Speciation of dissolved copper in human impacted freshwater and saltwater lakes

China's natural waters are experiencing an increasingly anthropogenic perturbation widely including acidification and hypoxia, and toxic metals including copper (Cu) are subject to a series of reactions including chemical speciation and transformation. However, there is still little information available regarding such alterations of metal behaviors in China's natural waters. By using solid phase extraction technique, this study for the first time measured total dissolved Cu, and different Cu species: toxic labile Cu (referred to those free cupric ions and some weakly organic compounds adsorbed onto Chelex-100 resins), the organic refractory Cu (referred to those adsorbed onto C18 resins after passing through Chelex-100 resins), and residual Cu (obtained by subtracting labile and organic refractory fractions from the total) in a freshwater lake (the Lover) and a saltwater lagoon (the Yundang) in Xiamen, China. Our results demonstrated that both waters were characterized with relatively low levels of total dissolved Cu (5-10 nM), as a result of a net removal process dominated by particle adsorption and precipitation. Relatively high proportion of organic refractory Cu (as high as 50 %) was observed in the saltwater Yundang lagoon as a result of organic matter production and/or discharges followed by complexation nearby. On the other hand, the toxic labile Cu accounted for >40 % of the total dissolved Cu pool in these waters, and particularly the increased proportion of toxic labile Cu (as high as 70 %) occurred in the bottom sulfidic Lover Lake. Our study provides clear evidence that toxic labile Cu could be transformed under reducing environments such as deep sulfidic waters of the Lover Lake (Xiamen, China), and the releases of toxic labile metals are increasingly threatening nearby aquatic ecosystems.