Environmental contextMetal toxicity to marirne organisms is largely controlled by organic ligands (L) although the source, structure and stimuli of most ligands remain unknown. We studied trends in Cd and Zn complexing ligands in a near-pristine Irish estuary to gain clues on the identity of L in natural waters. The evidence suggests the naturally occurring Cd ligands include fulvic acids whereas the Zn ligands are likely to be exuded from seaweeds. Further research is required to determine their exact identity.Trace metal interactions with organic ligands largely control metal bioavailability in marine systems, although little is known about the identity of the ligands. To gain insight into the identity of metal complexing ligands (L), surface water and four species of brown seaweed were sampled from four sites in a region of the Lough Furnace Estuary, Ireland with varying salinity. We measured metal (Cd, Cu, Pb, Zn) speciation, complexing ligands, stability constants (log K '), glutathione (GSH), cysteine (Cys) and seaweed metal contents. Although prevalent in seaweed tissue, dissolved Cu and Pb concentrations in water were below the detection limits. Both Cd and Zn occurred in seaweed tissue. In water, both Cd and Zn occurred predominantly complexed to ligands. Levels of complexed Zn increased with decreasing salinity, increasing from 77% at high salinity to 100% at low salinity. Total dissolved Cd showed a mid-salinity decrease in complexation. The concentration of zinc ligands (L-Zn) ranged from 27.41nM at high salinity to 95.81nM at mid-salinity. Cd ligands (L-Cd) occurred in the highest concentration, 8.72nM, at mid-salinity. Comparison of the log K ' values with known ligands provided evidence of their identity: similarities were identified for L-Cd and fulvic acid, and L-Zn and macroalgal exudates.