Similarity in Taxonomic Composition of Benthic Macroinvertebrate Communities in the Blagoevgradska Bistritsa River

Benthic macroinvertebrate communities in the Blagoevgradska Bistritsa River were examined as a part of a complex study of the river and its basin. Monthly in 2002 and seasonal in 2003 handnet samples were taken from seven stations along the river length. The similarity of the benthic communities at different stations—along the river continuum and at each station—in different months was determined in order to study the taxonomic (species) composition dynamics and to identify the main factors affecting it. Generally the species composition along the river continuum was changed slightly, which confirms the results about the dominant structure of benthic macroinvertebrate communities and the conclusion that the whole river possesses characteristics typical of upper river courses. Both in 2002 and 2003 the similarity in species composition between sampling stations from 1 to 6 was high (above 50% almost in all cases) but comparatively low (from 21.6 to 41.1%) between the first six stations and the last one where the saprobic pollution was quite high. The seasonal changes in composition between the stations and at each station were determined mainly by the river flow alterations. In periods with prolonged high flow (2002) the species composition similarity along the river continuum increased while in low flow period (summer-autumn 2003) the tendency was the opposite. During the summer-autumn low flow period in 2003 the composition of macrozoobenthos in the lower river course was strongly influenced by the altered characteristics of the basic hydrological parameters (velocity, temperature etc.) and was considerably changed.