Laboratory experiments have established the importance of complexation by organic ligands in determining the bioavailability of trace metals to marine phytoplankton, while electrochemical measurements with field samples have demonstrated that a large fraction of bioactive trace metals are complexed to strong organic ligands in seawater. Using the model organic ligands, EDTA and histidine, we show a quantitative correspondence between the bioavailability of Zn to the diatom Thalassiosira weissflogii, and its reduction at -1.2 V (vs Ag/AgCl) on a hanging mercury drop electrode. Equilibrium calculations and polarographic data indicate that Zn bound in inorganic complexes and the 1:1 Zn-histidine complex, but not in the 1:2 Zn-histidine complex or the Zn-EDTA complexes, is taken up by the organism and reduced at the electrode surface, confirming a previous report of the bioavailability of weak Zn complexes. Electrochemical measurements of Zn speciation in seawater do not generally reveal the presence of weak (and potentially bioavailable) complexes; but such measurements (particularly by Anodic Stripping Voltammetry) should nonetheless often provide good estimates of the bioavailable Zn concentrations. These results can likely be generalized to other bioactive divalent trace metals.