We conducted a study to characterize the primary production by benthic algae from the upper to middle reaches in the Yahagi River, Central Japan, using two methods, the light and dark bottle method in the laboratory and the in-situ bag method in Feb. (winter) and in Aug. (summer) 2018. The amount of the benthic algal community in the Yahagi River was large in winter when diatoms were predominant; however, the maximum photosynthetic rates on photosynthesis‒irradiance curves obtained from the light and dark bottle method and primary productions from the bag method were high in summer, when the cyanobacteria Homoeothrix jantina was dominant. It was assumed that in summer, the accumulation of benthic algae is suppressed by the predation pressure of algae‒eating fish, such as ayu Plecoglossus altivelis, and the riverbed disturbance caused by flooding. The average daily gross production (P_g‒d) by the bag method was 0.9 g C m^-2 d^-1 in summer and 0.4 g C m^-2 d^-1 in winter which were less than values of domestic and foreign rivers. To estimate the River Primary Productivity in the riffles, we determined the daily net production (P_n‒d) from the upper to middle reaches: 1,620 kg C d^-1 in summer and 180 kg C d^-1 in winter. In the Yahagi River, in summer, the primary production was just or slightly short for the estimated feeding amount of predators. But it was insufficient in winter. Therefore we believe that this River Primary Productivity is one of the essential indexes when considering the conservation of river ecosystems and the environmental impact assessment.

Lake Biwaʼs inflowing rivers often experience drought in summer. Periods of drought can severely affect fishes by preventing their migration. This study examined the effect of observed drought on under-yearlings of an amphidromous goby, Rhinogobius sp. OR, which seasonally moves between Lake Biwa and its tributaries. A survey of under-yearling fish was conducted at the end of the upstream migration season in 2008, from September to November, in the downstream and midstream sections of six rivers that flow into Lake Biwa. To assess migration success from the downstream to midstream sites, we gauged the upstream migration index of under-yearling fish (=CPUE in the midstream site / CPUE in the downstream site, hereafter "UM-UY index") and determined a past upstream migration index (hereafter "UM-P index") for individuals except for under-yearlings. The UM-UY index was lower in the droughted rivers. Multiple regression analysis showed that the UM-UY index serve as an indicator for distance of drought. Additionally, the UM-P index and the UM-UY index were similar for most of the rivers. The Echi River, in particular, had very low values of both indices. However, the UM-P index and the UM-UY index were different in Ado and Inukami River, this suggested that the condition of drought differed depending on the year, and thus resulted in different indices affecting the upstream migration of under-yearling fish. On the other hand, the Echi River was considered to experience a severe drought every year that caused a significant impact on the up-stream migration of under-yearling fish. These results indicate that drought negatively affected the upstream migration of under-yearling fish and that the index of upstream migration could be a useful indicator to evaluate the scale of drought conditions in a river habitat.

In Japan, Satoyama has become a habitat for many organisms, including endemic and rare species, by forming a unique secondary natural environment, contributing significantly to biodiversity. However, the satoyama environment has decreased and deteriorated recently. Reservoir ponds, one of the most important components of satoyama, have been decreasing significantly due to disaster prevention measures. In this study, we focused on six golf course ponds as an alternative to reservoir ponds. The six ponds were divided into deep and shallow ponds, and both environments were compared. The average number of aquatic plant species was 2.5 in the golf course ponds, comparable to the number of species in reservoir ponds reported by a previous study. Aquatic plant species appeared in shallow ponds but were not observed in deep ponds. The number of emergent plants was explained by the depth and dissolved oxygen. These results indicate that even shallow and small ponds, which tend to be removed from Satoyama, are important habitats for aquatic plants. The lack of artificial transportation of animals and plants would also affect the existence of aquatic plants. Therefore, it is necessary to proceed with golf course management as an alternative to Satoyama.

