Quantitative analysis of carnivory in the krill Nyctiphanes australis, with an examination of the effect of non-preferred phytoplankton alternative prey

The functional responses of Nyctiphanes australis feeding on the copepod Acartia spp. (hereafter Acartia) at 0.5 to 15 copepods I-' and on the diatom Chaetoceros gracilis at 1.7 to 14.8 pg chl a I-' were measured to provide the first quantitative informahon on carnivorous feeding rates for this krill. Predation rates on Acartia [0.004 to 0.144 Acartla (mg acetone-extracted dry weight, Edw)-l h-'] were comparable to those of more carnivorous knll and were up to 32 times higher than grazing rates on C. gracihs. Ingestion rates on C. gracilis were very low (0.45 to 3.45 ng pigment mg-' Edw h-'), but because of their small size, algae like C. graciljs may be suboptimal food for N. australis.