| In natural waters, copper (Cu) complexation by natural dissolved organic matter (DOM) can greatly influence Cu bioavailability to aquatic organisms by binding the free cupric ion (Cu2+), considered the most toxic Cu species. Increasing ultraviolet (UV) radiation may enhance the photochemical mineralization and alteration of DOM, thus altering the bioavailability of Cu. Also, it is unclear whether DOM from disparate sources photoreact or bind Cu similarly. To assess the effects of sunlight on DOM, and subsequently on Cu bioavailability, I photodegraded DOM from two contrasting sources: a lacustrine wetland and a river draining a grassland watershed. This project encompassed: (1) measurement of photochemical changes in DOM (as indicated by dissolved organic carbon (DOC) concentration, spectral slope, organic acid concentration, and fluorescence, which is a correlate of aromatic group content); (2) correlation of those changes with altered Cu complexation, and modeling of changes in cupric ion activity ({lcub}Cu2+{rcub}); and (3) prediction and testing of Cu-induced mortality during toxicity tests with larval fathead minnows (Pimephales promelas). After 72 h of irradiation, Cu-DOM conditional stability constants (K) of wetland DOM decreased while Cu ligand densities ([L]) increased significantly (thus, significantly decreasing {lcub}Cu 2+{rcub} per mg DOC at the same total Cu concentration) and varied positively with aromatic group content. In contrast, irradiation did not change the affinity of riverine DOM. However, decreases in [L], which significantly increased {lcub}Cu2+{rcub} per mg DOC, were strongly correlated with fluorescence, possibly indicating decreased aromatic group content. In toxicity tests, irradiated and unirradiated DOM had the same protective effect.{09}These results suggest that wetland and riverine DOM do not photodegrade similarly, that fluorescence is not predictive of Cu-binding affinity, and that some Cu-DOM complexes in irradiated the wetland DOM or unirradiated riverine DOM enhanced the toxicity of Cu beyond that due to {lcub}Cu2+{rcub} alone. |
