Effects of silver and zinc on tropical freshwater organisms: Implications on water quality guidelines and ecological risk assessment

Tropical freshwater ecosystems comprise valuable habitats and important biodiversity. There is an imperative need to build a framework for establishing appropriate water quality guidelines of chemical contaminants in order to safeguard freshwater ecosystems in the tropics. Silver (Ag) and zinc (Zn) are known to be toxic to freshwater organisms at elevated concentrations. However, toxicity data of tropical freshwater species for these metals are lacking. To address this issue, we first conducted standard acute toxicity tests of each of the metal using eight tropical freshwater species based on measured concentrations in test solutions. Subsequently, we tested the null hypothesis that there are no differences in species sensitivities towards Ag and Zn between tropical and temperate species by comparing their corresponding species sensitivity distributions (SSDs). The results of SSD comparisons indicated that tropical species were more acutely sensitive to their temperate counterparts to Ag, while temperate species were slightly more sensitive towards Zn. Temperate-to-tropic extrapolation factor of 5 was recommended for Ag, while acute-to-chronic ratios (ACRs) of 100 and 10 were advocated for Ag and Zn, respectively. Through comparison of taxon-specific SSDs, we observed that crustaceans were the most sensitive taxon to Ag and Zn, and thus suitable for toxicity testing and risk assessment. Based on our results, concentrations of 0.004 and 6.4 μg L⁻¹ (at hardness of 50 mg L⁻¹ CaCO₃) were recommended as interim chronic predicted no-effect concentrations for Ag and Zn, respectively for protecting freshwater ecosystems in tropical regions.