Characteristics Of Dissolved Iron Flux And Its Formation Mechanism During The Extreme Flooding Events In The Midstream Of Heilongjiang River Basin

Iron is an essential element limiting the primary production in the marine ecosystem, iron fertilization experiment proves that the increasing dissolved iron supply can promote the biomass of phytoplankton in the ocean, which may further retard the global climate changes. Iron is a hotspot in the Heilongjiang River Basin, due to its high flux in the river. Flooding is the key process of dissolved iron transportation from the continent to the ocean. At the background of global climate changes, the frequency of the extreme flooding events has been increasing, which may exert a significant influence on the dissolved iron output of the Heilongjiang River. In current study, the midstream of the Heilongjiang River Basin including the typical ecosystems was selected to study the variations of the concentration, species and flux of dissolved iron during extreme flooding events, reveal the main controlling factor and contribution of the wetlands and forests on dissolved iron formation, and discuss the formation mechanism of dissolved iron. The main conclusions are listed as follows:(1) During the 2013 extreme flooding in the midstream basin of the Heilongjiang River, the concentration of dissolved iron increased significantly, compared with that during the normal summer flooding seasons. The average content achieved 0.69 mg/L in the Songhua River and 1.11 mg/L in the midstream of the Heilongjiang River, with total flux of 0.36×105 tons and 1.81×105 tons, respectively. The main species was complexed iron in lowmolecular-weight. During the flash flooding caused by the Typhoon Bolaven, the increasing content of dissolved iron averaged at 0.25 mg/L, with an output of 33 ton/day. Low-molecular-weight complexed iron dominated the species of dissolved iron. During the extreme spring flooding event in 2013, the dissolved iron concentration did not show significant difference from the value during normal spring flood season, with a mean value of 0.28 mg/L and an output of 39 ton/day. The complexed iron in low-molecular-weight dominated the dissolved iron species.(2) The correlation analysis and simulation experiments indicated that the contents of suspended particles and dissolved organic matter(DOM) is the key controlling factors of dissolved iron content and formation during the extreme flooding event. DOC contents were significantly positive related to the dissolved iron concentrations, besides, DOM canrelease the dissolved iron in the suspended particles, resulting in the increasing concentration of dissolved iron in the river.(3) Wetlands exist widely in the midstream of Heilongjiang River Basin. During the extreme rainfall events, the mean concentration of dissolved iron was 2.08 mg/L in wetland waters of Carex lasiocarpa, 2.89 mg/L in Calamagrostis angustifolia, 3.98 mg/L in paddy land, which were significantly higher than that monitored in the rain season in 2012 and 2014. The low-molecular-weight iron was the dominant species in all the wetland waters. The fluctuation of dissolved iron concentrations was related to the precipitation. The simulated experiments revealed that the water depth, DOC concentration and alternation of drying and wetting can accelerate and enhance the release of dissolved iron from the wetland soils. Moreover, the DOC values in the wetland waters also increased significantly during the extreme rainfall events, indicating that the wetlands can contribute a large quantity of DOM into the river water during the extreme flooding events, resulting in the intensification of dissolved iron release from the suspended particles.Thus, the wetlands played an important role in dissolved iron formation during the extreme flooding events.(4) The dissolved iron concentration increased significantly in the forested river, the Yalu River, during the extreme flooding event, with the mean value of 0.24 mg/L and an output of 360 tons, suggesting the forests can play a proper role in iron formation. Low-molecularweight iron in complexed form dominated the species of dissolved iron during all the periods.The loading of dissolved iron was in a significant positive relationship with river runoff and DOC and the dissolved iron flux can be estimated based on the river discharge.(5) The extreme flooding event is the key process of dissolved iron transportation in the Heilongjiang River. Its formation mechanisms can be attributed to the high iron background value, the increase of suspended particles and DOC contents, as well as human activities, such as wetland reclamation, construction of agriculture ditches system and large reservoirs.