Revision of River & Lake Typology in Bulgaria within Ecoregion 12 (Pontic Province) and Ecoregion 7 (Eastern Balkans) According to the Water Framework Directive

A revision of river and lake typology in Bulgaria has been done within the Ecoregion No. 12 Pontic province and Ecoregion No. 7 Eastern Balkans according to the Water Framework Directive 2000/60/EC. Certain geographic changes between the Ecoregion No. 12 and Ecoregion No. 7 have been proposed based on existing biogeographic data, at the beginning. The whole part of the Danube and Black Sea River Basin district have been associated to the Pontic Province (ER No. 12) as well as the rest part of the Southern Bulgaria (Maritsa river basin, Mesta RB and Struma RB) has been allied to the Eastern Balkan (ER No. 7). A serious reduction of the total number of river types (from 33 to 16 types) was justified using clear hydromorphological and biological criteria, e.g. ecoregion, vertical factors (4 altitude zones, main substratum characteristics, slope, and other supplementary factors such as fish and vegetation zonation, climate maps) and horizontal factors (calcareous geology, salinity, size category). Similar reduction of the number of lake types (from 33 to 17 types) was proposed in parallel with a significant modification of the lake typology method. Lake typology in Bulgaria was based on the obligatory factors (4 altitude zones, size typology based on surface area, depth, salinity and geology) and optional factors (residence time, mixing characteristics, e.g. monomictic, dimictic and polymictic, presence of profundal zone, reference trophic status). Seven lake types were identified as “reservoir types”, which were only presented by heavily modified and artificial water bodies without any possibility for natural lake equivalent within the country or region. Four coastal lake types with various salinity (from freshwater <0.5 ‰ to hypersaline >40‰) have been reviewed as belonging to the category “transitional waters”.