Lipid biochemistry and physiology of Antarctic krill (Euphausia superba) in the present day and under future ocean acidification scenarios

Antarctic krill (Euphausia superba, hereafter ‘krill’) are lipid-rich euphausiids with an important role in the Southern Ocean, including as the primary prey of Antarctic megafauna (whales, seals, penguins), fish, squid and seabirds. They contain high levels of nutritious long-chain (≥C\(_{20}\)) polyunsaturated fatty acids (LC-PUFA), specifically eicosapentaenoic acid (20:5n-3) and docosahexaenoic acid (22:6n-3). The sheer abundance of krill in the Southern Ocean means that the ecosystem is largely driven by energy derived from krill lipids. In addition to their ecological importance, a Scotia Sea krill fishery harvests krill, including for commercial use of their LC-PUFA. The existence of this year-round krill fishery provides a unique opportunity to collect krill samples for research over large spatial and temporal scales, which is unfeasible using scientific research vessels. In this thesis, fishery caught krill samples were used to investigate the fatty acid content and composition of krill, during all seasons and over consecutive years (2013 – 2016). This research (presented in Chapter 2) aimed to fill knowledge gaps on the seasonal diet of krill (particularly in winter) in the Scotia Sea region, using fatty acids as dietary biomarkers. Krill were primarily herbivorous in summer (higher levels of 20:5n-3 and 22:6n-3, and low 18:1n-9c/18:1n-7c ratios) and became more omnivorous from autumn to spring (increasing ratios of 18:1n-9c/18:1n-7c and percentages of Σ 20:1 + 22:1 isomers). Seasonal proportions of herbivory and omnivory differed between years, and fatty acid composition differed between fishing locations. Selected samples were also used to investigate the composition of fatty acids in the structural (phospholipids) and storage lipids (triacylglycerols) of krill (Chapter 3). Triacylglycerol fatty acids (thought to better represent recent diet), reflected omnivorous feeding with highest percentages of flagellate biomarkers (18:4n-3) occurring in summer, diatom biomarkers (16:1n-7c) from autumn-spring, and greater carnivory (higher Σ 20:1 + 22:1 and 18:1n-9c/18:1n-7c ratios) in autumn. Phospholipid fatty acids were less variable and were higher in the essential membrane fatty acids 20:5n-3 and 22:6n-3. Percentages of the major krill sterol, cholesterol, were significantly higher in winter and spring compared with summer and autumn. Results presented in Chapters 2 and 3 highlighted the dynamic nature of krill lipids, and the flexible diet of krill, which likely contributes to their huge biomass and success as one of the most abundant organisms on Earth. Because krill are so important in the Southern Ocean food web, any decreases in krill biomass could result in a major ecological regime shift. Very little is known about how climate change will affect krill. Increasing anthropogenic carbon dioxide (CO\(_2\)) emissions are causing ocean acidification, as absorption of atmospheric CO\(_2\) in seawater alters ocean chemistry. Ocean acidification increases mortality and negatively affects physiological functioning in some marine invertebrates, and is predicted to occur most rapidly at high latitudes. Long-term laboratory studies are needed to understand how keystone species such as krill may respond to predicted future pCO\(_2\) levels. A long term experiment was conducted to test whether rising ocean pCO\(_2\) is likely to impact krill physiology and biochemistry (Chapters 4 and 5). Adult krill were exposed to near-future ocean acidification (1000 – 2000 μatm pCO\(_2\)) for one year in the laboratory. Krill reared in near-future pCO\(_2\) conditions were able to survive, grow, store fat, mature, and maintain normal respiration rates. Haemolymph pH, lipid and fatty acid composition were also maintained at the same levels as krill in ambient pCO\(_2\) (400 μatm). Negative effects on physiology and lipid biochemistry were only observed in extreme pCO\(_2\) conditions (4000 μatm), which krill will not experience in the wild. These results place adult krill among the most resilient species in ocean acidification studies to date. In summary, results in this thesis highlight the remarkable adaptability of krill in a changing environment, from short-term seasonal or annual scales, to longer-term decadal scales. Their flexible phenotype may aid their survival in an ocean that is rapidly changing with increasing anthropogenic CO\(_2\) emissions. The data obtained in this thesis can be used for fisheries management to guide fishing activities, and in fisheries models to predict how krill biomass may be affected by climate change. Krill lipid energy fuels the Southern Ocean ecosystem and to date, lipid data has not been included in Antarctic ecosystem models. The large scale of lipid data in this study makes it ideal for inclusion in such models, and it has important implications for the health of the wider Southern Ocean ecosystem