Data and R code from an exclusion experiment conducted across six streams of different temperature in the Hengill geothermal valley in Iceland. Fences were constructed at three points in each stream and all fish (brown trout) were removed, with no fish added to the upper reach and a set density of fish (7 individuals of similar size) added to the lower reach. The experiment ran for five weeks in August-September 2012. Sampling was conducted before and after the experiment to characterise chlorophyll, decomposition, diatom and invertebrate biomass, and food properties. A bioenergetic model was also used to determine the key mechanisms driving the patterns. This dataset provides all the information from the experiment underpinning the O'Gorman et al. Naature Ecology & Evolution paper "Warming indirectly simplifies food webs through effects on apex predators".