The Effect of Autumn and Winter Diet on the Physiology of Juvenile Antarctic Krill

As one of the most abundant animals on Earth, Antarctic krill (Euphausia superba) supports vast numbers of top predators, contributes to biogeochemical cycling, and is the subject of a rapidly growing fishery. Krill population numbers are largely determined by the ability of early life stages to survive the general food scarcity in winter. Warming in certain parts of Antarctica is changing the quality of the polar food web, resulting in a shift in the type of food available to young krill, with as yet unknown consequences to their physiology. We must understand the connections between food quality and krill winter physiology in order to understand the implications of climate change on the Antarctic pelagic ecosystem. In the austral winter of 2019, we ran a 22-week long feeding experiment to determine the effect of diet on the physiology of juvenile Antarctic krill. For the first half of the long-term experiment (May-June, considered autumn), dietary treatments consisted of a diatom culture and a freeze-dried, powdered mixed zooplankton assemblage (MZA). These treatments were split for the second half of the experiment (July-August, considered winter) and krill either remained on their initial diet, or transitioned to a restricted diet of natural seawater assemblage (NSA). Respiration rates were lowest [mean=0.25 µL O₂ (mg DW)^-1 hour^-1] at the start of the long-term feeding experiment, increasing significantly by the end of autumn [mean=0.42 µL O₂ (mg DW)^-1 hour^-1] - though we found no significant difference in respiration rate with diet at this point. By August, respiration rates of krill initially fed diatoms and then NSA were significantly less [mean=0.35 µL O₂ (mg DW)^-1 hour^-1] than those krill consistently fed diatoms [mean=0.47 µL O₂ (mg DW)^-1 hour^-1]. In contrast, the respiration rates of krill initially fed the MZA and then NSA were not different from those consistently fed the MZA. We discuss the implications of our results within the context of krill nutrition, physiology and condition.