Effect of salinity on acute copper and zinc toxicity to Tigriopus japonicus: The difference between metal ions and nanoparticles

•Increasing salinity reduced acute toxicity of Cu and Zn due to precipitation.•Increasing salinity decreased dissolution of CuO and ZnO NPs.•Increasing salinity enhanced aggregation of CuO and ZnO NPs.•Therefore, increasing salinity reduced acute toxicity of CuO and ZnO NPs. We investigated the effects of salinity (5‰, 15‰, 25‰ and 35‰) on metal ion (Cu and Zn) and nanoparticle (NP) CuO and ZnO toxicity to Tigriopus japonicus. Increasing the test media volume without renewal increased the 96-h LC50 for Cu (32.75mgL−1) compared to the reported value (3.9mgL−1). There was no significant difference in acute toxicity at different salinities between acclimated and unacclimated T. japonicus (p>0.05). Increasing salinity decreased the dissolved concentrations of Cu and Zn ions due to the precipitation of the metal ions, consequently reducing the acute toxicity to T. japonicus. The effect of salinity on acute CuO and ZnO NP toxicity was similar to that on metal ion toxicity. Since the aggregation of NPs generally enhanced at higher salinities, both the dissolution and aggregation of CuO and ZnO NPs may control the effect of salinity on acute toxicity to T. japonicus.