- The unique information provided by multiyear data sets can aid in detecting major long-term trends and making predictions with respect to environmental and biological variables. These trends are expected to differ among climatic regions but little information exists concerning long-term variability within a particular climate region.
- We analysed the long-term (14-year time span) patterns in species richness and composition of aquatic insects (EPT: Ephemeroptera, Plecoptera and Trichoptera) in relation to changes in temperature, precipitation and discharge in two reference streams in Catalonia (north-east Spain). Our study sites were located in the Mediterranean macroclimate region but in different mesoclimatic areas, one in the Wet Mediterranean climate area and the other in the Dry Mediterranean climate area (WM and DM, respectively).
- At the macroclimate scale, precipitation decreased and the frequency of drought events increased during the 14-year period. Although the mean number of taxa in each of the two sites remained similar, there was a replacement of species characteristic of riffle-like conditions by species characteristic of pool-like conditions over time in both sites.
- EPT composition differed between sites at the mesoclimate scale, demonstrating the importance of regional factors. In particular, changes in EPT composition were less variable in WM than in DM both in the short term and in the long term, indicating that the WM site was less constrained by climate than the DM site. Such differences between mesoclimates are related to differences in flood magnitude (higher in WM) and drought frequency (more frequent in DM) as well as local hydromorphological differences.
- Our findings indicate that the streams responded similarly to long-term patterns in large-scale variables (i.e. those affecting the overall macroclimatic region), but differed with respect to the effect of local-scale variables (i.e. those differentially affecting each mesoclimatic area). Predictive bio-assessment models that use reference conditions must incorporate long-term variability to accurately assess the ecological status of Mediterranean climate streams.