**2.1 Test organism culture**

268 Pesticides in the Modern World - Risks and Benefits

This is attributed to be relatively large amounts pesticides applied in paddy field, in addition to common practice of draining the paddy water in draining canals (Tejada, 1995). Around 95 % of freshwater in Thailand is withdrawn to irrigate the more than 5 million hectares of irrigated agriculture. Waste water from this activity may pose significant environmental hazards for aquatic ecosystem in particularly aquatic biota. Furthermore, this contaminate affect wildlife species ether by direct exposure or through bioaccumulation in food web. Pesticide contamination sites associated with paddy field activities may pose significant environmental hazards for terrestrial and aquatic ecosystems. They are important sources of agro-sourced pollution and may result in ecotoxicological effects, particularly following transfer of irrigation waters following use. Ecotoxicological effects occur at all trophic levels, from the molecular to the ecosystem level and effects may be observed via biomonitoring

Pesticide monitoring is traditionally based on evaluations of individual pesticides identified through chemical analyses. A variety of techniques may permit an examination of actual pesticides, herbicides and their metabolites that are present (Iwai et al., 2007). These techniques are based on sampling approaches that use concentration following collection or during collection. Although these techniques still are not able to show the direct response that ecotoxicity gives, they do give an indication of what is inducing the response of the organism. However, chemical analyses obviously do not reveal complex interaction phenomena and polar degradation products are often missed. In contrast to the use of chemical analyses, the ecotoxicity bioassay approach integrates the biological effects of all compounds present and factors such as bioavailability, synergism, or antagonism are

Ecotoxicological assessment of pesticides in paddy field are therefore expected to give a more comprehensive indication of environmental effects. The use of ecotoxicological assessment to evaluate the impact of pesticide residues in the paddy field is strongly recommended in order to have a more direct and integrated estimate of environmental impact. In fact, biological response to a complex mixture of chemicals integrates different factors such as pH and

Pesticides contamination associated with paddy field has been increased a big concern in Thailand. For risk assessment study on the impact of pesticides on aquatic environments that surrounding area, information about effect of pesticides on local species were limited, especially the ecotoxicological data on aquatic organism in Thailand, and it unknown, whether ecological effects test guideline developed elsewhere in the world (US. EPA, ATSM etc) may be use in Thailand. Countries located in the tropical zone rely, mostly, on data from temperate countries about ecotoxicity data. However, this data may be not suitable for tropical countries. Due to the difference organisms species, temperature, rainfall, and agriculture practices that might greatly influence pesticides behavior (Abdullah et al., 1997) and toxicity of pesticides on organisms. Considering the climate adaption of tropical species, assessment of effects of pesticide use on local ecosystem should be performed with local species since their sensitive to toxicants may differ considerably form temperate organism (Domingues et al., 2007). Differential response of organism representing diverse physiological capabilities and niches in aquatic system can help focus field studies where

Therefore, Thailand need ecological effects test guideline, this guideline typically derived data on toxicological response of local organism to environmental contaminant. The toxicity test is procedure that involves the exposure of organism to complex environmental sample under

controled condition to determine if adverse effects have occurred (Edmondson, 1959).

with both individual organisms and the ecosystem function and structure.

solubility, antagonism or synergism, and the bioavailability of substances.

nontarget effect due to off - site movement of pesticides are suspected.

reflected directly in the bioassay results.

*The Moina micrura* obtained from Fisheries Research Institute, Khon Khaen (Khon Khaen, Thailand) and have been maintained in cultured under control laboratory conditions in Ecotoxicology and Environmental Sciences Laboratory, Faculty of Agriculture, Khon Kaen University, Thailand. The culture was incubated at 25± 2 0C with 16:8 h light:dark photoperiod. *M. Micrura* were cultured using moderately hardwater and fed on singlecelled green alga, *Chlorella vulagaris* from axenic culture*.* The medium, used for zooplankton, as well as for experiments, was tap water at the Faculty of Agriculture, Khon Kaen University, Thailand. Water was filtered by using 0.45µm polymembrane filter. Dissolved oxygen concentration was between 5-7 mg/L and pH was 7-8. The culturing period for one generation was 2 weeks before testing.
