Influence of thermal regime and land use on benthic invertebrate communities inhabiting headwater streams exposed to contrasted shading.

Headwaters account for a high proportion of total freshwater stream-channel length in a drainage basin and are critical habitats for rare, endangered, and specialized species. In the context of climate warming, increasing water temperatures may be an ultimate threat to cold-adapted species even in temperate ecosystems. Climate change effects on streams may interact with other pressures such as pollution or habitat fragmentation, confounding their real impact on biological communities. Three headwater streams exposed to contrasted shading and land use conditions were sampled over a three-year period in spring and autumn (2010-2012). Five stations distributed along the longitudinal continuum were chosen in the upstream part of each stream. In addition to benthic invertebrate sampling, water temperature was recorded continuously using data loggers. Results showed that the riparian woodland associated with forested land use throughout the catchment clearly moderated winter temperature minima, summer temperature maxima and thermal variability compared to open river channels with narrow or absent riparian tree cover. Although, the variability in macroinvertebrate species distribution was mainly attributed to anthropogenic land use in the catchment, a significant part of the variability was explained by temperature descriptors such as the number of cumulative degree-days in summer and extremes in winter temperature. Trichoptera species preferring headwaters and cold water temperatures were found exclusively in the forested unimpacted stream. Conservation issues are discussed in relation to the predicted loss of the potential future distributions of these Trichoptera cold-adapted species.