Evidence for coupled iron and nitrate reduction in the surface waters of Jiaozhou Bay

Iron and nitrate (NO₃^-) are dominant physiologically required nutrients for phytoplankton growth, and iron may also play a key role in the marine nitrogen cycle. In this study, we investigated the temporal and spatial distributions of dissolved iron (DFe) and Fe(II) in the surface waters of Jiaozhou Bay (JZB) from April 2 to July 26, 2017. High concentrations of DFe and Fe(II) predominantly occurred in nearshore and estuarine stations and concentrations were generally higher in April and May. The highest DFe concentration was observed along the coast of Hongdao (51.55 nmol/L) in May, while the lowest concentration was observed in the western coastal region (2.88 nmol/L) in April. The highest and lowest Fe(II) concentrations were observed in the Licun estuary (22.42 nmol/L) and outer bay (0.50 nmol/L) in May, respectively. We calculated the proportions of nitrate, nitrite, and ammonium in dissolved inorganic nitrogen (DIN) as well as the ratio of Fe(II) to DFe in all four months. The mean Fe(II)/DFe ratio was 0.48 in April, 0.43 in May, 0.69 in June, and 0.32 in July. The mean ratio of NO₃^- to DIN was 0.78 in April, 0.54 in May, 0.20 in June, and 0.62 in July. NO₃^-/DIN continuously decreased in the first three months, while Fe(II)/DFe remained high, which suggests that the reduction of iron and nitrate occurred simultaneously in the surface waters of JZB.