Using the exposure-dose-response framework to formulate the lines of evidence for sediment ecological risk assessment

Abstract

The Lines of Evidence for Sediment Ecological Risk Assessment were investigated using the Exposure-Dose-Response framework to determine the impairment of estuarine sediments contaminated by copper, cadmium, lead and zinc. The Exposure-Dose-Response framework assesses environmental responses to contaminants as a function of the exposure as well as a function of the dose. Data were collected from around the world and from the Australian coastal lagoon Lake Macquarie on metal concentrations in sediments, molluscs, crustaceans and fish and on their responses, using toxicity tests and biomarkers as well as benthic community response, and used to investigate Exposure-Dose-Response relationships. The datasets from around the world showed that investigations of Exposure-Dose and Dose- Response relationships for trace metals in the sediment environments were more effective when conducted with organisms that live buried in sediments. Exposure-Response relationships showed high variability and the unknown influences of external confounding factors made results difficult to interpret. Piecewise regressions, used to investigate relationships between metals in sediments and benthic community indices, identified thresholds in the metal concentrations that led to a change in the benthic community indices. Relationships obtained using crustaceans and fish data were often not significant. These data showed that why a contaminant is of concern, what organisms is most appropriate to use to investigate metal contaminated sediments, whether it is more appropriate to relate to response total metal or its bioavailable fraction in the organism, and how to choose response endpoints and the way to use the Lines of Evidence need to be considered. Investigations of Lake Macquarie datasets, in which external influences are relatively known, reinforced the previous conclusions that the Exposure-Dose-Response relationships are best addressed using sediment-dwelling organisms. Investigations of Exposure-Dose, Dose- Response and Exposure-Response showed that Exposure-Response is possibly the least informative step. Exposure-Dose and Dose-Response provide information on the ability of the organisms to take up metals from the sediments and thus information whether the contaminant is a concern for the impairment of the sediments. Exposure-Dose and Dose- Response provide the link the response to the accumulation of the contaminant. Results obtained with the cockle Anadara trapezia showed its suitability for the investigations Exposure-Dose-Response relationships in sediments. Further, the subcellular fractionation identified the mitochondrial fraction as the site of effect. A meta-analysis was used to investigate the effect size of all data still using the Exposure- Dose-Response relationships. Again, this highlighted the significant differences among molluscs, crustaceans and fish in the Exposure-Dose relationships as well as the significantly different results obtained using different reference thresholds for both Exposure-Dose and Exposure-Response. Benthic community indices from Lake Macquarie, which had previously shown to follow the pattern identified by the highly contaminated datasets from around the world, showed significantly different results from the other datasets when pooled in the meta-analysis possibly because of external chemical-physical factors which acted upon the structure of the benthic community. The investigation of the Lines of Evidence for metal contaminated sediment using the Exposure-Dose-Response framework showed that dose should be used as the causal link between exposure and response when conducting a sediment ecological risk assessment, avoiding confounding factors due to the complexity of the environment which influence bioavailability and uptake, and thus extrapolation between contaminants in the sediments and organism response.