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 0.74-2.42 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 9.71-11.38 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.