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Climate change is affecting freshwater ecosystems worldwide. Warming temperatures have had profound impacts on the distribution of organisms, the timing of biological events, and the overall biodiversity of freshwater ecosystems. Much of the recent research investigating the effects of climate warming on freshwater ecosystems has focused on individual fitness and population dynamics. Much less focus has been on higher levels of organization, such as whole communities and food webs. The primary goal of this dissertation was to determine how warming affects community assemblages and the flow of energy through natural food webs. To address this, I used a whole-stream warming experiment in which I experimentally warmed an entire stream reach in Iceland by approximately 3.5 C using a geothermal heat exchanger, and examined the response of invertebrate communities in the warmed stream and a …