Water levels in the Watarase Reservoir, central Japan, were altered as part of a water quality improvement program. Specifically, the water level was lowered to near drying-out of the reservoir. This level exceeds the normal lowest watermark, and is hereafter referred to as “drying-out”. Since such treatments may affect the birds in the reservoir, a population survey was conducted in and around the reservoir from January 26th to May 15th, 2004. The survey was conducted in the three main ponds (Watarase Reservoir, Tatara-pond, and Joho-pond) throughout five water-level phases. Based upon levels in the Watarase Reservoir, these were defined as falling, maintaining the lowest level, drying-out, rising, and full. At each survey point, the number of birds present changed in relation to even slight changes in water level. However, covariance analysis indicated these population changes to be non-significant between each point and water level phase. No significant differences were detected between the bird populations at Watarase Reservoir and Tatara-pond over the duration of the study. Although a slight difference was detected between population decreases at Watarase Reservoir and Joho-pond, the decrease at the latter site where no water level lowering took place, was more conspicuous than at the former site. The decline in the population of birds detected in this study reflected the natural cycle of seasonal migration, and was not an effect of the drying-out of the reservoir.

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Investigation was conducted on relationship between vegetation of floodplain mire and fluctuation of water level, in order to clarify specific water level fluctuation and appropriate indices relating to their distribution. Water level was recorded every one hour from May to October in 2002 at an alder forest of back swamp, a fen of back swamp and a riparian herbaceous stands on natural levee at Kushiro mire in Hokkaido. Frequency and amplitude of water level fluctuations and seasonal water level were analysed for each vegetation site. Results showed that the water level of the riparian herbaceous stands changed frequently with high amplitude, and that of the fen was high from spring to summer compared to the alder forest. The water level of the fen and that of the alder forest showed similar fluctuation pattern in frequency and amplitude. Standard deviation of water level had high correlation with frequency, amplitude and seasonal water level, indicating that it was not appropriate index to characterize the water regime among vegetations. There were significant correlation between frequency and amplitude of water level, but seasonal water level was independent to them. In conclusion, we suggest that the indices independently representing amplitude and seasonal difference were effective even though measurement of water table was once a week or a month.

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Concrete revetment is widely applied in flood control to protect stream banks from a scouring. However, the utilization of concrete substrates by stream invertebrates is largely unknown to the present. In this study, we conducted a field survey on the physical habitat environment and community structure of stream invertebrates on concrete, stone and bedrock substrates, in order to characterize concrete substrate relative to natural substrates. Current velocity was lower on stone substrate than on concrete or bedrock, probably because of high roughness of stone substrate. Density of stream invertebrates was the highest on concrete substrate, while the evenness, one element of diversity, was lower on concrete and bedrock than on stone. In addition, the ordination using non-metric multidimensional scaling (NMS) revealed that the community structure of stream invertebrates on concrete was different from that on stone and can be characteristics by high density of Baetiella japonica, with bedrock showing intermediate characteristics between concrete and stone. Thus, stream invertebrate assemblage established on concrete substrate appeared to be distinct from those of natural substrates, possibly owing to low habitat complexity. High density, low taxon richness and low evenness of stream invertebrates were observed at the habitat with high current velocity. The value of NMS axis 2, which positively and strongly correlated with the density of Baetiella japonica, increased as the current velocity increased. Distinctive community structure, characterized by high density and low evenness, observed on concrete substrate would thus be caused by high current velocity and habitat preference of Baetiella japonica. Consequently, we suggest that concrete revetment modifies the community structure of stream invertebrates by simplifying and altering physical habitat environment, at the habitat patch scale examined in this study. Our findings could contribute to better understanding of the influences of flood control structures on stream invertebrates and, furthermore, provide basic information needed to the stream management and restoration.

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The purpose of this research is to explain the relationship between the winter habitat for juvenile masu salmon and three hierarchical structures: channel unit, subunit and microhabitat. The research was conducted at the Makomanai River and Shakotan River, southwest part of Hokkaido, from the middle to the end of December 2001. The natural stream reaches of both rivers are assigned as segment one. The three hierarchical stream structures are designated at the lower hierarchy under stream reaches. The channel unit is at the uppermost part, follows by subunit and then microhabitat. The results show that winter season environment of the juvenile masu salmon depends on two hierarchical stream structures, subunit and microhabitat. The winter season environment is related to combination of subunits close to the riverbank of the pool and glide that have relatively slow current velocity and small Froude number (Fr < 0.125), and the microhabitats with cover that have even slower current velocity. Not only is the microhabitat significant to winter season environment for juvenile masu salmon, but also the subunits around the microhabitat that have slow current velocity and small Froude number. We considered the subunits and microhabitats are very important habitat for the juvenile masu salmon before winter season in addition to the period during winter season. It is, therefore, suitable to regard the hierarchical characteristic of stream structure as contiguous habitat that shows seasonal change. Furthermore, we proposed river management methods with consideration of the winter season environment based on this research results as follows. We should be aware of conserving and creating channel unit with subunit and microhabitat for winter season by river management in the scale of the stream reach. In terms of microhabitat, we should pay attention to microtopography, vegetation and coarse woody debris.

