High metabolic zinc demand within native Amundsen and Ross Sea phytoplankton communities determined by stable isotope uptake rate measurements

Abstract. Zinc (Zn) is an essential micronutrient for most eukaryotic phytoplankton. Zn uptake by phytoplankton within the euphotic zone results in nutrient-like dissolved Zn profiles (dZn) with a large dynamic range. The combination of key biochemical uses for Zn and large vertical gradients in dZn implies the potential for rapid rates of Zn removal from the surface ocean. However, due to the ease of contamination at sea, direct measurements of dZn uptake within natural environments have not been previously made. To investigate the demand for dZn and for dissolved cadmium (dCd; a closely related nutrient-like element) within Southern Ocean phytoplankton communities, we conducted ⁶⁷Zn and ¹¹⁰Cd tracer uptake experiments within the Amundsen Sea, Ross Sea, and Terra Nova Bay into the >3 μm phytoplankton particulate size fraction. The highly productive Amundsen Sea and Ross Sea of Antarctica host large phytoplankton blooms in the austral spring and summer, during which macronutrient and micronutrient surface concentrations become significantly depleted largely due to phytoplankton uptake. In autumn and winter, nutrient levels are “reset” to high concentrations throughout the water column in these environments due to convective overturn, advancing sea ice cover, and darkness. This annual “resetting” of nutrient concentrations makes these Antarctic environments ideal locations to study the seasonal demand for Zn within these productive communities.

In this study, variations in metal uptake rates over depth and time and correlations with other oceanic parameters were examined. High total metal uptake rates (ρMetal) of both Zn and Cd were consistent with the observed depletion of dZn and dCd surface concentrations. Our findings suggest that high biomass and low seawater pCO₂ exerted primary control over increasing ρZn, which in turn led to increases in ρCd likely through the upregulation of shared transport systems. Overall, we observed a high magnitude of Zn uptake ( > 100 pmol dZn L^-1 d^-1) into the particulate phase within these Southern Ocean phytoplankton communities, suggesting that even in the Zn-rich waters of the Southern Ocean, high Zn uptake rates can lead to Zn depletion and potential Zn scarcity.