Impact of river dams on phosphorus migration: a case of the Pubugou Reservoir on the Dadu River in China

Unveiling the potential phosphorus retention effect in small-scale cascade reservoir systems: evidence from the Weiyuan River

The construction of artificial dams profoundly alters nutrient dynamics within reservoir systems, with the phosphorus (P) retention effect of large reservoirs well-established. However, the role of small cascade reservoirs in regulating P transport dynamics remains poorly characterized, and whether their biogeochemical impacts align with those of large-scale reservoirs requires systematic investigation. Traditionally, it is widely believed that reservoir systems retain P, preventing their export downstream and thereby reducing primary productivity downstream of the reservoir. Unexpectedly, our research on the Weiyuan River cascade reservoirs revealed elevated sediment total P (TP) levels of 1208.93 mg/kg, bioavailable P (BAP) at 623.14 mg/kg, and 0.23 mg/L of TP in the overlying water of downstream reservoirs, indicating that P gradually accumulates into a hotspot along the downstream path of the cascade reservoir, especially during the low-water season (LWS). P dynamics within cascade reservoir systems are primarily driven by three interconnected factors: (1) enhanced sediment P remobilization risks in downstream reaches, (2) anthropogenic P loading from external sources, and (3) cascade-induced sedimentological shifts toward elevated organic matter content and finer particle size distributions, which collectively amplify P bioavailability through modified adsorption-desorption equilibria. Notably, the combined effect of elevated P loading (0.17 mg/L) and prolonged hydraulic retention time (HRT: 13.13 days) during low-water seasons triggered pronounced P sequestration in upstream suspended solids (SS) of cascade reservoirs, retaining 30.35 kg (15.73 % of sediment TP). This far exceeds the P transport observed during the high-water season (HWS), where an increase of 34.34 kg (36.69 %) was recorded downstream. The observed sediment retention during LWS exhibits inconsistencies with reservoir-scale P biogeochemical dynamics, potentially driven by the limited P buffering capacity of small cascade reservoirs under hydrological perturbations and shortened sediment residence times. The study results challenged the conventional belief that a singular reservoir is greatly responsible for P retention, which underscores the importance of monitoring P pollution in areas downstream of cascade reservoirs. Our research offers new insights into how river dams affect nutrient cycling and ecosystem functions, aiming to provide theoretical guidance for river management.

Cascade damming alleviates imbalanced particulate C: N: P stoichiometry to regulate methane accumulation in the upper Yangtze River, China

Cascade damming and changes in land use are significant human activities that alter natural flow patterns and the biogeochemical cycles of carbon (C), nitrogen (N), and phosphorus (P). However, there is limited understanding of how damming and other terrestrial human activities affect stoichiometric imbalances, contributing to uncertainties in the processes and mechanisms of methane (CH4) emissions from river-reservoir systems. Here, we discuss the spatiotemporal patterns of particulate C: N: P mole ratios and dissolved CH4 in the upper Yangtze River, which covers 11 large dams along its main stem. The particulate C: N ratio increased with particle size, but C: P and N: P ratios were synchronous and showed adverse trends. Autochthonous (A-POC) and terrigenous (T-POC) particulate organic carbon to the bulk POC accounted for 44 % and 54 %. The POC/PON close to the Redfield ratio was predominantly influenced by coupling effects between damming and terrestrial anthropogenic activities. An increase in terrestrial anthropogenic activities tended to balance POC/PON but unbalance POC/PP and PON/PP. The cascade damming effect can alleviate stoichiometric imbalances and contribute about 15 %, 61 %, and 98 % for POC/PON, POC/PP, and PON/PP close to balance. The imbalances of POC/PP and PON/PP would be gradually amplified downstream in the cascade reservoir. A-POC can lead to short-term CH4 accumulation, but T-POC with higher POC/PON, lower POC/PP, and PON/PP could potentially regulate long-term CH4 accumulation. Particulate C: N: P close to balance and CH4 oxidation before the dams reduced CH4 accumulation in the upper Yangtze River. These insights have substantial potential implications for the adaptive management of cascade reservoir systems.

