A survey of the macrozoobenthos communities in the Maritsa River (South Bulgaria) was carried out in the summer of 2021. Benthic samples were collected and physicochemical parameters (water temperature, pH, conductivity, dissolved oxygen and nutrients) were measured at 15 sites located on the main river and its tributary system. The studied sites belonged to different river types and characterised the diversity of the ecological conditions—from unaffected to anthropogenically influenced river stretches. In addition, data from a study conducted in the summer of 2020 were used to analyse species–factor interactions in the river ecosystems and to assess the bio-indicative potential of the aquatic invertebrates. The dynamics of the taxonomic composition and abundance of the macrozoobenthos were analysed in relation to environmental factors. The physicochemical conditions of the water environment changed during the period of high water, which led to a reduction in the composition of the macrozoobenthos. Plecoptera and Trichoptera decreased in richness and abundance downstream and under human impacts. Ephemeroptera and Chironomidae were permanently present along the whole river. Oligochaeta increased in the lower river reaches and at sites with a greater amount of organic matter. The ecological status determined by the macrozoobenthos varied from high (site 1) to good, moderate and bad (site 13) at the studied sites.

The aim of this paper is to compare the taxonomic composition and species diversity of the macrozoobenthos in Maritsa River (Bulgaria) and Han River (South Korea). Samples were collected at 15 selected sites in each river, including some of their main tributaries in 2020 and 2021. The number of the taxa recorded in Maritsa River was more than twice as great as in Han River: 192 taxa belonging to 19 systematic groups compared to 88 taxa belonging to 18 groups respectively. The order Ephemeroptera had the highest species richness: 31 taxa in the Bulgarian rivers and 26 taxa in the South Korean ones. The macrozoobenthic communities responded and adapted to the various conditions and impacts in the water environment with changes in the species composition and structure. The analysis of the similarity in the taxonomic composition showed low resemblance between all study sites but displayed distinct separations between the rivers and the two years. In general, the species structure of the macrozoobenthic communities in Maritsa River and its studied tributaries was better than in Han River. In both years, high species diversity was recorded at the reference sites in the rivers, characterized by conditions closely resembling natural environments. The species richness and the evenness of macrozoobenthos were very low at sites downstream subjected to considerable anthropogenic pressure. Some of the communities in Han River were almost destroyed completely.

Two decades after the first record of the invasive mussel Corbicula fluminea (Müller, 1774) in Bulgaria in 2001, it has been found in many surface lotic and standing waters in the Bulgarian Danube River Basin. The aim of the current study is to report the finding of C. fluminea in lotic environment of the East Aegean water basin in Bulgaria. Six new localities of the mussel, all situated in the lower stretches of the Tundzha (two) and Maritsa (four) River were registered. The distributional patterns of the mussel and the abundance of its populations in relation to the environmental conditions were analyzed. The adaptive character of the Corbicula, the possible path for invasion as well as the probability of rapid spread of the mussel in the largest rivers and their tributary systems in south-eastern part of Balkan Peninsula was discussed.