The role of iron in surface waters
Iron plays a key role in aquatic ecosystems. It is a micronutrient essential for all organisms and affects the biogeochemical cycles of many important elements such as carbon, nitrogen, phosphorus, and various trace metals. Despite iron being one of the most abundant metals in soils and sediments, its bioavailability in well-aerated waters is low because of low solubility. Notably, iron has been shown to be limiting primary production in ocean waters. Moreover, iron affects the turnover of dissolved organic matter (OM) in aquatic systems by enhancing its susceptibility to photo-oxidation, flocculation and precipitation. Iron also cause precipitation of phosphorus, thereby decreasing the bioavailability and increasing the accumulation of phosphorus in sediment. More recently, iron has been receiving attention as one of the factors behind brownification (increased water color) of inland waters.
Recent research has shown that iron concentrations in Swedish and Finnish surface waters are increasing strongly over the last decades. On average in Sweden, iron concentrations have doubled over four decades, but there is a large variation among systems with increases between 20 and 470 %. This is an important finding, and given the decisive role of iron to a number of key functions, this is a phenomenon is bound to have far reaching implications for the aquatic systems. Current research is aiming to understand the driver/s and consequences of this trend.
Iron mobilization from soils is largely dependent on the redox state of iron, where ferric iron (III) has low solubility, while ferrous iron (II) has high solubility. One potential factor is that climate change, which brings more precipitation and higher temperatures in the catchment, results in more anoxic and reducing conditions that enhance the mobility of iron. Increasing precipitation also means shorter residence times in lakes that may reduce the loss of iron in suspension due to sedimentation. Another potential factor is the decreasing sulfate deposition, which may mean that less iron is bound by sulfides in soils and sediments.
As for the causes of this trend, research efforts focus on understanding the role to water color and also how increasing iron concentrations in riverine discharge may affect the receiving Baltic system. Effects on e.g. nitrogen fixation and phosphorus sedimentation are possible and depend on the behavior of the iron ones it reaches the estuary. Current efforts focus on the stability of the iron in the salinity gradient and how that is determined by the speciation of the iron and the interaction with organic matter.