This thesis provides new insights into the ecological functioning and trophic dynamics of a sub-tropical reservoir during a period of extreme hydrological variability. Australia is one of the driest continents on earth with the majority of its urban population relying on reservoirs for potable water supplies. Despite the importance of reservoirs in Australia, very little is known about the ecological functioning and trophic dynamics of these artificial ecosystems. This thesis investigates the trophic structure, dominant trophic interactions, key drivers of trophic dynamics and the influence of drought on these ecosystem features within a regionally important drinking water reservoir in South East Queensland, Lake Samsonvale ([27° 16' S, 152° 56' E]). Food web conceptual diagrams provide a means for describing resource-consumer relationships within an ecosystem and can highlight the importance of certain interactions between the biotic and abiotic components of the system in the flow of energy. Carbon and nitrogen are two elements that form the currency of these interactions. This study utilised stable isotopes of carbon and nitrogen to describe the food web and investigate the trophic dynamics under contrasting hydrologic conditions within Lake Samsonvale. The application of stable isotopes for describing the Lake Samsonvale food web conformed to a number of important prerequisites to successfully apply this technique. These included sufficient isotopic separation between available energy sources and between food web components such that they occupy unique carbon and nitrogen isotopic space. Secondly, isotope values of food web components were found to change in predictable ways according to the basic principles of isotope fractionation and mixing, including conservative δ13C fractionation, and trophic level δ15N enrichment. Additionally, isotopic description of diets of many Lake Samsonvale consumers conformed to literature based description of diets.