Dynamics and Ecological impacts of dissolved Iron on the Ross Sea Continental Shelf: Machine Learning Reconstruction

Abstract. Dissolved iron (dFe) availability is a key factor regulating phytoplankton productivity over the Ross Sea continental shelf, yet its spatial and temporal distributions remain poorly constrained because of sparse observations. We reconstructed monthly three-dimensional dFe distributions from 2005 to 2019 at 0.5° × 0.184° resolution on the Ross Sea continental shelf with XGBoost model trained by historical dFe observations and paired environmental variables and continuous fields produced by MOM6. The reconstruction demonstrated the spatial features of dFe, including elevated concentrations along the western coast and north of Franklin Island and widespread surface dFe depletion (<0.1 nmol L⁻¹) across the central Ross Sea from late spring with an overall R² of 0.79 and a mean absolute error of 0.29 in ln-transformed dFe. Based on our results, the pronounced regional differences of surface dFe could be explained by seasonal relationship between the ferricline and mixed layer. In winter, the mixed layer extended below the ferricline in the western Ross Sea and Ross Sea Polynya but generally remained shallower in the eastern Ross Sea. This pattern was consistent with regional differences in spring bloom development, suggesting that winter access to subsurface Fe contributed to bloom preconditioning. In summer, the relatively small separation between the mixed layer and ferricline in the western Ross Sea potentially favored continued vertical Fe supply and sustained high chlorophyll concentrations during the open-water period. Overall, this product provides accurate estimates of the regional dFe-phytoplankton coupled dynamics in the Ross Sea.