Seasonal characterisation of iron-binding ligands in the South Atlantic sector of the Southern Ocean (Scale cruises, 2019)

The Southern Ocean (SO) is an essential oceanographic region to fully understand the iron (Fe) biogeochemical cycle. This work presents the first study of the Fe-binding ligands (L_Fe) concentrations and their conditional stability constant (K^cond_Fe'L) in the South Atlantic sector of the SO in winter and spring of the same year, determining the importance of L_Fe sources and sinks in the region. Along the transect, the L_Fe concentrations ranged from 0.74 to 2.42 nmol L^-1 (mean = 1.29 nmol L^-1, n = 99) in winter and 0.57-2.03 nmol L^-1 (mean = 1.18 nmol L^-1, n = 113) in spring. The log K^cond_Fe'L ranged from 9.71 to 11.38 in winter and 9.75-11.21 in spring. Low L_Fe concentrations (winter: 0.89; spring: 0.93) and log K^cond_Fe'L (winter: 10.06 nmol L^-1; spring: 9.99 nmol L^-1) were measured in the marginal ice zone (MIZ), where seasonal sea-ice cover limits ligand production by reducing biological and atmospheric inputs and trapping ligands during ice formation. In contrast, the highest L_Fe concentrations occurred in deep waters near Bouvet Island were shallower in winter (1000-1500 m; 2.42 nmol L^-1) compared to spring (1500-2000 m; 2.03 nmol L^-1) and associated with higher log K^cond_Fe'L (winter: 11.24; spring: 10.83). These maxima reflect enhanced remineralisation, sediment resuspension, and the island mass effect on the Fe-binding ligands dynamics in the Southern Ocean. A significant positive correlation between L_Fe and apparent oxygen utilisation (ρ = 0.517, p < 0.001) confirms the key role of microbial remineralisation in ligand production in deep waters. Overall, seasonal differences in Fe-organic speciation were driven mainly by variations in ligand abundance rather than binding strength, shaped by the interplay of physical circulation, biological activity, and remineralisation.