A tractable approach to understand the effects of climate change on Ecosystems is the focus on "keystone" species whose biology is so pervasive as to dictate ecosystem functioning. A critical aspect in the understanding of the effect of climate change on pelagic systems is the need to progress beyond correlative studies towards a mechanistic understanding. How exactly do changes-oscillations-trends in the environment affect sensitive life cycle stages and thus dictate population size of key species, which in turn have a large impact on the environment? The Polar Regions represent one of the most rapidly warming areas on the globe. Therefore, a model polar organism to study and to understand the effect of climate change on the Southern Ocean ecosystem is Antarctic krill (Euphausia superba). This organism plays a key role in the food web of the Southern Ocean both as grazer of a variety of pelagic invertebrates and algae and as prey for a wide range of Antarctic fish, birds and mammals. The progressing climate change might alter krill's habitat in different ways (e.g. reduced sea ice coverage, changes in food quality and quantity, and a rise in sea water temperature). Long-term studies have already demonstrated that the krill population in the SW Atlantic decreased by about 80 % over the last 30 years and that this decrease is associated with a decline in winter sea ice coverage. It is suggested that the recruitment success of krill is related to seasonal sea ice dynamic but the knowledge of the mechanisms, which link krill population size to environmental variability is lacking. Up to now, the adaptability of krill to their extreme changing environment in short terms due to seasonality (daylight duration, food availability, sea-ice cover) and in long terms due to climate change (increasing sea-water temperature, decline in sea-ice cover, food quantity and quality) is unknown in many cases. However, this knowledge is essential to understand the impact of the progressing global warming trend on the population size of krill and consequently on the Antarctic marine ecosystem.