Zinc and cadmium speciation in subantarctic waters east of New Zealand

Abstract

Measurements of zinc and cadmium complexation by natural organic ligands in subantarctic waters east of New Zealand are presented for four separate cruises. Total dissolved zinc and cadmium concentrations are in the low picomolar range, while chemical speciation results indicate that a significant portion of both metals is bound to natural organic ligands. Ligand concentrations range between 1 and 2.5 nmol/kg for both metals. Conditional stability constants (log K ML,M′′) range between 9.75 and 10.28 for zinc and 9.82 and 10.93 for cadmium. Titration of crossflow filtered (100-kDa cutoff) seawater indicates that the zinc complexing ligand is truly dissolved and not associated with colloidal organic matter. Complexation of zinc and cadmium by natural organic ligands lowers inorganic zinc and cadmium concentrations to low picomolar (0.2–23 pmol/kg) levels within the mixed layer. Based that laboratory culture data, such low inorganic metal concentrations indicate that marine phytoplankton in this oceanic regime are close to levels that might limit growth. Results from an incubation experiment designed to investigate potential zinc limitation of the algal community showed that zinc additions had little affect on growth rates, chlorophyll a production and nutrient drawdown over that of the control. Rather, only ambient seawater spiked with iron showed an increase in growth, chlorophyll a production and nutrient drawdown over that of the control. These results suggest that this oceanic region is not zinc-limited, but is iron-limited. Interpretation of incubation results along with chemical speciation results suggests that phytoplankton in subantarctic waters either have a low requirement for zinc, lower than algae grown in laboratory culture experiments, or that these organisms are comfortably able to use other metals, such as cadmium and cobalt, to satisfy their metabolic requirements, or that they can utilize organically bound zinc.