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

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 Klcond, =“z+ = 11 .O) 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 ,OOO-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. The nutrient-type distribution of dissolved zinc in seawater indicates that it is removed from surface waters and transported to depth as a trace constituent of biogenic particles (Bruland et al. 1978; Bruland 1980). The low concentration of zinc in the surface photic zone (N 0.1 nmol kg-‘) led Anderson et al. (1978) to speculate that zinc is a biolimiting nutrient to phytoplankton in the open ocean. Based on the relationship between zinc concentrations in surface waters and zinc requirements of marine phytoplankton and their geographic distributions in the surface ocean, Brand et al. (1983) suggested that zinc (as well as iron Acknowledgments This work was supported by ONR contract NO0