Zoobenthos of Pools of Rapid Rivers: Composition, Abundance, and Trophic Structure (Based on the Example of Eastern Fennoscandia)

Abstract

The species composition and abundance of zoobenthos are studied to reveal the structure of bottom invertebrate communities under the effect of biotopical, hydrological, and climatic factors. One hundred and six quantitative samples were collected from 15 pools of rapid rivers (61.6°–67.1° N) from 2003 to 2017 in eastern Fennoscandia. The basic part of the fauna is formed by 151 species, including 36 Diptera species, 23 Trichoptera species, 21 Bivalvia species, and 19 Ephemeroptera species. Representatives of Chironomidae, Oligochaeta, Sphaeriidae, Ephemeroptera, and Trichoptera occurred most frequently in the samples (89, 72, 58, 40, and 28%, respectively). The diversity of bottom communities of the pools was low when compared to that in the riffles. The abundance of zoobenthos varies greatly from site to site (from 0.06 to 42.6 thousand spec./m2 and from 0.06 to 100.8 g/m2). The average values ​​(3106 ± 642 spec./m2 and 6.6 ± 1.27 g/m2) are comparable with the abundance of zoobenthos in the riffles. The basic biomass was formed by Bivalvia (Euglesa spp.), Gastropoda (Radix intermedia and Gyraulus borealis), Diptera (Chironominae spp. and Dicranota bimaculata), and Trichoptera (Brachycentrus subnubilus and Hydropsyche contubernalis borealis). The lowest zoobenthos biomass was detected in brooks (1.4 ± 0.37 g/m2) and the highest was in the medial zone of rivers (9.5 ± 2.52 g/m2). Local aggregations of large mollusks (Anodonta cygnea, Unio pictorum, and U. tumidus) with a multiple biomass (0.5–3.6 kg/m2) and a density of 80–120 spec./m2) were found in the study area (4% of samples, mainly in the medial zone). Gatherer–collectors were dominant in the trophic structure (32–60% of biomass); the proportions of groups varied depending on the characteristics of the biotope, in particular, the pattern of the bottom substrate. An increase in the zoobenthos biomass was recorded in the southern part of the region: from 2.7 ± 0.48 g/m2 north of 65° N to 8.6 ± 1.82 g/m2 south of 63° N. Comparison of the structure of the zoobenthos of rapid river pools in Fennoscandia with that in lowland rivers in other areas did not reveal any fundamental differences, which indicates the importance of local hydrological and other factors in the formation of the composition and abundance of bottom invertebrate communities.