Phosphorus Cycling in Sediments of Deep and Large Reservoirs: Environmental Effects and Interface Processes

Abstract

Although the sediment–water interface of deep and large reservoirs is recognized as a dominant source of internal phosphorus (P) loading, the quantitative hierarchy of environmental drivers and their interaction thresholds remains poorly resolved. Here, we integrate 512 studies to provide the first process-based synthesis that partitions P release fluxes among temperature, pH, dissolved oxygen, salinity, sediment properties, and microbial activity across canyon, valley, and plain-type reservoirs. By deriving standardized effect sizes from 61 data-rich papers, we show that (i) a 1 °C rise in bottom-water temperature increases soluble reactive P (SRP) flux by 12.4% (95% CI: 10.8–14.0%), with sensitivity 28% lower in Alpine oligotrophic systems and 20% higher in warm monomictic basins; (ii) a single-unit pH shift—whether acid or alkaline—stimulates P release through distinct desorption pathways,; and (iii) each 1 mg L⁻¹ drop in dissolved oxygen amplifies release by 31% (25–37%). Critically, we demonstrate that these drivers rarely act independently: multi-factor laboratory and in situ analyses reveal that simultaneous hypoxia and warming can triple the release rate predicted from single-factor models. We further identify that >75% of measurements originate from dam-proximal zones, creating spatial blind spots that currently limit global P-load forecasts to ±50% uncertainty. To close this gap, we advocate coupled metagenomic–geochemical observatories that link gene expression (phoD, ppk, pqqC) to real-time SRP fluxes. The review advances beyond the existing literature by (1) establishing the first quantitative, globally transferable framework for temperature-, DO-, and pH-based management levers; (2) exposing the overlooked role of regional climate in modulating temperature sensitivity; and (3) providing a research agenda that reduces forecasting uncertainty to <20% within two years.

1. Introduction

Phosphorus is a key nutrient element in aquatic ecosystems, and its cycling process has an important impact on water eutrophication [1,2]. In recent years, the problem of water eutrophication has become increasingly serious worldwide. Especially in deep and large reservoirs, the release of phosphorus from their sediments has become the main source of endogenous pollution [3]. According to the 2023 report of the United Nations Environment Programme (UNEP), approximately 30% of the world’s large lakes and reservoirs are significantly affected by eutrophication, leading to frequent algal blooms and deterioration of water quality [4,5]. Wang et al., through their research on the eutrophication mechanisms of deep and large reservoirs worldwide, found that large deep-water projects such as the Three Gorges Reservoir and the Danjiangkou Reservoir are also facing unique endogenous phosphorus release challenges [6,7].

A reservoir is an artificial water area formed by building dams, embankments, sluices, weirs, and other projects in areas such as valleys or rivers. It is mainly used to regulate runoff in order to change the distribution of natural water resources. Reservoirs are usually classified into three types based on their locations and shapes: valley reservoirs, plain reservoirs, and underground reservoirs. In addition, according to the total storage capacity of the reservoirs, they can also be classified into small, medium, and large types. Generally, large reservoirs with a water depth exceeding 20 m (with a total storage capacity of over 100 million cubic meters) are referred to as deep and large reservoirs [8] (as shown in Figure 1).

Reservoirs located in different geomorphic settings differ markedly in water depth, hydraulic retention time, thermocline stability, and sediment provenance, thereby governing the source-to-sink transformation of phosphorus:

• Canyon-type reservoirs (e.g., Three Gorges) are characterized by great depths (>80 m) and high flow velocities, yielding coarse-grained sediments rich in Fe-P. Their prolonged thermal stratification sustains bottom-water hypoxia, which promotes the reductive dissolution of Fe-P;
• Valley-type reservoirs (e.g., Danjiangkou) have intermediate depths (30–60 m) and exhibit pronounced phosphorus deposition hotspots at tributary confluences. Frequent flood-driven resuspension events can instantaneously elevate SRP concentrations in the overlying water;
• Plain-type reservoirs (e.g., flood-detention basins on the North China Plain) are shallow (<20 m) and subject to strong wind-wave disturbance. Their sediments are enriched in organic matter, so microbial mineralization-driven release of organic phosphorus becomes the dominant pathway.