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The effects of salmon carcasses on forest and stream ecosystems were determined by nitrogen stable isotope analysis in natural streams in eastern Hokkaido, Northern Japan, where numerous chum salmon (Oncorhynchus keta) were migrated upstream from ocean to spawn in autumn. The leaves and soils surrounding riparian forest and aquatic animals were collected at salmon present and absent stream before and after migration. The nitrogen stable isotope ratio (δ¹⁵N) of riparian trees (Salix spp. and Ulmus davidiana var. japonica) were not significantly different between the two sites. The grass leaves of Petasites japonicus, collected within 5 m at salmon present stream showed higher δ¹⁵N value (6 ‰) than another site (0.5 ∼ 3 ‰), however, this was not significantly different. The δ¹⁵N of A₀ soil layer at the salmon present site was 3 ‰ higher than absent site, but this was not significantly different. The C-N ratio was the lowest within 5 m of salmon sites, which was also not significantly different. The movement of carcasses was traced by telemetry, and three out of the five carcasses were removed within 10 m. However, one carcass was transported to 500 m distant hillslope ridge. The δ¹⁵N in four taxa of stream dwelling invertebrates and juvenile masu salmon (Oncorhynchus masou) were significantly different between the salmon present and absent stream. The temporal change of δ¹⁵N values of aquatic invertebrates were compared before and after migration, 0.5 to 3.5 ‰ increases were observed in grazer, collector-gatherer and predator, but not shredder. These results suggest that the salmon carcasses did not significantly effect on riparian vegetation, however, they strongly influenced on stream ecosystems by enhancing trophic levels of aquatic consumers.

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We evaluated the “salinity - water quality” and “salinity - macroalgal assemblage” relationships in summer on the polyhaline brackish Lake Nakaumi in western Japan. Temporal data on water variables at the lake center in 21 years (1984-2004 yr) and spatial data of 15 coastal sites within the lake in 2 years (2003, 2004 yr) were analyzed with attention to the salinity - water variables relationship (Secchi-depth, chl.a, TN, TP, DTN, DTP, DO). To evaluate the salinity - algal assemblage relationship, 2 years data at 15 coastal sites was analyzed to quantify the relationship between salinity and 3 assemblage characteristics (maximum vegetated depth, species richness, abundance of algae species). The results showed that the salinity increase resulted in both transparency improvement with decreasing chl.a and nutrient contents (TN and DTN), and the assemblage development with higher values of the three characteristics, both temporally and spatially. Previous studies on oligohaline lakes reported the opposite salinity - water quality/vegetation characteristics relationship, that is, the eutrophication and vegetation decline in higher saline years. This study and previous ones suggest that salinity increase affects many aspects of the ecosystem, and its consequences may differ depending on the location of lake (i.e. distance from sea).

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Conventional paddy farmland consolidation in Japan has been said to have negative impacts on biodiversity in rural areas. Especially, transforming earthen irrigation and drainage canals into concrete canals has caused catastrophic damage to aquatic animals. The canal structure and flowing water for the non-irrigation season in the farm drain of a paddy field is very significant for aquatic animals utilizing a paddy field as a spawning ground. The purpose of this study was to clarify the influence of canal structure, that is, the canal bed and side materials, on aquatic animals based on a field survey of the distribution of fishes and aquatic insects at two sites in the same farm drain of a paddy field in Shimodate City, Ibaraki Prefecture, Japan. The sampling interval was one month during the period from April 2002 to March 2003. The results indicated the followings: (1) Overall, the biomass of aquatic animals at an earth canal in the farm drain was larger than that at a concrete canal. (2) Misgurnus anguillicaudatus (Dojou) was the most frequently captured fishes. Orthetrum albistyrum speciosum (Shiokaratombo) was the most continually captured aquatic insects. (3) Misgurnus anguillicaudatus appeared to be affected by the canal bed material composed of mud but relatively unaffected by the side material. (4) Orthetrum albistyrum speciosum appeared to be affected by the particle size composition of the canal bed material and preferred argilliferous soil to sandy soil. (5) To enrich the biodiversity in the farm drain of a paddy field for the future, our results suggested that maintaining an earth canal is highly priority.

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