Sensitivity of a tropical micro-crustacean (Daphnia lumholtzi) to trace metals tested in natural water of the Mekong River.

Metal contamination is one of the major issues to the environment worldwide, yet it is poorly known how exposure to metals affects tropical species. We assessed the sensitivity of a tropical micro-crustacean Daphnia lumholtzi to three trace metals: copper (Cu), zinc (Zn) and nickel (Ni). Both, acute and chronic toxicity tests were conducted with metals dissolved in in situ water collected from two sites in the lower part of the Mekong River. In the acute toxicity test, D. lumholtzi neonates were exposed to Cu (3-30μgL), Zn (50-540μgL) or Ni (46-2356μgL) for 48h. The values of median lethal concentrations (48h-LC) were 11.57-16.67μg Cu L, 179.3-280.9μg Zn L, and 1026-1516μg Ni L. In the chronic toxicity test, animals were exposed to Cu (3 and 4μgL), Zn (50 and 56μgL), and Ni (six concentrations from 5 to 302μgL) for 21days. The concentrations of 4μg Cu L and 6μg Ni L enhanced the body length of D. lumholtzi but 46μg Ni L and 50μg Zn L resulted in a strong mortality, reduced the body length, postponed the maturation, and lowered the fecundity. The results tentatively suggest that D. lumholtzi showed a higher sensitivity to metals than related species in the temperate region. The results underscore the importance of including the local species in ecological risk assessment in important tropical ecosystems such as the Mekong River to arrive at a better conservational and management plan and regulatory policy to protect freshwater biodiversity from metal contamination.