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

Effect of sediment on fluvial phosphorus and aquatic water quality

Phosphorus (P) is an essential element in the aquatic environment that is closely linked to river sediment dynamics. The effect of sediment on P has been widely recognized, but has been limited to specific scenarios without considering its relation to water quality management under different sediment scenarios. Water samples (1139 samples) collected from eight rivers across China were analyzed and a consistent comet-shaped relationship between sediment and dissolved P (DP), the component most directly impacting aquatic ecosystems, was observed. It indicated that in rivers with high sediment concentration (S), sediment has a strong controlling effect on P levels in water quality, but with decreasing S, this effect gradually diminishes and water quality is mainly influenced by P input. Extensive laboratory experiments were conducted and substantial differences in P concentrations were found across different methods, reaching up to two orders of magnitude. Some official methods showed significant uncertainties influenced by S and particle size, as confirmed by sediment settling theory, compromising accurate water quality assessment in rivers with high S. Among the methods considered in this study, filtering proved to be the most operationally feasible and effective in terms of its impact on water quality. These results have important implications for the assessment and management of P in aquatic water quality, as well as for the development of future environmental policies and regulations.

Cascade reservoirs affected chemical compositions of dissolved organic matter and greenhouse gas dynamics in the Lancang River

Dissolved organic matter (DOM) is an important component in aquatic systems. There has been much debate about the potential effects of cascade reservoirs on the transport and transformation of DOM. Here, through a survey of source to leave-boundary section of Lancang River (LCR) in June and November of 2017-2018, our results revealed that weak spatiotemporal variations were observed for DOC content, whereas DOM parameters were significantly different between natural and reservoir reaches. And DOM showed higher humification degree from allochthonous sources with increasing autochthonous matter in reservoir reach, may due to high particulate organic matter and releasing autochthonous DOM from phytoplankton blooms in the LCR, which can be evidenced by depleted DIC, enriched δ13CDIC and higher BIX. A unique fluorescent fraction (C5) appeared in the reservoir reach and increased along water flow, which was strongly associated with dissolved CO2 and N2O. Meanwhile, BIX value decreased with increasing dam height, hydraulic residence time (HRT), and reservoir capacity, which may promote CH4 production, highlighting variation of DOM compositions in understanding the effect of greenhouse gas (GHG) dynamics in the LCR. The findings were essential for comprehending the influences of cascade reservoirs on carbon cycle, and informed policy development for the sustainable management of transboundary water resources like the LCR.

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.

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.

Biotic homogenization in multisystem cascade reservoirs: insights from sedimentary photopigment analysis

The existing literature provides limited insights into the dynamics of phytoplankton communities and the spatial heterogeneity of physicochemical parameters in multisystem cascade reservoirs (interconnected reservoirs derived from different rivers). The existing studies are concentrated on cascade reservoirs (interconnected reservoirs derived from the same river). To address this knowledge gap, the aims of the present study were as follows: (1) investigate the spatial heterogeneity, within and between reservoirs, of geochemical parameters associated with the eutrophication process, considering total phosphorus, chlorophyll-a, pheophytin, and metals (chromium, copper, nickel, lead, zinc, iron, and manganese); (2) evaluate sediment quality at the designated locations; (3) assess differences in the richness and concentration of sedimentary photopigments between the reservoirs. Application of principal component analysis revealed discernible gradients for the abiotic variables, although the differences were not statistically significant (one-way PERMANOVA test, p > 0.05). The observations suggested a tendency towards spatial homogeneity within and between the reservoirs. The metal concentrations were consistent with regional reference values, while phosphorus levels in the sediment approached the threshold for classification as pollution (~ 2000 mg/kg). Analysis of pigments indicated low dissimilarity among the reservoirs, which could be mainly attributed to the eutrophication process and high connectivity of the sampled areas. To counteract ongoing biotic homogenization, it is essential to reduce nutrient inputs and invest in ecological protection and restoration programs. The analysis of sedimentary photopigments provides an efficient and cost-effective alternative way to assess phytoplankton communities.

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.