Copper toxicity to blue mussel embryos (Mytilus galloprovincialis): The effect of natural dissolved organic matter on copper toxicity in estuarine waters

Abstract

Copper (Cu) is a naturally occurring micronutrient of eco-toxicological concern in aquatic ecosystems. Current knowledge of Cu-speciation and bioavailability in natural saline environments is insufficient to adequately inform environmental protection policy for estuarine systems. We assessed the combined effect of two of the main drivers of metal bioavailability, salinity and natural dissolved organic carbon (DOC), on Cu-speciation and associated Cu-toxicity to blue mussel (Mytilus galloprovincialis) embryos in a standard 48-h bioassay. We placed special emphasis on measurement of Cu-speciation rather than modelling. Cu-toxicity was found to be a function of DOC and salinity. The varying protective effect of different DOC-types suggests that estuarine DOC is more protective against Cu-toxicity than oceanic DOC. Salinity was negatively correlated with [Cu48-h-EC50], indicating a salinity-induced alteration in the physiology of the exposed mussel embryos and/or Cu-DOC-reactivity. These two assumptions were supported by (1) the relative uniformity of bioavailable copper ([Cu′]) across similar salinity treatments despite considerable variation in [Cu48-h-EC50] and DOC-concentrations, and (2) the fact that Cu-toxicity and [Cu′] were slightly higher in the 35 salinity treatment compared to the 25 salinity treatment. Stripping voltammetry studies determined the presence of only one strong Cu-binding ligand class (i.e., L1), either actively or passively released by the exposed embryos. [L1] was found to be proportional to the total dissolved Cu-concentration ([CuT]), suggesting a protective effect of Cu-binding-ligands, in addition to the protective effect of DOC. There was also a strong positive correlation between [L1] and [Cu48-h-EC50], implying that electrochemically defined ligand concentrations along with measurements of [Cu′], DOC-quality, and salinity can be used as proxies for 48-h-EC50 Cu-values in estuarine waters, which may result in a significant improvement to risk assessments of Cu in estuarine systems.