtures, using high performance liquid chromatography (BIPLC) to track concentrations of taxon-specific photopigments (Chapter 3). Finally, the degree of grazing by krill on Phaeocystis was estimated with an immunochemical assay of stomach extracts from field-collected krill (Chapter 4).;Clearance and ingestion rates by krill on small Phaeocystis Antarctica colonies (50--100 mum) were similar to those on T. antarctica, whereas rates on medium P. antarctica colonies (150--500 mum) and single cell P. antarctica were significantly lower than for T. antarctica. Mean carbon and nitrogen assimilation efficiencies for P. antarctica were 85% and 94%, respectively. Diatoms were selected over both prymnesiophytes (i.e. Phaeocystis) and cryptophytes in phytoplankton mixtures, even when Phaeocystis occurred as small colonies. Finally, krill grazed Phaeocystis at 20% of stations sampled, most of them nearshore.;My results suggest that Antarctic krill graze Phaeocystis, but to a lesser extent than they graze diatoms. This result should be considered when measuring and modeling food availability for Antarctic krill.