Links between size fractionation, chemical speciation of dissolved copper and chemical speciation of dissolved organic matter in the Loire estuary

Abstract

Environmental contextThe toxicity of copper to aquatic life is highly dependent on its chemical form. In the vineyards of the Loire valley, mixtures of copper compounds are increasingly being used as fungicides. Our study investigating the origin and behaviour of dissolved copper on the land-sea continuum of the Loire advances our understanding of the chemical forms and fate of copper in estuarine systems. AbstractWe present data on the organic speciation of dissolved copper (dCu) in the Loire estuary for the soluble (<0.02µm) and dissolved (<0.45µm) fractions. These results were interpreted according to the distribution of natural organic matter along the estuary. We observed a high concentration of dCu (80nmolkg−1) upstream of the tidal front (S=0.1, where S is the salinity), possibly induced by drainage from vineyards located on the watershed of the estuary, and a concentration typical of coastal seawater at the end of the salinity gradient (3–4nmolkg−1). dCu showed a non-conservative distribution along the estuary with a notably strong decrease in concentration at the very first stage of mixing (S ~ 0.5) that increased again at low salinity (S=4.7). The latter increase in the concentration of dCu was related to the supply of colloidal copper induced by particle desorption in the maximum turbidity zone. The organic speciation of dCu revealed that the complexing capacity of copper ligands (LCu, up to 147 nanomoles of Cu per kilogram of water) exceeded dCu in both the soluble and dissolved fractions, which kept the activity of cupric ions below toxic levels for most unicellular phytoplankton. Humic substances comprised up to 95% of LCu in the continental shelf sample, but their complexing capacity did not account for all of the LCu in the inner estuary. We conducted pseudopolarographic experiments and found two other additional ligand classes: thiol-like and amino-acids/proteins. While humic substances are the dominant component of dissolved organic matter (DOM), the study of DOM suggested the release of colloidal DOM from a wastewater treatment plant. This structure could be a major source of proteinaceous LCu in the system that changes the dCu speciation in the middle of the estuary.