Ontogenetic diet shift in Antarctic krill (Euphausia superba) in the Prydz Bay: a stable isotope analysis

As one of the most common and dominant species in the Southern Ocean, Antarctic krill (Euphausia superba) play a significant role in food web structure and the process of energy flow. The diet of Antarctic krill in the Prydz Bay during austral summer of 2012/2013 was investigated and the ontogenetic shift in krill diet was evaluated using the stable isotope method. The nitrogen stable isotope values (delta N-15) of adults ((2.78 +/- 0.58)aEuro degrees) were much higher than those of juveniles ((1.69 +/- 0.70)aEuro degrees), whereas the carbon stable isotope values (delta C-13) of adults (-(28.26 +/- 1.08)aEuro degrees) were slightly lower than those of juveniles (-(27.48 +/- 1.35)aEuro degrees). Particulate organic matter (POM) from 0, 25, and 50 m depth combined (0/25/50 m) represented phytoplankton food items. The results showed that phytoplankton food items in surface water and mesozooplankton were two essential food items for Antarctic krill in the Prydz Bay during summer. POM (0/25/50 m) contributes 56%-69% and 26%-34% to the diet of juvenile and adult krill, respectively, whereas mesozooplankton composes 13%-34% and 58%-71% of the diet of juvenile and adult krill, respectively. Thus, an ontogenetic diet shift from POM (0/25/50 m), which consists mainly of phytoplankton, to a higher trophic level diet containing mesozooplankton, was detected. The capacity for adults to consume more zooplankton food items may minimize their food competition with juveniles, which rely mostly on phytoplankton food items. This suggests "diet shift with ontogeny" which may somehow help krill keep their dietary energy budget balanced and well adapted to the Antarctic marine ecosystem as a dominant species.