Stepwise variations of nutrients and organic matter in the fragmented Changjiang River by two big “dams”: One visible and one invisible

The transport flux of biogenic elements from large rivers such as the Changjiang (Yangtze) River, played a critical role in determining the estuarine and coastal environments. However, the combined effects of human activities and natural factors had resulted in complex spatiotemporal variations of biogenic elements along the river. In this study, sampling was conducted on four campaigns through the Changjiang River in different seasons during 2021 and 2022. The results showed that two suspended particulate matter (SPM) Surge Zones with rapidly increased SPM concentrations emerged, one between Yichang and Yueyang, downstream of the Three Gorges Dam (TGD), and the other between Wuhu and Xuliujing, upstream of the estuarine area. The two SPM Surge Zones had acted as the two “dams” or “walls” that separated the Changjiang River mainstream and led to the formation of two SPM Deposition Zones, one in the Three Gorges Reservoir (between Chongqing and Yichang), and the other in the middle and lower reaches (between Wuhan and Wuhu). Within these SPM Deposition Zones, SPM concentrations slowly decreased, and the proportion of particulate organic carbon in SPM increased. This reservoirization of the river water hindered the concentration increase of nutrients with the river flow, despite their continuous supplies from river banks by anthropogenic activities. The different patterns of nutrients and organic matter variations in the SPM Surge Zones and in the SPM Deposition Zones highlighted a “stepwise” characteristic in their distribution. The Changjiang River showed a deeply reservoir-like characteristic at present, which, along with other anthropogenic activities, had also driven significant decadal trends of nutrients and organic matter in the river waters.