Spatiotemporal variability of adult Antarctic krill (Euphausia superba) lipids in relation to sea surface temperature and Chlorophyll a

Lipids are key biochemicals that form both cell membranes and energy stores. Lipids are of particular importance in energy poor environments where animals require stores to survive long periods of food shortage. In the Antarctic, food availability is dominated by extreme seasonal shifts in the environment and energy rich food is scarce for a substantial period of time each year. Antarctic krill (Euphausia superba) have adapted to have large lipid stores (over a third of their dry weight) during winter for this reason. Krill are a key species in the Antarctic environment; their biomass links lower and higher trophic levels and forms the main energy conduit for the system. Krill feed on diatoms, dinoflagellates and other algal species year-round, resulting in high omega-3 polyunsaturated fatty acids which are essential for krill health, growth and reproduction. Krill-derived omega-3 containing products (particularly eicosapentacnoic acid (20:5n-3) and docosahexaenoic acid (22:6n-3)) are sold as nutraceuticals for human consumption. Krill oil tablets (sold as an omega-3 supplement) are now the fastest growing nutraceutical globally. Understanding the krill life cycle is hampered by the restricted nature of scientific sampling. Knowledge of krill diet and krill lipid dynamics is lacking for the Indian and Pacific Ocean sectors, as most studies have focused on the South Atlantic Ocean sector where the krill fishery is based. Most scientific research voyages are conducted during summer months and all scientific studies are restricted in their spatial and temporal scale. Information on krill recruitment and reproduction in the Indian and Pacific Ocean sectors is also not as developed as in the South Atlantic, where fishery-derived samples are also available. A major gap in current ecosystem models is the link between environmental drivers (such as upwelling of nutrients, sea surface temperature and height, sea ice extent and thickness and salinity) and their impact on primary production and therefore food availability during extreme seasonal shifts in Antarctica. One way of measuring these environmental drivers is through remote-sensing via satellite, which can gather data over large geographic areas and over long timeframes. Satellite-derived data for biological and ecological measures is still developing as a tool for oceanographers and other end users. However, one area of growing importance is in the use of ocean colour data which can be converted into chlorophyll a concentrations (a proxy for primary production) via a standard algorithm. By linking the GPS locations of commercial krill harvesting, and therefore krill swarms, to environmental data obtained through remote-sensing from the same date, the relationship between the environment krill live in and their biochemical composition can be examined in ways not previously explored. My study used samples collected by a member of the krill fishery, Aker BioMarine, over a continuous three-year period in the South Atlantic Ocean to look at the seasonal and interannual trends in krill total lipids and lipid classes (such as those used for energy storage and the structure and function of cells). This dataset is unprecedented in its seasonal and spatial coverage in the South Atlantic Ocean. This study has been able to establish the sinusoidal shape of the seasonal and interannual trend in krill total lipids and its associated lipid classes. No samples from the fishery were available from other sectors. These South Atlantic Ocean krill samples were contrasted to krill samples collected from scientific expeditions in the other two ocean basins surrounding Antarctica (Pacific and Indian Oceans). Krill diet was investigated at a regional scale during the crucial late-summer spawning period. Results from my study revealed that krill diet varies between ocean basins, with Indian Ocean krill showing a distinctly different diet to Pacific and Atlantic Ocean krill, during the late-summer. The fishery-derived samples were also related back to environmental data collected via satellite, for both chlorophyll a and sea surface temperature, to investigate if environmental drivers influenced krill lipid biochemistry. This study showed that both sea surface temperature and chlorophyll a concentrations (derived from ocean colour data) can be related to krill lipid and fatty acid dynamics. Krill lipid composition and content were shown to be correlated to these environmental factors through simple models. The combination of results from this study will help fill the data gaps in ecosystem models and enable better determination of krill diet, recruitment and reproduction in all ocean basins surrounding Antarctica. These advances in krill knowledge will help improve fishery management policies.