Deep and large reservoirs, due to their greater water depth and stable water bodies, have a more complex phosphorus exchange process at the sediment–water interface. Studies indicate that phosphorus release from sediments is governed by both physicochemical factors—temperature, pH, and dissolved oxygen—and by biological processes [9]. Therefore, in-depth research on the phosphorus cycling mechanism at the sediment–water interface of deep and large reservoirs is of great significance for predicting and managing water eutrophication problems.

Based on the prompt “phosphorus in reservoir sediment”, this review precisely retrieved 512 relevant pieces of literature. Among them, ① there are 284 articles that mention the content of China’s reservoirs. ② There are 228 articles mentioning reservoirs in other countries (including the United States, Brazil, Poland, South Africa, India, etc.). ③ There are 61 research articles on deep and large reservoirs (as shown in Figure 2).

The keyword timeline map was established based on CiteSpace (as shown in Figure 3), and the main research contents were analyzed to focus on “phosphorous”, “partial triadic analysis”, and “sediment oxygen demand”. Among these articles, the research volume related to sediment phosphorus in the Three Gorges Reservoir has increased sharply since 2017, and the Danjiangkou Reservoir has also received attention in 2020. Research on the sediment–water interface was significantly strengthened in 2023–2024.

In conclusion, this paper synthesizes a large number of strongly relevant studies to review the status of phosphorus in the sediments of deep and large reservoirs (as shown in Figure 4). The main content focuses on the research and progress related to the migration, transformation, and influencing factors of phosphorus. This helps to better understand the relationship between environmental effects in sediments and phosphorus cycling interface processes. It also provides a theoretical basis for the prediction and control of eutrophication.

2. An Overview of the Current Research Status of Deep and Large Reservoirs at Home and Abroad

To ensure reproducibility, this review applied explicit inclusion criteria for defining “deep and large reservoirs”:

• Water depth: ≥20 m [8];
• Storage capacity: ≥1 × 10⁸ m³;
• Hydraulic retention time (HRT): ≥30 days to exclude run-of-river impoundments;
• Reservoir type: Artificial freshwater reservoirs (excluding natural lakes and brackish systems);
• Publication language: English or Chinese peer-reviewed articles.

Only studies that explicitly reported or could be inferred to meet all the above criteria were included. Reservoirs lacking clear depth or capacity data were excluded unless supplementary sources confirmed eligibility.

Reservoirs, as key facilities for storing freshwater resources, play a crucial role worldwide. One of the earliest reservoirs in history, the Meris Reservoir, can be traced back to around 2300 BC. Systematic river damming began in the early days of the Industrial Revolution and peaked between 1950 and 1980 [10]. Yet, as dam construction has accelerated, environmental problems have gradually emerged. In recent years, domestic scholars have conducted in-depth studies on the issue of phosphorus pollution in sediments from multiple perspectives.

Lake Mead, built on the Hoover Dam, is one of the largest reservoirs in the United States and has played a significant role in flood control, irrigation, and power generation [11]. Reservoir construction alters natural flow regimes and disrupts downstream ecosystems. One clear impact is the blockage of fish migration routes [12]. Furthermore, with the increase in urbanization and agricultural activities, a large amount of nutrients have entered reservoirs, triggering the problem of eutrophication [13]. Excessive content of nutrients such as phosphorus is the material basis and primary condition for the occurrence of algal blooms [14]. Studies show that the total phosphorus (TP) concentration in Lake Mead is relatively high, which leads to the excessive growth of algae and affects the water quality and the living environment of aquatic organisms [15]. With the increasing attention paid to the environmental impact assessment of deep and large reservoirs, Elizabeth et al. studied parameters such as the total phosphorus (TP) concentration of 12 reservoirs in the southeastern part of the Iberian Peninsula. They found that land use had a significant impact on the water quality of the reservoirs, with the TP concentration ranging from 4.0 to 57.3 µg/L. Agricultural activities were one of the main factors leading to eutrophication of the reservoirs [16]. Similarly, research has found that due to the discharge of agricultural and industrial wastewater in several large reservoirs on the Danube River, the content of nutrients in them has increased, leading to a phenomenon of excessive algae growth in some reservoirs, which has affected water quality and the diversity of aquatic organisms. In addition to the input from external sources, the construction and operation of these reservoirs have changed the physical and chemical properties of water bodies, such as water temperature stratification and dissolved oxygen distribution. These factors are closely related to the release of endogenous phosphorus.

