Dissolved zinc isotope cycling in the Tasman Sea, SW Pacific Ocean

The trace metal micronutrient zinc (Zn) act as a cofactor in several essential enzymes within phytoplankton. The uptake of Zn by phytoplankton is complex and depends on the concentration and chemical speciation of dissolved Zn; both of which affect Zn bio-availability. In some parts of the surface ocean dissolved Zn concentrations are sufficiently low, in the picomolar range, to potentially limit the growth of certain phytoplankton species. Here we investigated the relationship between Zn availability and primary production using the dissolved Zn isotopic composition of samples collected across high and low productivity waters in the Tasman Sea, SW Pacific Ocean. Maximum variability in the isotope composition of dissolved Zn was observed in the upper ocean (0–200 m) at two stations where productivity was dominated by eukaryotic phytoplankton. At these stations, heavier δ⁶⁶Zn values tended to coincide with the chlorophyll maxima indicating preferential uptake of lighter Zn isotopes by phytoplankton. A significant correlation was obtained between the dissolved Zn isotopic composition and the relative fluorescence in the upper 200 m strengthening the observation that isotope fractionation occurs during Zn uptake by phytoplankton. Immediately below the chlorophyll maxima, the isotope composition of dissolved Zn became lighter indicating the preferential regeneration of isotopically lighter Zn from sinking particulate organic material. A seasonal change in the Zn isotope signal was observed for a north Tasman Sea station with heavier Zn isotope values appearing to be coupled to an increase in biological production during the austral autumn. Based on the biological data, the ligand data (from a companion study) and the δ⁶⁶Zn values, we suggest that the composition of the resident phytoplankton community controls Zn isotope fractionation and Zn speciation in the upper water column for the Tasman Sea.