Influence of cascade reservoirs on the distribution, transport, and retention patterns of biogenic elements in the Jinsha River

Cascade reservoirs construction can greatly alter flow regime and sediment transport of rivers, further affecting migration and transformation processes of biogenic elements. The Jinsha River (JSR) is the China's largest hydropower base and the main runoff, sediment suspension, and nutrient source areas of the Yangtze River. However, the distribution, transport, and retention patterns of biogenic elements in the JSR are still unclear under the influence of cascade reservoirs. Therefore, monthly concentration monitoring work was conducted from November 2021 to October 2023 for various forms of carbon (C), nitrogen (N), phosphorus (P), and silicon (Si). Results showed that the concentrations and fluxes of total phosphorus (TP) and particulate phosphorus (PP) exhibited continuous decreasing trends along the reservoirs cascade, whereas N exhibited contrasting trends. The concentrations of dissolved total carbon (DTC), dissolved inorganic carbon (DIC), and total silicon also showed decreasing trends from upstream to downstream, whereas their fluxes were primarily influenced by runoff and exhibited upward fluctuations. Compared with other biogenic elements, there was a more pronounced retention effect on TP and PP by reservoirs, with average retention rates of 8.29 % and 16.01 %, respectively. Longer hydraulic retention time (HRT) can retain more TP and PP. Meanwhile, the retention rates of DTC, DIC, and particulate silicon were positively correlated with HRT, while the retention rate of dissolved silicon (DSi) showed a positive correlation with reservoir age. Moreover, the higher ratios of dissolved inorganic nitrogen to dissolved inorganic phosphorus (DIP) and DSi to DIP have occurred, resulting in apparent P limitation, particularly during the non-flood season due to lower DIP concentration. Overall, cascade reservoirs construction exists great influences on the spatial allocation, fluxes transport, and biogeochemical cycles of biogenic elements, potentially affecting the stability of rivers ecosystem along the food chain network.

Quantifying the impact of damming on phosphorus reallocation: Finer particles offset the reduction in soluble reactive phosphorus (SRP) by decreasing suspended sediment concentration

In this research, an innovative approach to quantify the impact of damming on phosphorus (P) reallocation between suspended sediments (SS) and water was proposed. P allocation can be described by the surface complexation model, with the impact of damming quantified by four variables: P load, suspended sediment concentration (SSC), particle size, and pH. Iron/aluminium (Fe/Al) oxide-adsorbed P (Fe/Alo-P) was identified as the exchangeable P during adsorption/desorption equilibrium with a series of heterogeneous sediment samples from two large Asian rivers, the Mekong River and the Yellow River. In both rivers, the Fe/Alo-P concentration increased from the tail towards the dam of the reservoirs, primarily attributed to the decrease in particle size from the tail towards the dam of the reservoirs. The Fe/Alo-P concentration in the Lancang River was higher than that in the Yellow River, ranging from 14.5 to 119.9 mg kg-1 and from 14.5 to 22.1 mg kg-1, respectively. The soluble reactive P (SRP) concentration decreased with decreasing SSC, while finer suspended sediment particles containing more Fe/Alo-P greatly offset the reduction in SRP concentration. When the maximum Fe/Alo-P concentration in the finest particles of SS was assumed to be 100 mg kg-1, the P equilibrium concentration (ce) decreased from 0.028 mg L-1 to 0.008 mg L-1 when the SSC decreased from 64 g L-1 to 1 g L-1 for SS with a median grain size (D50) of 32 μm and an Fe/Alo-P concentration of 11 mg kg-1. However, ce increased from 0.008 mg L-1 to 0.021 mg L-1 when the D50 of SS decreased from 32 μm to 4 μm with an SSC of 1 g L-1 and an Fe/Alo-P concentration of 76 mg kg-1 for 4-μm SS. The SRP concentration is sensitive to the Fe/Alo-P concentration in SS, and the P allocation ratio between sediments and water is comparable.

