Trophic dynamics of Antarctic shelf ecosystems - food webs and energy flow budgets

Predicting impacts of environmental change on organisms and on the communities they belong to are of considerable concern to current ecological studies. The Southern Ocean is a useful model system for examining the responses of species and systems to environmental change, because as well as presenting a continuum of ecological complexity, the geographical location and isolation makes monitoring environmental change straightforward.This thesis is an attempt to achieve deeper insights into the functioning of polar communities and ecosystems by analyzing trophic dynamics and energy flow patterns primarily in the high-Antarctic Weddell Sea. In order to achieve this goal, a number of approaches targeting different system aspects as well as system levels had to be developed: (i) Stable isotope signatures and diet information were combined to determine trophic position of and trophic interactions between organisms. (ii) A new multidimensional approach was developed to measure and to compare species trophic niche position and width. (iii) Food web descriptors were estimated to allow comparisons between different communities. (iv) A Mass balanced flow model was used to assess importance of trophic linkages and compartments.The high trophic complexity of the food web of the Weddell Sea observed, results from the high trophic generality of most of the populations as well as their ability for vertical niche expansion. The numerous closely connected species with their trophic flexibility affect overall system properties such as stability and resilience.It remains to be seen if the 'loose' connectivity observed in the Weddell Sea food web leads towards stability, but it is likely that there are different ways of being robust related to different types of perturbations.