Selection and validation of reference sites in small river basins.

Type-specific reference conditions that describe sites with no or only very minor anthropogenic disturbances are a basic requirement of the European Water Framework Directive. The reference condition approach implies a previous determination of criteria acceptable for the definition of near-natural stretches. In this paper, a methodology based on selection and validation procedures is applied to a Portuguese watershed to guide the identification of reference sites. The methodology consisted of three phases: (1) a preliminary site inspection through the use of maps, available data and an extensive screening field campaign that resulted in the selection of 52 potentially undisturbed small- and medium-sized rivers stretches, ranging from 43 to 1069 m in altitude, with catchment areas between 4 and 641 km2 and representing siliceous and calcareous riverbeds; (2) a detailed site selection based on ten pre-defined criteria involving physical/social/biological attributes that indicated catchment land use changes and alien vegetal riparian species as major human impacts; (3) a site validation procedure involving detailed investigation of benthic macroinvertebrate communities, riparian vegetation, in-stream habitat quality and chemical parameters that showed riparian wood-related problems as a major restriction to validate a stretch as a near-natural site. Only about 12% of all investigated river sites, accounting for 600 m of all 5200 m studied, could be considered as near-reference stretches or sites retaining essential natural functions. Selection and validation procedures can identify different sites as reference, which reinforces the need for applying both procedures. The results presented can help to accomplish the requirements of the EU – Water Framework Directive by selecting reference sites as the first step to establish biological reference conditions and, simultaneously, to form a basis for nature conservation strategies. [ABSTRACT FROM AUTHOR]