Variations of total dissolved iron and its impacts during an extreme spring flooding event in the Songhua River

• TDFe output increased significantly during the extreme spring flood event, whereas TDFe concentration maintained steadily.
• Fe(III) with the molecular weight < 10 kDa was the dominant species.
• DOM was the controlling factor of TDFe species and migration.

Total dissolved iron (TDFe) is a controlling factor in primary productivity in marine ecosystems, hence the variation of riverine TDFe output is attracting an increasing attention. Spring flooding is a key hydrological process in cold regions and the extreme spring flooding event (ESE) is thought to influence the output of TDFe considerably. In 2013, an ESE arose in the Songhua River, to reveal its impact on TDFe output and its species, water samples were collected in the river during this ESE as well as normal spring flooding period (NSP) in 2014 and 2015 to analyze the concentration and species of TDFe as well as other basic parameters. The speciation analysis was conducted by filtration and ultrafiltration methods. The results indicated the concentration of TDFe did not represent a significant difference during ESE, with an average concentration of 0.28 mg/L in ESE and 0.30 mg/L in NSP. The steady trend of TDFe concentration can be contributed to the limited intensity and duration of erosion by the snowmelt runoff and the construction of hydraulic works. Whereas, ESE intensified TDFe output increasing from 27.93 ton/day during NSP to 48.56 ton/day during ESE due to its high discharge. The species of TDFe was dominated by Fe(III) with the molecular weight lower than 10 kDa. The correlation analysis indicated that content and property of DOM were the main controlling factors of TDFe species. Furthermore, the dynamics and migration of TDFe were accompanied by the nutrient and heavy metal transport which may potentially influence the water quality in the river basin.