To show effective measures against filamentous green algae that grow in abundance in regulated rivers, in-situ experiments on the effects of atmospheric exposure were carried out using communities of Spirogyra sp. and Cladophora sp. as subjects. In addition, the changes in biomass attributable to atmospheric exposure and discharge were compared. Morphological changes of the chloroplast occurred in most cells of Spirogyra sp. and Cladophora sp. with atmospheric exposure. The biomass of Spirogyra sp. communities decreased by about 90—98% in cases of one-day atmospheric exposure, three-day exposure, and discharge after three-day exposure. The latter case exhibited the greatest biomass decrease. It was the only case in which the ash free dry mass decreased. The biomass of Cladophora sp. communities decreased by 84% in both cases of discharge after one-day atmospheric exposure and three-day atmospheric exposure. In those cases, the ash free dry mass also decreased markedly after atmospheric exposure. Although the relative abundance of filaments of Cladophora sp. accounted for about 80% of the communities before atmospheric exposure, the relative abundance of other algae increased more than 75% after atmospheric exposure. Both filamentous green-algal communities were abundant after discharge, however they were decreased markedly by atmospheric exposure, suggesting that atmospheric exposure is an effective method for control of filamentous green algae in regulated rivers.

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The purpose of this study is to conduct a simple and comprehensive stream assessment based on a “Degree of Departure from Reference (DDR)” in physical environments. We define ‘departure’ as the distance between a reference site and each test site in multiple principal component dimensions of physical conditions. The assessment was conducted at the reach scale. The three categories were anthropogenic alteration, habitat diversity and stream & floodplain structure, with each consisting of specific indices representing physical conditions of the stream and floodplain. We established 68 test sites in a lowland, valley bottom segment of the Shibetsu River, Hokkaido, northern Japan, where relatively undisturbed reference sites were available. Physical conditions for each reach were measured, using a survey method modified from the River Habitat Survey, a quantitative assessment system that has been developed and practiced in the United Kingdom. The test sites were classified into principal components separately for three categories. A group of least disturbed sites was determined as a reference. The departure in physical conditions was smallest within the reference group and was large in other reaches which exhibited differences from the reference group in terms of multiple indices. These sites departing from the reference flow through urban areas and show a high level of alteration, low habitat divesity and sinuosity and less frequent occurrence of riffles, pools and bars. The Degree of Departure from Reference is an objective measurement of differences between reference conditions and a target site for assessment, which provides a comprehensive, accurate evaluation of stream physical environments. Our assessment method is applicable to other rivers. It can be also more easily used for spatially large-scale assessment relative to the ones with biotic indices, thereby providing an effective tool for ‘mass-screening test’ of river environments.

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Genetic structure of the caddisfly Stenopsyche marmorata populations was studied in the Sakawa River catchment containing Miho Dam in Kanagawa Prefecture, based on the mtDNA COI region by polymerase chain reaction-single-strand conformation polymorphism (PCR-SSCP) method. Primer set (product size: 160 bp) was designed based on 771 bp of mtDNA sequences from 59 individuals of S. marmorata. Five SSCP types were recognized in 330 individuals from 11 sites located in the Sakawa River and its adjacent (the Sagami River) catchments. Frequencies and diversities of the SSCP types were clearly different between the populations of upper sites of Miho dam and those of other sites (including tributary sites). This result suggests that Miho dam is a prominent barrier for the movement of S. marmorata.

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Paddy farmland consolidation in Japan has been widely recognized as having a negative impact on biodiversity in rural areas. In particular, drainage ditches made of concrete that are introduced to lessen farmers’ burden during maintenance activities to expell vegetation raise concern about their potential strong impact on the ecosystem, as some of the vegetation has established itself in the ditches. In this study our goal was to clarify the conditions for vegetation intrusion into concrete drainage ditches so that appropriate drainage canals can be selected that will lessen the environmental burden. A field survey was performed from 2004 to 2006 in a consolidated paddy field in Joso City, Ibaraki Prefecture, Japan (139°58’ E, 36°2’ N). The results were as follows: (1) Three-year vegetation distribution in concrete drainage ditches was almost steady, with the same dominant species. (2) Water depth and sediment deposition measured at all points of vegetation observed in two selected ditches suggested that most vegetation was found in water less than 10 cm deep. (3) An index of average vegetation coverage (AVC) was introduced to obtain the spatial average of vegetation coverage. High AVC occurred in water less than 15 cm deep, regardless of the wide range in sediment deposition. (4) Based on a germination test for soils in different conditions, deep water appeared to suppress germination in the drainage ditches. (5) The found conditions of vegetation suppression in the two selected ditches can explain the distribution of vegetation over the whole study area.

