Complexation of zinc by natural organic ligands in the central North Pacific

Abstract

The complexation of zinc by natural organic ligands in the upper 600 m of the central North Pacific was determined with differential pulse anodic stripping voltammetry at a thin mercury film, rotating glassy‐carbon disk electrode. Of the dissolved zinc in surface waters >98% was bound in strong complexes (log K′cond, Zn2+ = 11.0) by relatively zinc‐specific organic ligands existing at low concentrations (1.2 nM). Although the vertical distribution of this ligand class is relatively uniform, total dissolved zinc varies from concentrations close to 0.1 nM in surface waters of the central North Pacific to 3 nM at 500 m. Complexation of dissolved zinc by this class of organic ligands causes the concentration of inorganic zinc to vary from 2 pM in surface waters to 2 nM at 500 m, a 1,000‐fold variation. The free zinc ion activity, expressed as pZn, is calculated to be of the order of 12.7 in the surface waters of the central North Pacific. This low free zinc ion activity may influence distributions of oceanic and neritic phytoplankton species.