Abstract
Iron is an important trace element for the growth of phytoplankton in natural waters. Primary production in coastal water is considered to be maintained by the supply of terrigenous iron that migrates downstream via surface water and ground water flow. In this review, we summarize previous studies on iron biogeochemistry in coastal and freshwater systems with particular emphasis to the influence of dissolved organic matter and its quality on the iron transformation kinetics and the iron availability to phytoplankton in coastal waters. A number of previous studies, particularly those in the field of ocean science revealed that in air-saturated water at circumneutral pH, the solubility of inorganic ferric iron is at the subnanomolar level, whereas the presence of dissolved organic matter substantially increases the iron solubility. In addition to iron coordination by organic ligands, other reactions, including photochemical and thermal redox reactions, are also related to iron bioavailability. Given that iron bioavailability to phytoplankton is tightly associated with its speciation, studies on iron chemical speciation is necessary to properly assess the contribution of terrigenous iron in coastal primary productivity. Finally, to understand the connection between terrigenous iron and coastal primary productivity, integrated studies that cover not only the iron speciation and availability in freshwater and coastal waters but also the hydrochemical processes associated with terrigenous iron runoff are recommended.
