Water-retention at a dam reservoir affects water quality in the lower reaches of the river. Those effects were assessed by comparison with a nearby river of similar size. Reservoir drainage operation ware conducted to repair dam equipment during this investigation period. For both rivers, it was confirmed that the NO₃-N concentration tended to rise and that the EC and SiO₃^2- tended to decline during the snow-melt period. Nitric acid from the atmosphere attached to the snow, which then flowed out as surface water without dissolving soil minerals into the melting snow water. The SS value rose when the river was swollen by rains, during the snow-melt period, and during the drainage operation period. The SS mainly comprised inorganic matter, with low and small changes in VSS. The Chl-a value tended to rise by the dam reservoir, but it was around 5μg/L even when it rose. Little influence was noted by the increase of phytoplankton. The BOD value was higher than that of a control river, but its mean value was less than 1 mg/L, which might be attributable to the small dam capacity and the low phosphorus content of inflowing rivers. The water temperature rose around 5°C in the dam downstream. During drainage operation, the values of SS and NH₄-N rose. However, those values were not sufficiently high to affect aquatic organisms of the lower reaches.

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Numerous sediment-control dams are installed in most headwater catchments in Japan. These dams cause habitat fragmentation of stream-dwelling salmonids, raising the risk of local extinction of their isolated populations. We examined such an effect of habitat fragmentation due to dams on red-spotted masu salmon (Oncorhynchus masou ishikawae) in Shikoku, southwestern Japan. We hypothesized that their local extinctions would occur from uppermost reaches (uppermost margins of their distribution range), where populations are completely isolated, and then extend downstream. Our purpose was to clarify habitat size (catchment area upstream of dams) at which local extinctions becoming apparent in the present state. Classification tree analysis using presence/absence data at 69 study sites suggested that local extinctions due to dams were apparent in stream reaches at catchment area 0.49-0.89 km². In addition, regression tree analysis showed that masu salmon density increased upstream, suggesting that upper reaches have higher habitat quality. Estimations of stream length and population size based on catchment area indicated that stream reaches facing extinction threat are within 500-600 m from the headwater and have only 40-50 individuals of adult population size. Our results emphasized the priority of uppermost reaches in conservation and restoration of connectivity for populations of stream-resident salmonids. We proposed four management classes (critically endangered area: catchment area < 2 km² ; least-required area: 4 km² ; required area: 10 km² ; target range: 40 km²) based on previous knowledge and the present results to facilitate conservation and restoration of the headwater connectivity.

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[in Japanese]

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[in Japanese]

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Relationships between land use and the carbon and nitrogen stable isotope ratio of riparian plants, aquatic algae and masu salmon juveniles of seven rivers in the coastal area of Ishikari Bay Hokkaido, Japan were examined. Principal component analysis was conducted to characterize the land use of these rivers. The correlation between the principal component scores and the stable isotope ratio of riparian plants, aquatic algae and masu salmon juveniles in these rivers was analyzed. The first principal component (PC1) accounted for 47 ∼ 57% of the variance of land use variables. Component loadings of PC1 were positive high in the land use of agricultural land or artificial forest, and were negative high in the land use of natural forest and natural grassland. A positive correlation was observed between the principal component scores of PC1 and the nitrogen isotope ratio of aquatic algae and masu salmon juveniles. These results suggest that the nitrogen stable isotope ratios of aquatic algae and masu salmon juveniles are influenced by land use. These stable isotope ratios provide a useful index of the human impact on stream ecosystems.

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