Climate change is now recognised as one of the major global threats facing natural ecosystems. Freshwaters are likely to be particularly vulnerable, as many are 'islands in a terrestrial sea', making it difficult for species to migrate to more amenable areas as temperatures rise and habitat fragmentation intensifies. Those that are unable to move elsewhere must face altered thermal regimes, with potentially severe consequences for individual metabolism. This, in turn, can induce emergent effects at higher levels of organisation - including altering ecosystem process rates and food web structure. Unfortunately, few empirical data exist on climate change effects on freshwater communities and fewer still have measured ecosystem-level responses. Most current data are inferential, based on large-scale temporal and/or spatial surveys across thermal gradients, whereas cause-and-effect relationships remain largely unknown beyond laboratory microcosm trials, especially because artificially warming natural ecosystems is extremely expensive and logistically challenging. We propose to take advantage of 'natural experiments' in Icelandic and Scottish streams, which are heated above ambient conditions by geothermal and human activity, respectively. The student will therefore be able to characterise both food web architecture (e.g., linkage complexity, food chain length) and key ecosystem process rates (e.g. primary and secondary production, decomposition and nutrient fluxes) to test the predicted effects of warming derived from general ecological theory and, more specifically, Brown et al's (2004) recently proposed 'metabolic theory of ecology'. The student will be given access to extensive datasets and samples held by the CASE Supervisor (Dr. N. Friberg), which have already quantified important characteristics (abundance, standing stocks and biomass growth rates) of individual 'nodes' within the Icelandic food webs. The feeding links between these nodes are, however, still unknown, and the student will characterise these using detailed gut contents analysis of consumers (invertebrates and fishes) from archived and newly-collected samples, in combination with stable isotope analysis. Thus, a core aim of the project is to construct highly-resolved, quantified food webs that can be used to test our hypotheses. Because these streams are species-poor (about 35 species) and the nodes are already well-characterised, this task is relatively straightforward and the student can follow protocols developed by the main supervisor during his work in similarly simplified food webs in acidified freshwaters. We will also track the flux of important elements (carbon, nitrogen, phosphorus) and nutrients (e.g. fatty acids) through the food web, as these are likely to be disrupted in thermally-altered ecosystems. The same techniques used in Iceland will be applied to the Scottish sites in the second year of sampling. In addition to empirical survey work, we will also carry out additional field and laboratory experiments under controlled conditions, including measuring microbial activity in C-T rooms.