Diet of the Antarctic krill ( Euphausia superba Dana) : I. Comparisons of grazing on Phaeocystis antarctica (Karsten) and Thalassiosira antarctica (Comber)

Diatoms and prymnesiophytes are major constituents of spring phytoplankton blooms in the Antarctic. While diatoms are known to be well grazed by krill, the role of prymnesiophytes in the krill's diet is unknown. Clearance and ingestion rates of Euphausia superba grazing on three size classes of Phaeocystis antarctica were compared to rates on the diatom Thalassiosira antarctica in parallel, unialgal experiments. Clearance rates were calculated from disappearance of chlorophyll a during grazing, and converted to carbon and nitrogen ingestion rates based on elemental (CHN) analysis of phytoplankton cultures. Clearance and ingestion rates of krill on small P. antarctica colonies (50–100 μm) were similar to rates on T. antarctica, whereas rates on medium P. antarctica colonies (100–500 μm) and single cell P. antarctica were significantly lower than for T. antarctica. Overall, clearance rates on P. antarctica decreased with increasing colony size. Clearance rates on T. antarctica were variable, and correlated with degree of chain formation. These results suggest that E. superba effectively grazes small colonies of P. antarctica, but not larger colonies or single cells.