Least Disturbed Condition for European Mediterranean rivers

• Mediterranean Least Disturbed Streams (LDS) show various types of hydromorphological alterations.
• Common LDS thresholds were found for all river types in water quality and land use.
• But a lower threshold value for DO (60%) was retained for temporary streams.
• Invertebrate, diatom and macrophyte data were used to settle biological Least Disturbed Condition.

The present report describes a three-step approach that was used to characterize and define thresholds for the Least Disturbed Condition in Mediterranean streams of four different types, regarding organic pollution and nutrients, hydrological and morphological alterations, and land use. For this purpose, a common database composed of national reference sites (929 records) from seven countries, sampled for invertebrates, diatoms and macrophytes was used. The analyses of reference sites showed that small (catchment < 100 km2) siliceous and non-siliceous streams were mainly affected by channelization, bank alteration and hydropeaking. Medium-sized siliceous rivers were the most affected by stressors: 25–43% of the samples showed at least slight alterations regarding channelization, connectivity, upstream dam influence, hydropeaking and degradation of riparian vegetation. Temporary streams were the least affected by hydromorphological changes, but they were nevertheless affected by alterations in riparian vegetation. There were no major differences between all permanent stream types regarding water quality, but temporary streams showed lower values for oxygenation (DO) and wider ranges for other variables, such as nitrates. A lower threshold value for DO (60%) was determined for this stream type and can be attributed to the streams' natural characteristics. For all other river types, common limits were found for the remaining variables (ammonium, nitrate, phosphate, total P, % of artificial areas, % of intensive and extensive agriculture, % of semi-natural areas in the catchment). These values were then used to select the list of reference sites. The biological communities were characterized, revealing the existence of nine groups of Mediterranean invertebrate communities, six for diatoms and five for macrophytes: each group was characterized by specific indicator taxa that highlighted the differences between groups.