The Remobilization and Removal of Fe in Estuary—A Case Study in the Changjiang Estuary, China

Dissolved iron (dFe) delivered by rivers plays a vital role in supporting primary productivity in coastal ecosystems. River-borne Fe is generally removed in estuaries due to the salt-induced flocculation. However, effects of the river mouth bar area on the variations of dFe concentration are frequently overlooked. Here we show the results of quasi-synchronous observation conducted in three water channels in the mouth bar area of the Changjiang Estuary during both spring and neap tides in July 2014. dFe showed high spatial variation, from 2.63 to 690 nM, with higher values commonly found in the inner study area. An episode of significant Fe remobilization was observed in the inner mouth bar before the removal in the subsequent fresh-saline water mixing process. Averaged dFe concentration increased from 45.2 ± 22.9 nM at Xuliujing to 188 ± 185 nM in the inner salinity (S) ≤ 1 water, then decreased to 10.3 ± 9.53 nM in the outer S > 1 water. Budget calculations revealed that the Changjiang input was the largest terrestrial source for dFe, while desorption from particles was the main reason for dFe enhancement in the study area. The entire mouth bar area was identified as a net sink for dFe, whereas, its S ≤ 1 and S > 1 zones were a source and a sink, respectively. We suggest that the mouth bar area diversifies the biogeochemical behavior of Fe in the estuary and enhances Fe output flux. Our study offers a new insight for better understanding the behavior of Fe and other particle-reactive elements in large dynamic estuaries.