|dc.description.abstract||The Hienuki River, located in Iwate Prefecture, Japan, originates on Mt. Hayachine, and joins the Komata River at a confluence downstream from the Hayachine dam. Construction of the Hayachine dam on the Hienuki River was completed in 2000. Previous to this study little research had been done on these rivers.
This study found the overall species diversity of aquatic insects to be high in both the Hienuki and Komata Rivers. Twenty-five families, 50 genera, and 125 species of Ephemeroptera (mayflies), Plecoptera (stoneflies), and Tricoptera (caddisflies) were identified. Among them, two families and 15 genera are unknown in North America. Many taxa may have immigrated into the sampling area from the north (Hokkaido, Sakhalin and Kamchatka) or south (Honshu) through landbridges during the Pleistocene.
The aquatic community was negatively affected by the Hayachine dam at the outlet, but recovery was seen within 4 km. Water chemistry was similar both upstream and at the outlet. Among the causes of community change may have been (1) the elimination of drifting aquatic insect fauna from upstream, (2) fluctuation of water flow and (3) destruction and alteration of food sources. The changes in food sources included (1) changes in algal growth and (2) reduction of coarse particulate organic material, (settled out in the reservoir) which resulted in the elimination of shredders. Flow manipulation may have caused the alteration of the algal community at the outlet, which in turn caused a reduction of scrapers. Furthermore, the abundance of blackfly larvae increased as the abundance of the caddisfly species (Hydropsyche orientalis Martynov) gradually decreased beginning in late May 2002. This decrease may have been the result of competition, since both blackflies and Hydropsyche are filter feeders.
Cyanobacteria species that were commonly found in subaerial and aerial habitats were abundant at the outlet from late May 2002. This suggests that low water and fluctuation in water levels occurring between the middle and end of May 2002 had an effect on the cyanobacteria community. The presence of blackfly larvae and hydropsychid larvae are characteristic of the outlet of natural lakes. Their presence at the outlet of the Hayachine dam indicates a similarity with natural lakes.
This study showed that species diversity and community similarity indices may be unsuitable for ecological study. Species diversity indices showed little species richness at each site, and tended to focus attention on the few most dominant species. Community similarity indices generally showed the similarity between upstream and downstream mayfly and the overall communities (mayflies, caddisflies, and stoneflies combined). However, these indices failed to detect differences for the caddisfly community. In addition, these indices were unable to quantify (1) the impact of the dam at the outlet and (2) recovery downstream. Rank abundance curves and tables may provide better pictures of species diversity and the community change of aquatic insects and thus be preferable to these indices.
It is concluded from this study that the surface-release features of Hayachine dam prevented the extreme damage that is done to river communities by hypolimnion dams. Hayachine dam appears to be similar to a natural lake on a river, where changes appear to be minimal, and recovery is very rapid - within about 4km, compared to lOO's of km in the case of dams with hypolimnion drains.
It is recommended that from the point of view of natural communities, dams installed in geographic areas with large seasonal fluctuations in temperature should be designed with surface or temperature regulated release. This recommendation is especially critical in areas where winter temperatures drop below 4°C and reservoirs and lakes are stratified.||en_US