The high N 2 fixation rate observed in the Lau Basin of the western tropical South Pacific Ocean (WTSP) is fueled by iron (Fe) released from shallow hydrothermal systems. Understanding Fe bioavailability is crucial but the controls on the stability and bioavailability of hydrothermal Fe inputs are still poorly understood. Here, we provide new data on the spatial and vertical distribution of the soluble ubiquitous humic-like li...