Riparian plants can use nitrogen (N) from soil and river water, but the use of river water N might be limited in higher floodplain environments of the Chikuma River. The purpose of this study is to reveal the relationship between N uptake by riparian plants and the floodplain topography (relative height and distance from a river channel). We examined the hypothesis that surface sediment removal from the higher floodplain increases river water N uptake by riparian plants by using a stable isotope analysis. The d 15 N value of river water samples (ca. 8%) were significantly higher than those of the soil extracts (ca. 3%) in the study area. The d 15 N value of riparian plants increased from +3.0% (standard deviation, SD ±2.1%) before sediment removal to +9.6% (±2.1%) after sediment removal, although there was no significant change in the d 15 N value in N sources of soil and river water. The sediment removal enhanced frequency of flood disturbance, relative ground water level, and river water N uptake by riparian plants on the floodplain.

Agricultural channels can be important habitats for aquatic species inhabiting agricultural areas. However, water supply in agricultural channels can vary substantially with types of agriculture and irrigation/drainage system, and thereby affect the viability of local populations of aquatic species. In this study, we focused on the amphidromous freshwater atyid shrimp Caridina leucosticta. To clarify whether C. leucosticta utilizes agricultural channels as a habitat in relation to fluctuations in the water supply, we conducted a series of field surveys on the distribution of this shrimp in a rice paddy drainage channel (spring-fed channel) around the Furu River, central Japan, during July and December 2008. Large numbers of C. leucosticta had migrated into that drainage channel. Water from the adjacent rice paddies had disappeared by October, because irrigation was stopped, but the water level in the drainage channel was maintained by a supply of spring water. By mid-November the supply of spring water had almost disappeared, then the channel nearly dried up, and many shrimps were concentrated in an isolated water pool in the channel. Thus, habitat fragmentation of C. leucosticta occurred in terms of water (dis-)continuity. This water pool dried up completely by mid-December. We found many dead individuals of C. leucosticta in the dried-up area of the channel. These results indicate that agricultural channels can be an important temporal habitat for amphidromous freshwater shrimps, but that the drying-up of the channels can lethally affect migrating atyid shrimps. RÉSUMÉLes canaux agricoles peuvent être des habitats importants pour les espèces aquatiques rurales. Cependant l'alimentation en eau des canaux agricoles peut varier substantiellement avec le type 4 )

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To conduct the water quality improvement properly, it is necessary to estimate the dynamics of pollutant loads at watershed scales. We have attempted to quantify the watershed information by GIS to estimate the relation between water quality and watershed imformation, and carried out the field observation and sample collection , and then we have analyzed the run-off of NO3-N in Chikuma Basin. As a result , it became clear that the land use of watershed had significant influence on NO3-N dynamics in the river. Since, the amount of NO3-N from the vegetable field changes , NO3-N in the river water changed seasonally. Also, nitrogen stable isotope ratio increased in downstream of the river , where N discharge from city area increase.