Environmental contextIdentifying the source and stimuli responsible for organic ligands in seawater is crucial to understanding trace metal availability. Voltammetric techniques were employed to characterise the water chemistry of seaweed cultures exposed to low levels of Cu or Zn over 7 days. The results suggest that seaweeds are a potential source of metal complexing ligands and Cu and Zn appear to stimulate ligand production; further research is required to determine if this is applicable to macroalgae and metals outside this study.It is widely accepted that organic ligands control metal speciation in seawater, although little is known about their source, stimuli or identity. To gain insight on the possible environmental controls of metal complexing ligands (L), three brown macroalgae common in Irish waters (Ascophyllum nodosum, Fucus vesiculosus and Laminaria hyperborea) were cultured under low levels of Cu (0, 7.86 or 15.7nM) or Zn (0, 15.2 or 91.7nM) exposure. Seaweed chlorophyll-a fluorescence (F-v/F-m), metal speciation (Cu, Zn), complexing ligands (L-M), conditional stability constants (logK ' L-M), glutathione (GSH), cysteine (Cys) and seaweed metal contents were monitored over 7 days. Although there was no effect on the internal seaweed metal concentrations, Cu and Zn additions significantly altered the water chemistry of each culture. Metal additions increased the total dissolved metal concentrations for all three species. Significantly higher [L-M] values in cultures with added metals than the relevant controls point to both metals as stimuli of L production. All species released ligands in response to Cu or Zn exposure, indicating each seaweed is a relevant source of L. Comparison of logK ' values to those of previously determined ligands provides little evidence that the ligands reported here belong to the compounds identified as L in the literature.