Coupling Between the Subantarctic Seasonal Iron Cycle and Productivity at the Southern Ocean Time Series (SOTS)

Abstract In the Subantarctic Southern Ocean, primary productivity is predominantly limited by seasonal changes in light and iron (Fe) availability, shaping the phytoplankton community and impacting the magnitude of the biological carbon pump. However, quantifying the seasonal iron cycle is challenging, as observations of bioavailable, dissolved iron (DFe) from individual campaigns rarely span a full seasonal cycle. Here, we present a composite seasonal cycle constructed from 27 years of DFe observations at the subantarctic Southern Ocean Time Series (SOTS) south of Australia. Iron measurements are paired with time series data to explain the iron cycle contextualized to broader Southern Ocean biogeochemistry. Three distinct phases were revealed with clear coupling between iron and productivity in the first two phases. In the first phase, light limitation initially controls spring to summer primary production with shoaling of the mixed layer, accounting for around half of annual net community production (ANCP). In the second phase and remaining half of ANCP, rapid biomass increases and near‐complete drawdown of DFe drive iron limitation, evidenced by maximum fluorescence‐to‐chlorophyll ratios. A subset of this period covering a third of ANCP exhibits a mean Fe:C uptake ratio of 31.08 ± 8.88 μmol:mol. During the third phase, iron is weakly coupled to productivity as the system transitions to net heterotrophy and biomass declines despite increased Fe supply associated with the east Australian current system. Together, 27 years of continuous monitoring draws a comprehensive picture of how and when iron fuels subantarctic productivity, providing a critical baseline for model validation and continued monitoring in a rapidly changing climate.