Euphausiids are distributed ubiquitously across the globe and many krill species are represented in extraordinarily high numbers in various ecosystems. Their biomass, physiological plasticity and importance as a trophic link between primary production and higher trophic levels enable these animals to fill key positions in marine ecosystems. Knowledge about their distribution, abundance, life-history traits, growth patterns, physiological and behavioural adaptations to short- and long-term changes in their environment will help to understand both spatial and temporal variability found in krill populations. Changes in krill distribution and abundance may significantly impact the food web and ecosystem structure and functioning. One ecosystem, where euphausiids play a pivotal role and can dominate the zooplankton biomass, is the Northern Benguela Current (NBC) also called the northern Benguela upwelling system (nBus). The NBC is a nutritionally poly-pulsed, highly productive and stratified coastal upwelling system. High fishery yields and large populations of fish, birds and seals point to the economic and ecological importance of this area. Several physical and biological changes have occurred over the last decades in this Eastern Boundary Upwelling Ecosystem (EBUE). As a component keystone species in the food web of the NBC the dominant krill species may serve as indicators for ongoing changes in the environment since they are known to react quickly to changes in abiotic and biotic conditions. It is therefore crucial to enhance our understanding of the biology and life-history of the dominant krill species in this area and to investigate their physiological constraints and behavioural adaptations to the prevailing conditions. Most notable and important areas for further investigations are adaptations to highly fluctuating abiotic parameters, such as temperature and dissolved oxygen , as well as metabolic and life-history adaptations to short-term upwelling pulses and seasonal differences in terms of food availability. A better understanding of how these parameters impact the variable distribution and biomass of euphausiids in the challenging northern Benguela upwelling system will help to predict possible climate induced changes in ecosystem structure and functioning. To date, ecophysiological adaptations of euphausiids from the NBC have received little attention, despite their important role for this ecosystem. The aim of this thesis is to fill some of these gaps and to elucidate the influence of temperature, oxygen and food availability on the physiology and behaviour of the dominant krill species, principally Euphausia hanseni (Zimmer 1915). Furthermore, a comparison between the physiological adaptations of krill to this nutritionally poly-pulsed system and those to mono-pulsed systems, such as the North Sea, will help to understand the impact of the productive regime (timing and duration of pulses) on physiological adaptations of zooplankton in general. In this regard, euphausiids may serve as useful model organisms with a great capacity for applying general concepts to other similar organisms. This thesis compiles five chapters focusing on (1) thermal constraints, (2) diel vertical migration behaviour, (3) physiological performance and (4) trophic position of krill species in the nBus. Furthermore, a chapter concerning the anaerobic capacity of E. hanseni is included (5). In summary, the nBus euphausiid community is highly diverse with respect to physiological and behavioural adaptations to prevailing and fluctuating environmental conditions. Species-specific adaptations to temperature, oxygen and food availability show that the predominant species have distinct “autecological strategies”, leading to a highly structured euphausiid community in the NBC. Physiological and behavioural adaptations were driven by short-term differences in environmental conditions mediated by upwelling and non-upwelling events rather than seasons. Furthermore, krill species in the NBC are useful indicators of the prevailing environmental conditions, both on short-term and long-term scales. This may allow euphausiids to be used as suitable model organisms for comparison between mono- and poly-pulsed systems.