Saline streams occur frequently in arid and semiarid areas with an important presence of evaporitic rocks, being common in some places of the Mediterranean basin. While freshwater Mediterranean streams have been largely studied and even classified into ecotypes, saline streams are often forgotten and their major ecological features are still unknown.

Therefore, we have carried out a comparison between freshwater and saline streams to assess how they differ with regard to their macroinvertebrate community structure. Data were obtained from GUADALMED project database and samples collected from unpolluted streams placed in the Iberian Peninsula, Morocco and Sicily where both freshwater and saline streams are widespread.

Basin features (geomorphology, climate and lithology) and conductivity values were recorded in each stream in order to single out the variables which may explain the observed community structure. Results showed that saline stream macroinvertebrate communities are significantly different from freshwater ecotypes (temporary streams, evaporitic-calcareous streams, calcareous headwaters and siliceous headwaters). Non-metric multidimensional ordination plot revealed that Mediterranean streams are disposed along a gradient mainly governed by conductivity. The family richness of Mediterranean streams decreased with osmotic stress and flow permanence.

Maximum values of taxonomical richness were found in calcareous headwaters while hypersaline streams displayed lower values. The dominant orders and families in saline streams are Diptera (Ceratopogonidae, Chironomidae, Ephydridae, Stratiomyidae), Coleoptera (Dytiscidae, Hydrophilidae, Hydraenidae) and Hemiptera (Corixidae), while Trichoptera (only Hydropsychidae, Limnephilidae families) and Ephemeroptera (Baetidae, Caenidae) orders are rare and Plecoptera being absent. Using the macroinvertebrate information, saline streams were classified into 3 ecotypes (hyposaline, mesosaline and hypersaline) that were related to the conductivity gradient.

Lithology (percentage of evaporitic outcrops in the stream basin) accounted for 45% of the conductivity variation. In conclusion, this study reveals that saline streams are peculiar ecosystems that contribute noticeably to Mediterranean biodiversity and, therefore, their inclusion in the protection and biomonitoring networks is a conservation priority.