Wintertime trace metal (Zn, Cu, Ni, Cd, Pb and Co) and nutrient distributions in the Subantarctic Zone between 40-52°S; 155-160°E

Abstract

In high nitrate low chlorophyll (HNLC) oceanic regions, trace metals play a key role in regulating phytoplankton productivity; however large knowledge gaps remain in the biogeochecmical cycling of trace metals in these areas. Here I present the first comprehensive winter dataset (0-1000 m depth) for dissolved zinc, cadmium, copper, nickel, cobalt, and lead along with the macronutrinets phosphate and silicon, between 40°S-52°S, which traverses the Subantarctic zone (SAZ), east of Australia. Dissolved lead concentrations were conservative with depth with values ranging between 12 and 33 pmol kg- 1. Surface dissolved concentrations for cadmium, copper, nickel, cobalt and phosphate showed large north-south gradients, with lower concentrations at the Subtropical Front (∼ 43°S) and higher concentrations at the Subantarctic Front (∼ 51°S). Profiles of dissolved cadmium, copper, nickel and cobalt concentration increased with depth in a manner similar to that of phosphate. Dissolved cadmium was strongly correlated to phosphate at concentrations greater than 0.74 μmol kg- 1; below this concentration cadmium was depleted (< 40 pmol kg- 1). The depletion of cadmium before that of phosphate is consistent with the hypothesis that the 'kink' in the cadmium-phosphate relationship is linked to iron limitation in Southern Ocean waters. For zinc and silicon, surface dissolved concentrations were low across the transect. Profiles of dissolved zinc and silicon concentration increased with depth and were similar in nature, indicating a close coupling between these two elements. The coupling of zinc and silicon implies that diatoms may control the global zinc-silicon relationship. These results also suggest that mode water and intermediate waters, which are important conduits for the export of silicon depleted waters to low latitudes, play major roles in regulating the concentration of zinc in the main thermocline outside of the Southern Ocean.