Global warming effects on benthic macroinvertebrates: a model case study from a small geothermal stream

The aim of this study was to predict global warming effects on benthic macroinvertebrate community structure by using a small temperate geothermal stream as a model system. We collected benthic macroinvertebrates, measured physical and chemical water properties at eight localities up the Kudoski stream steep water temperature gradient, and used 11 metrics and indexes to characterize community structure. Species richness and evenness decreased, but total abundance increased with the increase of average annual water temperature (t av), with species richness being most and total abundance least sensitive to this parameter. The increase of Gastropoda relative abundance and the decrease of Ephemeroptera, Plecoptera and Trichoptera richness, respectively, were the earliest responses of taxonomic groups to t av increase. Relative abundance of Orthocladiinae decreased and that of Chironomini increased with the increase of t av. This indicates that Chironomidae are not reliable predictors of global warming effects in running waters, and that lower taxonomic levels, subfamily or tribe, are more suitable for that purpose. Changes in community structure did not linearly follow t av increase, since a great community shift was observed at t av ≈ 20°C indicating that present trends of community responses to changes in climatic conditions should not be linearly extrapolated to future warming.