Intercalibration of Macroinvertebrate-Based Method for Status Assessment of Bulgarian Tributaries of the Danube River

Bulgaria has joined the official intercalibration (IC) in order to complete the state commitments for the classification of rivers based on Biological Quality Elements (BQE). The objective of this paper was to verify whether the present Bulgarian classification method for ecological status of large lowland (national type R7) and medium-sized lowland (national type R8) rivers was compliant with the normative definitions of the Water Framework Directive. Thus, a two-year study (2014 - 2015) on macroinvertebrate communities was conducted at 43 sites (18 from R7 and 25 from R8). Differences in qualitative and quantitative composition in the above- mentioned river types, as well as indicative potential of BQE in relation to different anthropogenic pressures were analysed. Based on alternative benchmark sites (identified using the criteria defined by the EU), the High/Good boundary and a reference value for the Biotic index were defined. Class boundaries were in line with the results of the completed intercalibration exercise. We found a significant association between the biotic index and all three groups of pressures (land use, chemistry and hydro-morphology) and these results could be used for assessment of the ecological status. The national assessment method (based on our data) was in good agreement with the accepted methods from other member states who share the same river types within the Eastern Continental Geographical Intercalibration Group (EC-GIG).