Abstract
We compared macroinvertebrate assemblages between two sites in the Otani River, namely, upstream and downstream from the treated wastewater outlet of the Ikuno silver mine. The concentrations of heavy metals (i.e., copper, zinc, cadmium and lead) at the upstream site were remarkably high, based on the water quality criteria of the US Environmental Protection Agency for the protection of aquatic organisms. At the downstream site, the concentrations of heavy metals were lower by 60-80%. The water hardness was approximately 6 times higher at the downstream site than at the upstream site. Thus, it appeared that the toxicity of heavy metals to macroinvertebrates was reduced after the discharge of treated water. However, we observed no significant changes in most variables, indicating species richness and the abundance of macroinvertebrates. At the downstream site, heavy-metal concentrations were not reduced and/or there could have been some adverse effects of physicochemical changes, so that assemblage diversity and abundance cannot increase. We observed thick slime deposition on the riverbed at the downstream site, which is likely to have significant adverse effects on macroinvertebrates. Because such slime deposition is often observed in several rivers that received mine treated wastewater effluent, we should clarify its effects on aquatic organisms for the conservation of the river ecosystem around abandoned mines.
