This study adds further critical information to the limited body of knowledge on the ameliorative ability of Australian dissolved organic carbon (DOC), reinforcing the importance of DOC source and concentration as significant factors controlling the risk copper poses to organisms in freshwater systems. The ameliorative ability of five unstudied DOCs on the chronic toxicity of copper to the tropical alga Chlorella sp. was compared. Sensitivity to copper varied dramatically; effect concentrations at the 50 percent effect level (EC 50 ) increased by up to 22-fold in the high DOC treatment compared to controls and more than 2-fold between DOCs at the same concentration. The analytical techniques, diffusive gradients in thin films (DGT) and Chelex column, were used to understand whether differences in copper toxicity could be explained by copper lability. Labile copper mirrored the trends seen in the toxicity tests; lability decreased with increasing DOC concentration and varied between DOCs at the same concentration. The equilibrium model, WHAM VII, was also used to better understand the role of the free copper ion on the toxicity observed. Disagreement between EC 50 values derived using the WHAM-predicted free Cu 2+ concentrations and agreement between DGT-labile and the maximum dynamic concentration (cmax dyn) suggest free copper is not the sole contributor to toxicity and that the source of the specific DOCs also plays a role.