Deciphering the impact of cascade reservoirs on nitrogen transport and nitrate transformation: Insights from multiple isotope analysis and machine learning

Construction of cascade reservoirs has altered nutrient dynamics and biogeochemical cycles, thereby influencing the composition and productivity of river ecosystems. The Lancang River (LCR), characterized by its cascade reservoir system, presents uncertainties in nitrogen transport and nitrate transformation mechanisms. Herein, we conducted monthly monitoring of hydrochemistry and multiple stable isotopes (δ15N-NO3-, δ18O-NO3-, δ18O-H2O, δD-H2O) throughout 2019 in both the natural river reach (NRR) and cascade reservoirs reach (CRR) of the LCR. Through the monthly detection of nitrogen forms and runoff in the import (M2) and export (M9) section, the average annual retention ratios for Total nitrogen (TN), Nitrate nitrogen (NO3--N), Particulate Nitrogen (PN) and Ammonium Nitrogen (NH4+-N) were about -35%, -53%, 48% and -65%, respectively. The retention rates were positively correlated with hydraulic retention time and negatively correlated with reservoir age, especially in the flood season. Compared to the NRR, the reservoir had significantly affected the nitrogen transport characteristics, especially for the large reservoirs (like Xiaowan and Nuozhadu), which enhanced phytoplankton uptake of NO3--N to form PN capabilities in the lentic environment and subsequently to precipitate or intercept it at the reservoir. This led to the overall decreasing trend of TN and PN concentrations along the CRR. The Bayesian stable isotope model quantified NO3--N sources from the NRR to the CRR. During this transition, soil nitrogen (SN) ratios decreased from 69.3% to 61.8%, while Manure & sewage (M&S) increased from 24.0% to 31.3%. Anthropogenic and natural factors, including urban sewage discharge, population density, and precipitation, were selected as key predictor variables. The eXtreme Gradient Boosting (XGBoost) model exhibited superior predictive performance for NO3--N concentrations, achieving an R2 of 0.70. These findings deepen our understanding of the impact of reservoirs on river ecology.

The underestimated role of manganese in modulating the nutrient structure in a eutrophic seasonally-stratified reservoir

Accumulation and subsequent release of nutrients have great potential to trigger algal blooms in lakes and reservoirs. We conducted high vertical resolution (2 m interval) monitoring at ∼monthly intervals over a year for hydrological parameters, Chl-a, ammonium (NH4+), nitrate (NO3-) and different species of phosphorus (P) and manganese (Mn) in a 40-meter-deep subtropical reservoir (Shanmei Reservoir) in Fujian, southern China. In this seasonally stratified reservoir featured with high nutrient loading, the consistent trend in the ratio of dissolved inorganic nitrogen (DIN) to dissolved inorganic phosphorus (DIP) between the euphotic zone and the hypolimnion, coupled with its mirrored correlation with Chl-a concentration indicates that upward flux from the hypolimnion affects phytoplankton growth in the euphotic zone. The monthly variation of the depth-integrated multiple species of N and P indicates that during the stratification period in the hypoxic hypolimnion, approximately 80% of the DIP is removed, leading to a remarkable decoupling phenomenon between NH4+ and DIP. This process effectively increases the ratio of DIN to DIP in the hypolimnion, thereby significantly reducing the potential of algal blooms caused by the upward flux. A robust positive linear correlation between iron-manganese bound phosphorus (CBD-P) and particulate Mn was observed during stratification period implying that DIP was scavenged by sediment-released Mn throughout the water column. Vertical profiles during stratification showed that upward diffusion of Mn2+ facilitated the formation of Mn oxide zones near the oxycline. The most significant decrease in DIP inventory occurs when Mn oxide zones migrate either upwards from the bottom or downwards from the oxycline, indicating that the migration of Mn oxides on the vertical profile is a key factor in the decoupling of NH4+and DIP. Our findings underscore the importance of Mn cycling as an underappreciated DIP self-immobilization process in the water column of reservoirs characterized by high nutrient loading. Furthermore, we propose that denitrification and Mn cycling establish a consecutive feedback mechanism, preventing excessive nutrient accumulation in low oxygen bottom water. In the context of global changes, we anticipate a heightened prominence of this feedback mechanism.

