How trophic dynamics of adult Antarctic krill Euphausia superba responses to the condition of no ice in the water during the winter: a case study at South Georgia?

Understanding the variation in krill diet during winter is crucial to elucidate the overwintering of krill in the Southern Ocean. The information on trophic variation of krill under ice-free waters during the winter can also provide the insight for understanding the response of krill to global warming. Stable isotope analyses (¹⁵N/¹⁴N and ¹³C/¹²C) were utilized to explore the variation in diet of adult krill with size and month during the winter season (June to September 2016) at the South Georgia. Stable isotope signatures indicated that there were no significant differences in δ¹³C and δ¹⁵N values between sexes of krill. Nitrogen isotope signatures suggested increasing carnivory with size of adults throughout winter. Carbon isotope signatures revealed reduced food sources for the larger adults during early winter, but all adults could feed on similar food items during middle to late winter. The trophic niche of adults was similar during the winter season at the South Georgia. Our results provide insight for the diet of adult krill in the food-limited season and should be useful in understanding the energy transfer in the marine food web in the South Georgia ecosystem. Moreover, the information derived from this study can also be used to support krill fishery management and understand the interaction between krill population, top predators and fishery.