Jean C. Loock

Combined observations of phytoplankton abundance, community structure, and trace metals in winter are scarce but crucial for understanding Southern Ocean biogeochemistry. This study provides the first early winter insights of phytoplankton‐micronutrient dynamics in the Indian sector of the Southern Ocean. Depth‐resolved measurements of chlorophyll‐ a , phytoplankton composition, and micronutrients iron (Fe), manganese (Mn), cobalt (Co), nickel (Ni), copper (Cu), zinc (Zn), and cadmium (Cd) reveal low phytoplankton abundance (0.27–0.08 μg L ⁻¹ chlorophyll‐ a ) and variable phytoplankton–micronutrient dynamics between the zones. In the Subtropical Zone, cyanobacteria, and haptophytes dominated, driven by higher temperatures, limiting macronutrients, Fe and Zn, and shaped Co and Mn through biological uptake. In the Subantarctic and Polar Frontal Zones, cyanobacteria contributions declined but continued to play a key role in shaping Co dynamics. Diatoms were limited by Mn, Zn, and silicic acid, enabling the dominance of prasinophytes. Other groups such as coccolithophores may have benefitted from the ability to substitute Zn for Cd and contributed to the formation of particulate Cd. Coccolithophores along with Phaeocystis also contributed to particulate Cd in the Antarctic Zone, where Fe and Mn limited the abundance of the dominating diatoms. Diatoms, in turn, strongly contributed to particulate Zn. This perspective on phytoplankton‐micronutrient relationships shows that despite their low productivity in the harsh low light wintertime Southern Ocean, phytoplankton continue to influence trace metal distribution and that there are strong phytoplankton group‐specific associations with various essential metals, rather than Fe alone, that vary latitudinally.

Zinc (Zn) is vital to marine organisms. Its active uptake by phytoplankton results in a substantial depletion of dissolved Zn, and Zn bound to particulate organic matter replenishes dissolved Zn in the ocean through remineralization. However, we found that particulate Zn changes from Zn bound to phosphoryls in cells to recalcitrant inorganic pools that include biogenic silica, clays, and iron, manganese, and aluminum oxides in the Southern Ocean water column. The abundances of inorganic pools increase with depth and are the only phases preserved in sediments. Changes in the particulate-Zn speciation influence Zn bioavailability and explain the decoupling of Zn and phosphorus and the correlation of Zn and silicon in the water column. These findings reveal a new dimension to the ocean Zn cycle, implicating an underappreciated role of inorganic Zn particles and their impact on biological productivity.