Increasing cascade dams in the upstream area reduce nutrient inputs to the Three Gorges Reservoir in China

The upstream cascade dams play an essential role in the nutrient cycle in the Yangtze. However, there is little quantitative information on the effects of upstream damming on nutrient retention in the Three Gorges Reservoir (TGR) in China. Here, we aim to assess the impact of increasing cascade dams in the upstream area of the Yangtze on Dissolved Inorganic Nitrogen and Phosphorus (DIN and DIP) inputs to the TGR and their retention in the TGR and to draw lessons for other large reservoirs. We implemented the Model to Assess River Inputs of Nutrients to seAs (MARINA-Nutrients China-2.0 model). We ran the model with the baseline scenario in which river damming was at the level of 2009 (low) and alternative scenarios with increased damming. Our scenarios differed in nutrient management. Our results indicated that total water storage capacity increased by 98 % in the Yangtze upstream from 2009 to 2022, with 17 new large river dams (>0.5 km3) constructed upstream of the Yangtze. As a result of these new dams, the total DIN inputs to the TGR decreased by 15 % (from 768 Gg year-1 to 651 Gg year-1) and DIP inputs decreased by 25 % (from 70 Gg year-1 to 53 Gg year-1). Meanwhile, the molar DIN:DIP ratio in inputs to the TGR increased by 13 % between 2009 and 2022. In the future, DIN and DIP inputs to the TGR are projected to decrease further, while the molar DIN:DIP ratio will increase. The Upper Stem contributed 39 %-50 % of DIN inputs and 63 %-84 % of DIP inputs to the TGR in the past and future. Our results deepen our knowledge of nutrient loadings in mainstream dams caused by increasing cascade dams. More research is needed to understand better the impact of increased nutrient ratios due to dam construction.

Enhancing phosphorus source apportionment in watersheds through species-specific analysis

Phosphorus (P) is a pivotal element responsible for triggering watershed eutrophication, and accurate source apportionment is a prerequisite for achieving the targeted prevention and control of P pollution. Current research predominantly emphasizes the allocation of total phosphorus (TP) loads from watershed pollution sources, with limited integration of source apportionment considering P species and their specific implications for eutrophication. This article conducts a retrospective analysis of the current state of research on watershed P source apportionment models, providing a comprehensive evaluation of three source apportionment methods, inventory analysis, diffusion models, and receptor models. Furthermore, a quantitative analysis of the impact of P species on watersheds is carried out, followed by the relationship between P species and the P source apportionment being critically clarified within watersheds. The study reveals that the impact of P on watershed eutrophication is highly dependent on P species, rather than absolute concentration of TP. Current research overlooking P species composition of pollution sources may render the acquired results of source apportionment incapable of assessing the impact of P sources on eutrophication accurately. In order to enhance the accuracy of watershed P pollution source apportionment, the following prospectives are recommended: (1) quantifying the P species composition of typical pollution sources; (2) revealing the mechanisms governing the migration and transformation of P species in watersheds; (3) expanding the application of traditional models and introducing novel methods to achieve quantitative source apportionment specifically for P species. Conducting source apportionment of specific species within a watershed contributes to a deeper understanding of P migration and transformation, enhancing the precise of management of P pollution sources and facilitating the targeted recovery of P resources.