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Dam construction has negative impacts on river ecosystems, but we do not understand well how and why it influences riverine macroinvertebrate assemblages. The purpose of this study was to determine the influences of the Oishi Dam (Sekikawa Village, Niigata Prefecture, Japan) on downstream macroinertebrate assemblages with particular focus on net-spinning caddisflies. I conducted a monthly field survey at one site upstream and three sites downstream of the dam in Oishi River from April to December in 1994. The upstream macroinvertebrate assemblage was dominated by Ephemeroptera and Plecoptera, whereas the downstream assemblages by Trichoptera, especially net-spinning caddisflies Stenopsyche. Net-spinning caddisfly species were more abundant at the downstream sites than at the upstream site. Sestonic organic matter concentration in river water increased in summer at the downstream site that received much water directly from the reservoir, probably due to a large amount of drifting phytoplankton. Early instar larvae of Stenopsyche showed remarkably high density in summer at this site. Phytoplankton originated from the reservoir might enhance downstream population of Stenopsyche.

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We surveyed fish fauna from 2001 to 2006, and crab and insect fauna in 2006 in an artificial wando-pool in the Kitagawa River, Gokasegawa River System, Miyazaki Prefecture, Japan. The artificial pool is located 4 km upstream of the river mouth, and it is 50 m in width, 400 m in length, and average of 1.5 m in depth. This pool was constructed in 2001 to replace the natural wando-pool lost that was due to river improvement. In this survey, we recorded 72 fishes, 12 crabs, and 7 insects. The number of fish species captured in the artificial pool was almost identical to that in the earlier natural pool. In addition, various species of fishes, crabs, and insects were captured in the tidal flat and the Zostera japonica vegetation zone present along the waterfront of the pool. These results indicate that the species diversity in the artificial pool is identical to that in the natural pool, and this species diversity was maintained despite the diversified environments at the waterfront. In addition, this artificial pool also varied in other aspects and exhibited roughness of the bottom, substantial differences in water depth and salinity, and formation of a halocline. We assume that the diversified environment of not only the waterfront but also other places contributed to maintenance of the species diversity in this artificial pool.

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Freshwater mussels (Order Unionoida) constitute one of the important biological components of freshwater ecosystems particularly because some fish species require them as spawning beds whereas mussels need other fish species to reproduce themselves. Furthermore, as their longevity and poor mobility render them exceedingly susceptible to environmental changes, they could serve as a useful indicator of habitat conditions. Unionoid mussels used to widely inhabit in both lentic and lotic environments and 18 species have been reported in Japan; approximately 1,000 species exist worldwide. However, decreases in species diversity, abundance, and habitat range of Unionoida are common throughout the world, and major causes include alterations of physical, chemical, and biological characteristics of habitat environments through human activities such as harvesting of individuals, river works, landuse change, and invasion of alien species. Comparisons of major habitat types reported for unionoid mussels between Japan and other countries show that one of the most common habitats for lotic species in Japan is agricultural ditches that are much smaller in size and more regulated compared to common habitat types (mostly large rivers) in other countries. Based on direct observation of ditch habitat over alteration works, habitat degradation resulted from reduced habitat heterogeneity in both longitudinal and lateral directions. An experimental case with placement of lateral structures and creation of artificial riffles may provide one possible approach in restoring degraded ditch habitat for Unionoida. There is a need for more studies on Unionoida in ditch habitat because various underlying processes may be different from those in ordinary river ecosystems. Most importantly, efficient and rapid dissemination of various type of information collected at the local level to those interested in the topic in other areas is crucial for conservation of unionoid habitat at the country level and beyond.

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This paper presents a review on analysis methods for eutrophication phenomenon or growth of phytoplankton, which is one of typical water quality problems in dam reservoirs. Methods and models on predictions of phytoplankton growth in dam reservoirs have long been studied and basic methodologies seem to have been confirmed for practical uses. However, there are issues for improvement of prediction models requested from the needs of the public, as well as from mere problems of technique. The prediction methods applied in practical business are categorized into two types: simple analysis methods and ecosystem simulation models. Simple analysis methods are mainly based on statistical data analysis collected from many dam reservoirs. A representative example is the phosphorus load model by Vollenweider. Ecosystem simulation models have also long been studied and various types of the model have been proposed. A basic structure of an ecosystem model is consisted of a hydraulic model and water quality — ecosystem model. Issues to be tackled on phytoplankton prediction in dam reservoirs are also discussed. More precise predictions on occurrence of water bloom and water quality disorder and on effects of water quality preservation facilities are needed. On the other hand, simple analysis methods are needed to be improved. These issues are expected from a view point of accountabilities as well as mere technical viewpoint.

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