Abstract
Recently, the stable isotope analysis is recognized as analytic methodology for examining food web. Until now, however, this method has scarcely been applied to organisms in paddy field areas. In this study, this method was utilized to study the food web in small ditches used for irrigation and drainage within hill-bottom paddy fields. The authors investigated small ditches three times (early spring, early summer, and autumn) in 2004, and measured the carbon and nitrogen stable isotope ratio of ecosystem components ranging from substances at low trophic level to fishes at high trophic level. The results obtained were as follows; 1) throughout the year, the food web of ditches was based on terrestrial C3plants or particulate organic matter (POM: Particulate Organic Matter) generated by C3plants as trophic level one substance, and the food web was subject to nutritive supply out of the ditches. 2) in terms of food chain system, explicit differences between paddy fields and ditches were observed, and 3) a food chain system based on algae (Batrachospermumsp.) was observed in the ditched in early spring. From these results, it was considered that the stable isotope analysis can be used effectively to analyze the food web in small ditches used for irrigation and drainage systems.
