Long-term impacts of reservoir operation on the spatiotemporal variation in nitrogen forms in the post-Three Gorges Dam period (2004–2016)

Abstract

Nitrogen (N) is an essential nutrient limiting life, and its biochemical cycling and distribution in rivers have been markedly affected by river engineering construction and operation. Here, we comprehensively analyzed the spatiotemporal variations and driving environmental factors of N distributions based on the long-term observations (from 2004 to 2016) of seven stations in the Three Gorges Reservoir (TGR). In the study period, the overall water quality status of the river reach improved, whereas N pollution was severe and tended to be aggravated after the TGR impoundment. The anti-seasonal reservoir operation strongly affected the variations in N forms. The total nitrogen (TN) concentration in the mainstream of the Yangtze River continuously increased, although it was still lower than that in the incoming tributaries (Wu and Jialing rivers). Further analysis showed that this increase occurred probably because of external inputs, including the upstream (76%), non-point (22%), and point source pollution inputs (2%). Besides, different N forms showed signi cant seasonal variations; among them, the TN and nitrate nitrogen concentrations were the lowest in the impoundment season (October-February), and the ammonia nitrogen concentrations were the highest in the sluicing season (March-May). These parameters varied likely because of internal N transformation. Redundancy analysis revealed that the water level regulated by the anti-seasonal operation was the largest contributor. Our ndings could provide a basis for managing and predicting the water quality in the Yangtze River.

Nutrient Dynamics under the Coupling Effects of Damming and Urbanization in the Three Gorges Reservoir: Status, Sources, and Driving Factors

“…DIN (77.9 ± 64.1 μmol/L) accounted for 71.6% of TDN, which was slightly lower than that in 1997 (87.5%), 2009–2010 (90.4%), and 2016 (93.9%) . The average concentration of DON (19.8 ± 28.1 μmol/L) was approximately twice as high as that in previous years (6.7–12.7 μmol/L), while NO 3 – –N (77.9 ± 64.1 μmol/L) fell within the range from 2003 to 2016 (75.3–102.8 μmol/L) and higher than the natural background (0.08–0.16 N mg/L). , Additionally, NO 3 – –N dominated the composition of DIN, accounting for approximately 89–94% during wet season. , …”

“…The significant negative correlation between NO 3 – –N/TDN and NH 4 + –N/TDN ( p < 0.01, r = −0.418, Figure a) implied the occurrence of transformation between NH 4 + –N and NO 3 – –N. Previous studies also showed that NH 4 + –N is easily converted to NO 3 – –N by the ammonia oxidation process, resulting in an increase in NH 4 + –N and a decrease in NO 3 – –N in TGR river water . Compared to YL, the hydrological process of the Chongqing-TGD reach is more vulnerable to the regulatory impacts of the TGD .…”

“…Previous studies also showed that NH 4 + −N is easily converted to NO 3 − −N by the ammonia oxidation process, resulting in an increase in NH 4 + −N and a decrease in NO 3 − −N in TGR river water. 49 Compared to YL, the hydrological process of the Chongqing-TGD reach is more vulnerable to the regulatory impacts of the TGD. 66 Meanwhile, there was a negative correlation between d-excess and NO 3 − − N/NH 4 + −N (p < 0.05, r = −0.343, Figure 4b).…”

“…The findings of this study suggest that the retention by damming may have facilitated the denitrification over past two decades, 24 thereby accelerating the removal of riverine N. Meanwhile, rising water levels also make it more difficult to upward release of stored N in the bottom layer. 49…”

Biogeochemical Processes of Dissolved Nitrogen in the Backwater Zone of a Tributary in Three Gorges Reservoir, China: Implications from the Hydrologic Processes and Isotopic Tracing

“…Since the operation of the Three Gorges Dam (TGD), the hydrodynamics and nutrient load have been clearly affected. , Meanwhile, algal blooms occur frequently in the backwater zone of 22 tributaries during the wet season . Surface water quality in the TGR basin has been widely concerning .…”