Coupling between iron and humic substances in a land-sea transitional zone in the northern Baltic Sea

Abstract: In marine ecosystems around the world, iron (Fe) is a well-studied and important trace element which has been found to stimulate phytoplankton and cyanobacteria growth, as well as limit nitrogen fixation and being essential for Legionella pneumophila which is known as the main cause of Legionnaires’ disease. Fe also contributes to the brownification of freshwater and has recently been documented to increase in concentration in rivers in Sweden. The aim of this study was to investigate the link between Fe and humic substances (HS) in rivers and a coastal area in the northern Baltic Sea. In late winter, surface samples from rivers and the coast were analyzed for Fe, HS, dissolved organic carbon (DOC), pH, salinity, and temperature. Overall, the results showed a positive correlation between Fe and HS, indicating a terrestrial input of Fe. Other variables affecting Fe and HS including pH, salinity, and temperature were analyzed using principal component analysis (PCA), which showed spatial variations between river and coastal samples. Fe is known to be transported in water as two main carrier phases, Fe-OM complexes and Fe-oxyhydroxides. Which carrier phase is the most contributing depends on multiple factors, including catchment characteristics, seasonal variations, and water chemistry. Understanding the link between Fe and HS in land-sea transitional areas is important since terrestrial Fe is identified as a source of bioavailable Fe in marine waters such as the Baltic Sea.