Trends in the Change of the Ecological Condition of the River Mesta After 10 Years Period of Research

Long-term trends of the ecological status assessed by biological quality element macrozoobenthos, of the transboundary Mesta River (South Bulgaria) during the period 1978-2020 were analyzed. Macrozoobenthos samples were taken and physico-chemical parameters were measured twice at 7 sites situated on the main river and its tributaries during the low water period (August-September 2019 and 2020). Selected sites represented various ecological situations that are characterized by different types of human impact. Data on the current status of the river were compared with the last studied periods 2009-2010. The investigation was an extension of long-term research of the Mesta River that associated the river with the European Long Term Ecological Research Network (LTER). During the last decades, the self-purification processes and recovery of the bottom invertebrate communities and processes of stabilization of the aquatic ecosystem have been observed, after stopping the heavy organic pollution from industries in the Razlog town. This resulted in improvement of the water quality and the Mesta River ecological status. The current study demonstrated local sources of load on the aquatic ecosystem (agriculture, lack of urban wastewater treatment plants), which has a negative effect on the ecological situation in the river. Measures aimed at stimulating environmentally friendly agricultural practices and the construction of treatment plants are needed to preserve the integrity of the aquatic ecosystem and to maintain the regulated good ecological status.