Winter dynamics of phytoplankton and micronutrients in the Southern Ocean

Combined observations of phytoplankton abundance, community structure, and trace metals are limited but crucial for understanding Southern Ocean biogeochemistry. While summer studies exist, winter research is scarce. This study provides a fundamental first overview of phytoplankton-micronutrient (i.e. trace metals) dynamics during early winter in the Indian sector of the Southern Ocean. Depth-resolved measurements were taken for chlorophyll-a, phytoplankton composition, and micronutrients iron (Fe), manganese (Mn), cobalt (Co), nickel (Ni), copper (Cu), zinc (Zn), and cadmium (Cd). Phytoplankton abundance (0.27 - 0.08 µg L-1 chlorophyll-a), and presumably, metal uptake were low in winter. In the Subtropical Zone, cyanobacteria and haptophytes dominated, driven by the interplay of higher temperatures, limiting macronutrients, and trace metals such as Fe and Zn, while also influencing Co and Mn dynamics through biological uptake. In the Subantarctic and Polar Frontal Zones prasinophytes dominated, cyanobacteria were linked to Co uptake, while Mn and Zn deficiency and silicic acid limitation constrained diatoms. In contrast, flagellate groups, such as the dominant prasinophytes, may have compensated through substitution. In the Antarctic Zone, Fe and Mn were the primary limiting nutrients for the dominating diatoms, as silicic acid was no longer limiting. In turn, diatoms strongly influenced Zn particulates through biological uptake, while Phaeocystis and coccolithophores appear to have contributed more to Cd uptake. This winter perspective on phytoplankton-micronutrient relationships shows that phytoplankton continue to influence particulate trace metal distribution despite their low productivity and that there are strong phytoplankton group-specific associations with various essential metals, rather than iron alone, that vary latitudinally.