| Summary: | PhD Climate change and the accompanying increase in global surface temperatures pose a major threat to freshwater ecosystems, especially at high latitudes where warming is predicted to be particularly rapid. To date many aspects of how rising temperatures can impact fresh waters remain unknown. Information about temperature effects on the level of communities, food webs, ecosystems is especially scarce. The few studies focusing on higher levels of organisation have used either laboratory microcosm experiments, which can lack realism or space-for-time substitution across large ranges of latitude, which can be confounded by bio-geographical effects. This study aimed to overcome these shortcomings by using a “natural experiment” in a set of 16 geothermally heated streams in the Hengill area, South-West Iceland, with water temperatures ranging from 4ºC to 49ºC (mean temperature). Data were analysed for two seasons, August 2008 and April 2009. The principal goal of this study was to assess the effects of temperature on the structure and functioning of food webs. Additionally the persistence of the community structures along the temperature gradient was examined through time (comparison of previously collected data in August 2004 and August 2008). Abundances of cold-stenotherm species decreased whereas those of eurythermal species increased with increasing temperatures leading to knock-on effects on abundances of other species. Species community overlap between streams declined as temperature difference between streams increased. The persistence of species composition through time was weakened at the extremes of the temperature gradient. Food webs showed a clear size structuring in analyses of trivariate food webs, abundance and biomass size spectra. Analysis of connectance, complexity, mean link length, mean 2-span, mean community span and slopes and intercepts of linear regressions fitted to the trivariate foods or size spectra revealed the impact of temperature change on freshwater ecosystems.
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