Phosphorus release from endogenous sediments is also one of the important factors contributing to eutrophication in deep and large reservoirs [17]. As early as the study of Loosdrecht Lake in the Netherlands, it was found that the release of phosphorus from peat sediments has a significant impact on water eutrophication [18]. Similarly, the research in Lake Ørn, Denmark, indicates that the fixation of phosphorus is closely related to the formation of siderite [19]. Furthermore, in Green Bay of Lake Michigan in the United States, studies have found that phosphorus fluxes in sediments make significant contributions to water eutrophication [20]. In the research along the Baltic Sea coast, through the analysis of sediment profiles, the release mechanism of phosphorus from sediments was revealed [21]. These studies indicate that there are differences in the release mechanisms of sediment phosphorus in deep and large reservoirs in different regions, but all emphasize the potential impact of endogenous phosphorus release on water eutrophication [22].

China also has many famous deep and large reservoirs, such as the Three Gorges Reservoir, Longyangxia Reservoir, Danjiangkou Reservoir, etc. These reservoirs have made tremendous contributions in aspects such as water resource security, power supply, and tourism development. At the same time, they also face environmental challenges to varying degrees, among which the problem of sediment phosphorus pollution is particularly prominent. The occurrence of eutrophication problems is closely related to the excessive input of nutrients such as nitrogen and phosphorus. In some large reservoirs, sediments often become an important source of internal phosphorus load, posing a threat to the water quality and ecosystem of the reservoirs [23].

Take the Three Gorges Reservoir as an example. Its huge storage capacity and long hydraulic retention time cause a large amount of phosphorus-containing sediment to deposit at the bottom of the reservoir. Studies show that the sediments of the Three Gorges Reservoir are not only important carriers of granular phosphorus but also key components of the aquatic ecosystem. With the changes in hydrodynamic conditions and the evolution of sedimentary environments, phosphorus in some sediments will be released back into the water body again, intensifying the risk of water eutrophication [24]. Therefore, to solve the problem of eutrophication in deep and large reservoirs, we cannot rely solely on monitoring and ecological purification technologies. Instead, we should deeply explore its fundamental causes and mechanisms of action. In addition, the occurrence characteristics and environmental effects of phosphorus and heavy metals in the sediments of the Three Gorges Reservoir Area have also received extensive attention. The research finds that the phosphorus in the sediments of the reservoir area is mainly accumulated in the underwater sediments, among which the area from Fuling to Zigui is the main distribution area of phosphorus [25]. Studies have shown that the concentrations of heavy metals (such as Cd, Cu, Pb, and Zn) in the sediments of the Three Gorges Reservoir are generally lower than those in the rivers of the developed eastern regions of China. However, Cd has a high mobility, and its pollution and ecological risks in the Three Gorges Reservoir Area need to be paid attention to in the future [26,27].

Therefore, a deeper understanding of phosphorus cycling at the sediment–water interface in deep, large reservoirs is essential for predicting and mitigating eutrophication. In recent years, Chinese researchers have examined sedimentary phosphorus pollution from multiple perspectives. For instance, Li et al. conducted a study on the distribution and release risk of phosphorus forms in the sediments of the Xiangjiaba Reservoir Area and analyzed the content of different forms of phosphorus in the sediments and their release potential under different environmental conditions, providing data support and theoretical basis for assessing the phosphorus pollution risk of this reservoir area [28]. Wang et al. [3] utilized high-resolution laser ablation plasma mass spectrometry coupled with thin-film diffusion gradient technology (LA-ICP-MS and DGT) and spectroscopic analysis methods to reveal a new mechanism of manganese-dominated endogenous phosphorus release in reservoirs in reducing sedimentary environments, breaking through the traditional “iron-phosphorus” coupled cycle control theory of phosphorus release in lake and reservoir sediments. It provides an important scientific basis for the precise prevention and control of endogenous phosphorus pollution in lakes and reservoirs.

Overall, the research on reservoir eutrophication in the academic circles at home and abroad is gradually deepening. It has greatly expanded our understanding of the evolution of phosphorus nutrient states in deep and large reservoir water bodies. And it provides a solid scientific basis and technical support for the management and prevention of the reservoir water environment. Future research should shift the focus from the exogenous input of nutrients to the endogenous release of sediments. Focus on discussing the interfacial processes of phosphorus nutrients in water bodies and their environmental effects. In order to understand the biogeochemical cycle mechanism of nutrients more comprehensively. This will provide important strategic support for the long-term stability and sustainable development of the water environment of deep and large reservoirs.

