Proposal of integrative scores and biomonitor selection for metal bioaccumulation risk assessment in mine-impacted rivers

Abstract

Development of sound criteria for metal and metalloid bioaccumulation risk assessment in river basins affected by mining activities is a necessary tool to protect the aquatic communities. The aim of this study is to propose integrative scores for tissue residues that are suitable for surveillance programs and readily interpreted in terms of risk assessment in mining impacted rivers. Tissue residues of 7 trace metals and 2 metalloids were measured in ten macroinvertebrate taxa from the Nalón River basin (Spain), affected by Hg, Cu and Au mining activities. Compared with reference sites, biomonitor taxa from Hg and Au mining districts showed the highest bioaccumulation. However, low or non-significant bioaccumulation was found in sites influenced by historical Cu mining. Multivariate analyses (ANOSIM) performed on individual taxa revealed significant differences in tissue residues between sites classified according to their ecological status. The bioaccumulation risk assessment was based on the average ratio of the actual metal tissue residues in each macroinvertebrate taxon to the corresponding Ecological Threshold tissue concentration (Tissue residue Ratio to Threshold, TRT). The suitability of the biomonitors was evaluated using linear regression models fitted to the relationships between TRT scores and site sediment pollution or ecological status scores. Biomonitor selection also considered differences in invertebrate functional traits, which can influence metal and metalloid bioavailability. Site bioaccumulation risk was assessed on an Integrated Tissue concentration score (INTISS), calculated over a selection of the most relevant chemicals (As, Cu and Hg) and 3 biomonitor taxa (Baetidae, Hydropsychidae, Microdrile oligochaetes) comprising a set of feeding styles. Based on INTISS, it was possible to predict community alteration scores, using linear regression models. A comparison of site bioaccumulation and ecological status assessments based on the departure from reference conditions showed that operational monitoring programs in basins impaired by mining can be optimized by combining both approaches.