Salt-marsh areas as copper complexing ligand sources to estuarine and coastal systems

► Tagus waters show a constant (20 nM) copper concentration during the estuarine mixing. ► Most of it complexed by a strong ligand (L1): 19-55 nM and a log K′ (14.14-15.75). ► L1/Cu ratios are constants and close to 1 indicating the same source. ► Salt-marsh areas are important and continuous sources of copper complexing ligands. ► This input determines copper distribution along the salinity gradient in Tagus waters.