Krill worldwide: A comparison of hypoxia tolerances of euphausiid species from Atlantic, Pacific and Polar Regions

Modern Oxygen Minimum Zones (OMZs) are extensive and prevail in most of the world oceans. Anthropogenic and climate induced changes will cause OMZs to expand, while coastal hypoxia is assumed to increase in extent and severity. Particularly in Eastern Boundary Upwelling Ecosystems (EBUEs), such as the Humboldt and the Benguela upwelling system, severe OMZs prevail. Animals living in these areas have to physiologically and/or behaviourally adapt to the low oxygen levels or will be excluded from these areas or at least their vertical distribution ranges will be limited. It is assumed that some areas may experience a shift from an abundant and diverse regime to one that is lean and dominated by vertical migrators. Further, temperature is known to impact the hypoxia tolerance of animals negatively. Euphausiids are known as pronounced diel vertical migrators, thus facing different levels of oxygen and temperature within 12 hours and are an important trophic link between primary producers and higher trophic levels throughout the world oceans. The critical oxygen pressure (Pcrit) and the regulation index (RI), as a method to differentiate between oxygen conformity and regulation, were used to assess hypoxia tolerances of different species from different ecosystems. Furthermore, diel vertical migration behaviour was monitored in some species. We will present and compare hypoxia tolerances of different dominant euphausiids species, show how oxygen availability may affect their vertical migration behaviour and assess how future climate scenarios (warming waters and decreasing oxygen content) may alter horizontal and/or vertical distribution of these species.