| Summary: | We studied the effects of warm effluents from the Nesjavellir geothermal power plant on benthic invertebrate communities along a depth gradient of 4 m in lake Þingvallavatn, Iceland. Four study sites were selected: 2 heataffected sites and 2 cold reference sites. Thermal pollution was detected down to 0.4 m at both heat-affected sites, which were 7-12 °C higher than at the same depth at the reference sites. Sustained temperatures ≥27 °C, which is 10-17 °C above ambient water temperatures, seems to be detrimental to the gastropod Radix peregra and several of the littoral chironomid species. Such effects were, however, confined to a small area inside a sheltered rift pool at one of the heat-affected sites. Moderate warming up to ~10 °C above ambient lake temperature in summer, seems to benefit R. peregra, potentially through both a higher survival rate and a shorter life cycle. At the heat-affected sites, sustained high temperatures transformed a relatively diverse chironomid community to a species-poor community, dominated by Paratanytarsus sp. and Cricotopus sylvestris. In contrast, the chironomid Eukiefferiella minor was much more prominent at one of the reference sites. To separate thermal from biological effects, more studies are needed on speciesspecific attributes, including time-series data on growth, survival, and reproduction of the main littoral zoobenthic species. Although our research examined short-term responses of aquatic invertebrates to local thermal stress, our results may also be relevant to long-term responses of freshwater ecosystems at high latitudes to climate change and warming.
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