Toxicity of tri- and penta-valent arsenic, alone and in combination, to the cladoceran Daphnia carinata: the influence of microbial transformation in natural waters.

The acute toxicity of arsenic(III) and arsenic(V) alone and in combination to a cladoceran, Daphnia carinata, was studied in both cladoceran culture medium and natural water collected from a local suburban stream. As(III) was found to be more toxic than As(V) to Daphnia survival. The LC(50) values for As(III), As(V), and As(III) + As(V) were 0.554, 1.499, and 0.692 mg l(-1), respectively. Although various species of As, particularly As(III) and As(V) co-exist together in natural waters, the existing guidelines for water quality are based on individual As species. The results of this investigation suggest that As(III) and As(V) can interact either synergistically or additively resulting in an increase in the overall toxicity of the mixture compared to individual As species. Also, indigenous microorganisms in natural water may play a significant role in the transformation of As, thereby influencing the toxicity of As in receiving waters. This study clearly suggests that the joint action of As species should be considered in the development of water quality guidelines. To our knowledge this is the first study on the interactive effect of As(III) and As(V) to a cladoceran. Thus, this study suggests that these two species of As, when present together above 0.1 mg l(-1) concentration, are toxic to fresh water invertebrates; therefore, pollution with these compounds may adversely affect natural ecosystems.