The Mekong River is one of the world's greatest river systems and sustains human life and ecosystems. The livelihoods of 60 million people who live along the Lower Mekong Basin (LMB) rely on both the economic resource and the ecological health of the river. In this study, US EPA method was used for the acute toxicity with different water dissolved organic carbon (DOC) of Mekong River in Cambodia on Chironomus javanus and fish Nile tilapia (O. niloticus) to modify the effecting of DOC on copper

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... oxicity. Both C. javanus and Nile tilapia were significantly less sensitive to copper in water high DOC (5.74 mg/L DOC), compared to water low DOC (1.12 mg/L DOC) exposures. The effect of DOC, as humic acid source on the acute toxicity of copper (Cu) to C. javanus and Nile tilapia also was investigated. The mortalities for both species increase with increasing copper concentration, but LC 50 value decreased as more toxic on Nile tilapia and C. javanus. This gave an order of toxicity of copper in water with low DOC > water with high DOC at the end point of LC 50 . DOC might provide protection against Cu toxicity in the freshwater in term of completive between copper form and DOC. The result of the LC 50 with 95% confidence limit obtained at 48 hr in tap water on Moina macrocopa, C. javanus, Grass Carp (Ctenopharyngodon idella) and Nile tilapia were 12 µg/L, 16399 µg/L, 118 µg/L and 1383 µg/L, respectively. This gave an order of toxicity of copper in tap water with M. macrocopa> Grass Carp > Nile tilapia> C. javanus. Also, it could be noted that Moina was the most sensitive followed by Grass Carp, Nile tilapia, and C. javanus to copper. Present study indicated that water chemistry parameters can influence on copper toxicity to tropical freshwaters biota. Exposures in this series of laboratory experiment will provides a worst-case scenario and useful for determine the risk assessment of copper on Mekong tropical freshwater animals.