Iron is an essential element in biochemical processes. Iron is insoluble in seawater, and its behavior is strongly influenced by its chemical speciation. Most dissolved iron exists as organic iron complexes; however, ferrous iron (Fe(II)), which is more soluble than Fe(III), can reach high concentration levels in reducing environments such as hypoxic waters. In addition, high concentrations of Fe(II) have been reported in surface waters during the daytime. However, Fe(II) oxidation is extremely rapid and is influenced by various environmental factors such as temperature, salinity, pH, dissolved oxygen (DO), and organic ligands. Ariake Sea and Omura Bay are located on the western coast of Japan. Ariake Sea is characterized by high tidal range and river water input, and Omura Bay is a strongly enclosed coastal area. At both regions, hypoxic waters have been formed near the bottom. We investigated the distribution and oxidation kinetics of Fe(II) in these areas.
 Seawater samples were collected in August 2024. Samples were obtained using acid-cleaned X-Niskin bottles and Teflon pumps, followed by filtration through a 0.2 µm-pore-size filter. Fe(II) concentrations were determined on board using flow injection-luminol chemiluminescence. Oxidation rates were measured in the laboratory by monitoring the decay of the added Fe(II) over time. To investigate the effect of organic matter, samples from the outer Ariake Sea (April 2025) were analyzed before and after UV irradiation.
 The dissolved oxygen concentration near the bottom layer in the inner part of the Ariake Sea was low but not hypoxic (2.4 – 3.0 ml/L). On the other hand, in Omura Bay, the bottom water at the central part of the bay was found to be almost anoxic. Fe(II) concentrations were elevated in the surface low-salinity waters in the inner part of the Ariake Sea, suggesting riverine iron input and the influence of photochemical reduction. In addition, nitrite maxima was found in the bottom waters, likely due to the link between iron reduction and benthic denitrification. Conversely, in the near-anoxic waters of central Omura Bay, Fe(II) was undetected, likely due to precipitation as iron sulfides under sulfate-reducing conditions. Fe(II) half-life in the Ariake Sea and Omura Bay was short, showing a negative correlation with temperature and a positive correlation with salinity. Notably, UV-irradiated samples exhibited a longer half-life than untreated samples, suggesting that organic ligands in the Ariake Sea accelerated Fe(II) oxidation. These findings highlight that while hypoxia preserves reduced iron, the subsequent oxidation rate upon re-oxygenation is heavily mediated by natural organic matters, necessitating further experiments under strictly controlled dissolved oxygen levels.
 

Fe(II),Fe(II) oxidetion,Iron,hypoxic water,anoxic