The changes of water environment and aquatic biota were investigated over a three-year period in an artificial wetland biotope (Tebika Biotope) constructed by excavating fallow field. We identified 93 aquatic animal species in 18 orders and 4 submerged plants, so it was thought that this biotope was effective in biodiversity conservation. There was a marked seasonality in the number of aquatic insect species, with an increase in summer (mainly in August) and a decrease in winter (mainly in February). The species number and diversity index (Shannon-Wienerʼs Hʼ) was greater at a site of still water with ecotones. On the other hand, some unique species were found in the monotonous flowing water environment. Almost all the aquatic insects identified in this biotope were found in the ponds within 2 km of the biotope. This suggests that the aquatic biota of an artificial wetland biotope is highly dependent on the surrounding biota, and that ecotones and diverse flow regimes are important for the conservation of biodiversity. The growth of submerged plants and changes in the number of aquatic insect species suggest that the biota of this biotope may have been adversely affected by the invasive alien species, a crayfish Procambarus clarkii, and a snail, Pomacea canaliculata

Recently in Japan, land consolidation projects of paddy fields have been carried out considering the environment. In such projects, frogs are often selected as conservation target species. To contribute to effective conservation of frog habitats, this paper reviews the ecological characteristics of frogs inhabiting rural areas, the effects of land consolidation on frogs, and available countermeasures to conserve their habitats. Then, future related research areas are discussed. First, 13 domestic species and subspecies are summarized based on their migration characteristics between habitats and breeding sites. Second, four reasons are presented to explain potential causes of decreased frog population after such projects: (1) frogs die after the topsoil of paddy fields is peeled, and existing earth agricultural canals are destroyed during construction; (2) concrete canals restrict migration among habitats; (3) quantitative and qualitative changes in levees of paddy lots affect their habitats owing to land readjustment and conversion to concrete levees from earth ones; and (4) breeding and overwintering sites disappear due to reformation into well-drained paddy fields. Third, five countermeasures to conserve frog habitats are explained: (1) protecting and moving individuals before construction; (2) installing lid structures to prevent individuals from falling into the concrete canals and slopes to enable frogs to escape from the canals; (3) creating nature-friendly canals and biotopes to provide suitable inhabiting, breeding, and overwintering sites to frogs; (4) environmental consideration of water management and field management in the paddy fields, such as cultivating with less or no agrochemicals; and (5) dealing with alien species. In future, changes in the consolidation approach, such as the further enlargement of paddy fields and the introduction of smart agricultural technology, are expected to become increasingly common in rural areas. It is necessary to evaluate the impacts of such changes on frog habitats and to consider countermeasures against negative impacts.

This paper reviews fish breeding in paddy field water areas and discusses the relationship between the development level of paddy field water areas and fish breeding in floodplains and compares the frequency of flooding. Traditional paddy field water areas include water bodies with diverse environments and hierarchies. Owing to their low drainage capacities, widespread flooding occurs in paddy fields when it rains. Accordingly, fish have several opportunities to move from rivers and other sources to paddy field water areas to breed. Conversely, the characteristics of consolidated paddy field water areas as a floodplain water area are likely to be significantly altered by the reduction in channel density, the fragmentation of water areas owing to enhanced drainage capacity and drop-offs, and the associated simplification of the hierarchical structure and channel environment. Most paddy fields are privately owned, are primarily maintained by farmers, and used as a place of livelihood. This makes it difficult to conserve organisms, including fish, in these fields. Therefore, here in, a case study of the three types of fish conservation measures is presented: 1) the installation of fishways after the project is completed, 2) focusing on waterlogging in fallow fields and creation of biotopes, and 3)the conservation of traditional paddy fields in suburban areas. Finally, the method used in the conservation of fish in paddy field water areas is discussed on the basis of recent changes in social conditions.

To clarify the function of rice ecosystem as a habitat for birds, we organized previous studies conducted mainly in Japan. Rice fields provide foraging habitats for birds throughout the year. Few bird species complete their life history in rice fields alone, and thus habitat heterogeneity with the presence of grassland, forest, and other (semi-) natural environments support high diversity of bird species. The post-war intensification of agriculture deteriorated the habitat quality of rice ecosystem and had a serious impact on bird diversity. The vegetation succession following the fallow and abandonment of cultivated rice fields since the 1970s has significantly changed species composition of birds. It was suggested that various types of wildlife-friendly farming implemented in Japan, such as organic farming, winter flooding, and the installation of shallow ditches (Ye) and fishways, are useful to conserve bird species in rice ecosystem. The knowledge can be useful for members of Ecology and Civil Engineering Society for planning future conservation management of rice ecosystem.