Degree of ontogenetic diet shift and trophic niche partitioning of Euphausia superba and Thysanoessa macrura are influenced by food availability

Euphausia superba and Thysanoessa macrura are dominant krill species in the Southern Ocean and their habitats are often overlapped reportedly. Studies of the feeding strategies of these two krill species will help us better understand the coexistence mechanisms and estimate the roles that krill played in the food web of the Southern Ocean. The trophodynamics of E. superba and T. macrura at different ontogenetic stages (furcilia, juvenile, adult) were studied using fatty acid and stable isotope biomarkers in the samples collected in Amundsen Sea during austral summer of 2017/2018 and 2018/2019. Diatoms like Fragilariopsis spp. was the most abundant phytoplankton species in the summer of 2017/2018, while the abundance of phytoplankton in the summer of 2018/2019 was dominated by Phaeocystis sp. The gradual increase of the carnivorous index 18:1n–9/18:1n–7 with ontogeny of both species in 2018/2019 indicated more carnivorous feeding of adults compared with juveniles and larvae. Meanwhile, greater δ15N values of T. macrura than that of E. superba were more significant in the juvenile and adult stages during the summer of 2018/2019. Our results indicate that the trophic niche differentiation between the two krill species appeared in postlarval stage and can be influenced by food availability. Compared with E. superba, T. macrura was more prone to feed omnivorously or carnivorously responding to food availability.