Fig. 1: Key relationships across the Southern Ocean.

a Sampling stations are shown superimposed on nitrate concentrations to illustrate the contrasting sites sampled during the Antarctic Circumpolar Expedition (ACE). b Component loading from the Principal Component Analysis for the ACE dataset (0–1000 m, n = 70). This figure represents the relationship between dissolved Fe (DFe, nmol L⁻¹), in situ iron-binding ligands (Fe-L, nmol L⁻¹), the conditional stability constant (log K_Fe’L), electroactive humic substances (eHS, µg L⁻¹ Suwanee River Fulvic Acid (SRFA) equivalent), hydrolysable carbohydrates (Carb, µg L⁻¹ glucose equivalent) and depth (m). Principal Component (PC) 1 and 2 together explain 64.33% of the dataset. The variances of the data are explained at 77% for depth, 80% for DFe, 81% for Fe-L, 22% for log_KFe’L, 53% for Carb, and 75% for eHS. log K_Fe’L and Carb are not statistically represented.