Qualitative and quantitative analysis of organochlorine insecticide residues were conducted in water, sediment, aquatic plants and animals from 3 freshwater reservoirs. Ninety water samples, 90 sediment samples, 397 aquatic plants samples and 121 aquatic animals samples were collected during March-April and August-September 1989. There were about 14 kinds of aquatic plants, such as water hyacinth, water lettuce, algae and water lilly etc. Aquatic animals are fish, shrimps and snails, but most of them are fish. They are striped snake-head fish, carp and catfish etc. Total samples of 698 were analyzed by using gas chromatographic techniques at Agricultural Toxic Substances Division. The results indicated that 5 kinds of insecticides were found in most samples. They were lindane, heptachlor, aldrin, dieldrin, DDT and derivatives. Dieldrin was detected at higher concentration and found in all samples. The residue level of dieldrin ranged from 0.01-0.12 ppb in water, 0.005-0.036 ppm in sediment, 0.001-0.138 ppm in aquatic plants and 0.001-0.037 ppm in aquatic animals respectively. The accumulation of dieldrin residue in water and aquatic plant samples from all reservoirs are not different but the level of dieldrin residue in sediment samples from Bueng Boraphed is higher than the other 2 reservoris. However the levels of dieldrin residue in sediment, aquatic plant and aquatic animal samples higher than those in water samples respectively.

For the purpose of examining on role of irrigation and cultivation system to paddy ecosystem, we study species and number of Odonata larvae and analyze the stable isotope ratios of the Odonata larvae and the plankton/detritus in water of paddy fields and irrigation ponds. As a results, cultivation system clearly effects to composition of species and number of Odonata larvae. delta sup(13)C indicates that Orthetrum albistylum speciosum larvae and Coenagrionidae spp. larvae which are dominant species in paddy water, depend on specified food sources, each other. delta sup(15)N shows that the trophic level of Odonata larvae is higher than that of the plankton/detritus in paddy water. It is considered that analyze the stable isotope ratios application is effective to examine water ecosystem and food web in paddy.