河川間隙動物（特にコナガカワゲラ属幼虫）の生息環境の特徴

Abstract

We reexamined numerous specimens of nymphs, exuviae and adults of the genera Flavoperla and Paragnetina (Inscta, Plecoptera, Perlidae; both emerge in summer and have more than 2 years of nymphal period) collected mainly in the Yahagi River system, central Honshu, Japan, from 2000 to 2022 and preserved in Aichi Institute of Technology. The number of Flavoperla nymphs was far fewer than that of Paragnetina nymphs. However, for exuviae and adults, the number of Flavoperla was similar (exuviae) to, or fewer (adults) than that of Paragnetina. We excavated dry riverbeds just off the water at seven sites in the Yahagi River system from 2019 to 2021 to collect hyporheos. At six of the seven sites, we collected animals that could be considered hyporheos (nymphs of Flavoperla and Leuctridae, Plecoptera, larvae of Stenelmis, Elmidae, Coleoptera and Ceratopogonidae, Diptera) or groundwater animals (Bathynellacea, Pseudocrangonyctidae, both Crustacea). The excavations in which nymphs of Flavoperla and Leuctridae were collected tended to have high oxygen saturation and low dissolved nitrogen and dissolved phosphorus. The Pseudocrangonyctidae and the larvae of Stenelmis and Ceratopogonidae were collected mostly in the excavations with high oxygen saturation and low dissolved nitrogen and phosphorus, but a small number of the animals were also collected in the excavations with low oxygen saturation and high dissolved nitrogen and phosphorus. The Bathynellacea were collected only in the excavations with low oxygen saturation and high dissolved nitrogen and dissolved phosphorus. There was no clear relationship between the estimated amount of sediment transport and the number of these animals collected. At Odo in the Yahagi River, benthic animals were collected on the adjacent riverbed near the excavations where the hyporheos were collected. Larvae of Stenelmis and Ceratopogonidae, which were collected in large numbers in the excavations, were also collected on the adjacent riverbed with benthic animals, but in much smaller numbers. These results suggest that the low numbers of Flavoperla nymphs collected by the usual methods for benthos may be due to their habitat in hyporheic zone. Other candidates for hyporheos include the nymphs of Leuctridae and the larvae of Stenelmis and Ceratopogonidae. Most of these animals showed a preference for the waters with high oxygen saturation. Although no relationship was found between the numbers of these animals and the estimated amount of sediment transport, strong disturbance of the riverbed is thought to increase oxygen saturation in hyporheic zone, and therefore, Flavoperla nymphs etc. could be used as indicators for strong riverbed disturbance.