Organic iron complexes enhance iron transport capacity along estuarine salinity gradients of Baltic estuaries

摘要

Rivers discharge a notable amount of dissolved Fe(1.5×109 mol yr−1) to coastal waters but are still not consideredimportant sources of bioavailable Fe to open marine waters. The reason isthat the vast majority of particular and dissolved riverine Fe is consideredto be lost to the sediment due to aggregation during estuarine mixing.Recently, however, several studies demonstrated relatively high stability ofriverine Fe to salinity-induced aggregation, and it has been proposed thatorganically complexed Fe (Fe-OM) can “survive” the salinity gradient,while Fe (oxy)hydroxides are more prone to aggregation and selectivelyremoved. In this study, we directly identified, by X-ray absorptionspectroscopy, the occurrence of these two Fe phases across eight borealrivers draining into the Baltic Sea and confirmed a significant butvariable contribution of Fe-OM in relation to Fe (oxy)hydroxides among rivermouths. We further found that Fe-OM was more prevalent at high flowconditions in spring than at low flow conditions during autumn and thatFe-OM was more dominant upstream in a catchment than at the river mouth. Thestability of Fe to increasing salinity, as assessed by artificial mixingexperiments, correlated well to the relative contribution of Fe-OM,confirming that organic complexes promote Fe transport capacity. This studysuggests that boreal rivers may provide significant amounts of potentiallybioavailable Fe beyond the estuary, due to organic matter complexes.