Features of the Macrozoobenthos Structure in Rivers of the Southern Part of the Olonets Isthmus

The study of the composition, abundance, biomass, and trophic structure of macrozoobenthos in watercourses of various sizes in the southern part of the Olonets Isthmus, most of which belong to the basin of the River Svir, was conducted during the summer seasons of 2011–2023. A total of 207 species of macrozoobenthos were identified in the rivers, with a dominance of chironomids, caddisflies, and mayflies. The riffle areas were characterized by significantly higher abundance (5970 individuals/m2) and biomass (18.21 g/m2) compared to the pools (1540 individuals/m2 and 2.16 g/m2, respectively). All groups were represented in the trophic structure, except for scrapers. An increase in the proportion of collector-gatherers was noted in the pools. Riffle areas exhibited higher β-diversity and expected species richness (278 species) compared to pools (165 species). It was shown that, under conditions of relatively thin layers of sediment, the macrozoobenthos of the rivers combines features of both riffle (high species richness and abundance) and pool rivers (low species richness and abundance).