Abstract

The adequate management and protection of aquatic ecosystems is based on solid knowledge, including various multidisciplinary methods, reflecting work at various levels (scales). Rarely, distant methodological approaches contribute to scale mismatches, even for the same issue. The case of the Mesta (Nestos) River is such an example: in different official documents the whole catchment is accepted as in the Alpine biogeographical zone. In parallel, part of the catchment belongs to R5 and R13 national type rivers (lowland rivers). In order to clarify this discrepancy, the controversial fish communities were compared with typical lowland ones of the same type (R5 and R13), as well as typical alpine ones (R1 national river type). It was established, that typology aligns more closely with the observed fish community structure than biogeographical zonation, in terms of species composition and abundance. Thus, an essential part of the Lower Mesta River catchment should be placed in the Continental biogeographical zone, and relevant regulatory documents should be changed accordingly.