Abstract
The study aims to monitor the state of two rivers from the Topolnitsa River sub-basin – the Elshishka and Medetska Rivers. For the study, samples of macrozoobenthos and waters from the lower reaches of both rivers were collected. The monitoring was carried out in the spring, summer and autumn of 2024 according to a methodology approved for the country and the European Union. A total of 14 taxa of macroinvertebrate organisms belonging to 9 orders (Amphipoda, Coleoptera, Decapoda, Diptera, Ephemeroptera, Hemiptera, Odonata, Sphaeriida, Trichoptera) were collected. The main metrics are presented, including the total number of taxa, EPT taxa, species diversity indices, trophic index, saprobic index, and biotic index. During the study period, no macroinvertebrate taxa were detected in the Medetska biotope. During the spring, no macroinvertebrate organisms were detected in the Elshishka biotope. The dominant macrozoobenthos taxon in spring and summer in the Elshishka biotope is Gammarus pulex (Linnaeus, 1758), indicating ꭓ-β-mesosaprobic conditions. In both seasons, the Elshishka biotope is dominated by macroinvertebrate taxa belonging to group C (relatively tolerant forms) and no organisms belonging to groups A and E (sensitive forms and most tolerant forms, respectively) were found. Based on the calculated indices and metrics, the ecological state of the Elshishka River in spring is very bad, while in summer and autumn it varies from very good to very bad. The comprehensive ecological assessment for the two studied biotopes shows a very bad ecological state for the entire period of the study.
Keywords: Bulgaria, comprehensive ecological assessment, indices, Maritsa River basin, metrics