Impact of cascade reservoirs on nutrients transported downstream and regulation method based on hydraulic retention time

Cascade reservoirs construction has modified the nutrients dynamics and biogeochemical cycles, consequently affecting the composition and productivity of river ecosystems. The Jinsha River, as the predominant contributor to runoff, suspended sediment (SS), and nutrients production within the Yangtze River, is a typical cascade reservoir region with unclear transport patterns and retention mechanisms of nutrients (nitrogen and phosphorus). Furthermore, how to regulate nutrients delivery in the cascade reservoirs region is also an urgent issue for basin water environment study. Therefore, we monitored monthly variations in nitrogen and phosphorus concentrations from November 2021 to October 2022 in the cascade reservoirs of the Jinsha River. The results indicated that the concentrations and fluxes of total phosphorus (TP) and particulate phosphorus (PP) decreased along the cascade of reservoirs, primarily due to PP deposited with SS, while opposing trends for total nitrogen (TN) and dissolved total nitrogen (DTN), which might be the consequences of human inputs and the increase of dissolved inorganic nitrogen discharged from the bottom of the reservoirs. Moreover, the positive average annual retention ratios for TP and PP were 10% and 16%, respectively, in contrast to the negative averages of -8 % for TN and -11% for particulate nitrogen (PN). The variability in runoff-sediment and hydraulic retention time (HRT) of cascade reservoirs played crucial roles in the retention of TP and PP. A regulatory threshold of HRT = 5.3 days in the flood season was obtained for controlling the balance of TP based on the stronger relationship between HRT and TP retention ratio. Consequently, the HRT of these reservoirs could be managed to control nutrients delivery, which was of particular significance for basin government institutions. This study enhances our comprehension of how cascade reservoirs influence the distribution and transport patterns of nutrients, offering a fresh perspective on nutrients delivery regulation.

Quantifying cumulative changes in water quality caused by small floodgates in Taihu Lake Basin - A case in Wuxi

Small floodgates in the river network area own some characteristics: considerable quantity, wide range and short adjustment time, and intercepts the one-dimensional constant flow of rivers, which induce a great impact on riverine water quality. In this study, a typical urban floodgate-controlled reach was selected, and analyzed through the monthly data of four pollutant indicators TN, TP, CODMn and NH3-N at six sampling sites S1-S6 in 2016-2018. The principal component analysis and correlation analysis showed that TP was a representative indicator and there was a positive correlation between various pollutants. The difference test and linear regression showed that the concentration of pollutants at different sampling points varied greatly, and the pollutant concentrations in the longitudinal direction of the river showed a cubic-linear regression. The cluster system and CCME WQI showed that the water quality in the urban floodgate-controlled reach is "marginal" state, and TN and NH3-N are severely exceeding the standard. The "cumulative changes" of the floodgate on the pollutant input to the environment appeared spatial heterogeneity.

Evaluation of sediment phosphorus dynamics in cascade reservoir systems: A case study of Weiyuan River, China

Reservoirs tend to accumulate phosphorus (P) originating from agriculture, industry, and other upstream sources in sediment, with this stored P later released. However, the spatiotemporal dynamics of sediment P release in reservoirs remains unclear. This study investigated the spatiotemporal dynamics in P of the sediment and water of three cascade reservoirs in the Weiyuan River (Tuojiang tributary). The results showed elevated P in sediment [total P (TP): 1208.93 mg kg-1] and water (TP: 0.23 mg L-1) during the low-water season (LWS), which could be attributed to notably higher organic matter content (9.65%), finer particle size (20.95 μm), and extended hydraulic retention time (HRT: 13.13 days) downstream of the cascade reservoirs. Further study employing static in-situ diffusive gradient in thin films (DGT) and dynamic ex-situ adsorption kinetic experiments confirmed that the downstream release of P from sediments [diffusion flux (Fd): 1.67 mg m-2 d-1, equilibrium P concentrations (EPC0): 0.22 ± 0.10 mg L-1] greatly exceeded those upstream (-0.66 ± 0.17 mg m-2 d-1, 0.07 ± 0.001 mg L-1), Fe (II) was a critical factor in regulating sedimentary P release. The combined effects of high P in overlying water and sediment significantly stimulated downstream phytoplankton growth, particularly among cyanobacteria (26.48%) and green algae (8.33%). Further regulatory steps are needed to regulate LWS algal blooms downstream of cascade reservoirs.