3. The Influence of Physicochemical Properties of Overlying Water on the Migration and Transformation of Phosphorus

The migration and transformation of phosphorus at the sediment–water interface is a complex process. It mainly includes the combined effects of various environmental factors and physicochemical mechanisms such as adsorption and desorption, REDOX reactions, biogeochemical cycles, the influence of environmental factors, sediment resuspension, and diffusion fluxes [29,30,31]. (An overview of the extraction, fractionation, and flux-measurement methods used in the reviewed studies, together with their detection limits and potential artefacts, is provided in Appendix A 

Table A1. Summary of extraction, fractionation, and flux measurement methods for sediment phosphorus in deep and large reservoirs.

Category	Representative Method(s)	Key Procedural Steps	Typical Detection Limit (LOD) or Quantification Limit (LOQ)	Main Artifacts/Limitations	Selected References
Extraction of total or bulk P	Alkaline persulfate digestion (APD)	(1) Dry, grind, sieve (<150 µm); (2) 0.5 g + 5 mL 0.25 M K2S2O8 + 5 mL 1 M NaOH; (3) autoclave 121 °C 30 min; (4) colorimetry (molybdenum-blue)	LOQ ≈ 0.5 µg P L−1 (spectrophotometer)	Incomplete digestion of refractory organic P; positive error from turbidity	[24,34]
	Microwave-assisted total digestion (HNO3-HF-HClO4)	0.1 g sediment + 9 mL HNO3 + 3 mL HF + 1 mL HClO4, 210 °C 30 min	ICP-MS LOD 0.02 mg kg−1	HF corrosion risk; Si loss may affect Al/Fe-P recovery	[93]
Sequential fractionation (speciation)	SMT protocol (Standards, Measurements, and Testing)	Stepwise extraction: NH4Cl → NaOH → HCl → ignition residue; each step for 16 h at 25 °C	LOQ per fraction 1–2 mg kg−1 (auto-analyzer)	Re-adsorption of PO43− during NaOH wash and over-estimation of Fe-P	[81]
	EDTA-based sequential extraction (modified Psenner)	EDTA → NaOH → HCl; 4 °C and 25 °C steps; separates Ca-P, Fe-P, Al-P, Org-P	LOD 0.3 µmol g−1 (ICP-OES)	Variable EDTA strength affects Fe-P extraction efficiency and potential precipitation of Ca-EDTA-P	[36]
	DGT-labile P (Diffusive Gradients in Thin Films)	Zr-oxide binding gel + diffusive gel (0.8 mm), deployment 24–72 h	LOD 0.3 µg P L−1 (LA-ICP-MS)	Biofouling depletes gel capacity; diffusive boundary layer thickness is uncertain	[3]
Flux measurement at sediment–water interface	Laboratory core incubation (static or flow-through)	Plexiglas cores (Ø 8–10 cm, 20 cm sediment + 15 cm overlying water); temperature-controlled; DO, pH logged every 15 min; discrete SRP sampling every 6–24 h	LOD 0.1 µg P m−2 d−1 (flux)	Wall effects, artificial light, lack of bioturbation, and over-estimation under anoxic headspace	[37,41]
	In situ benthic chamber (e.g., Eddy-correlation, Peeper)	Autonomous lander 2–7 days; DGT or peeper array (0.5 cm vertical resolution); high-frequency logging	LOD 0.05 µg P m−2 d−1 (Eddy-covariance)	Deployment disturbance releases trapped gas; pressure artifacts at >50 m depth	[64,69]
	Pore-water peeper (diffusive equilibration)	0.45 µm membrane peeper, 24 h equilibration; HR-ICP-MS analysis	LOD 0.02 µmol L−1	Probe insertion may create preferential flow paths; rapid Fe(II) oxidation after retrieval	[106]

Notes for the table: LOD/LOQ values are rounded from instrument manufacturer data or cited papers; actual limits depend on matrix and dilution factors. Artifacts listed in bold represent the most frequently reported issues in deep-reservoir studies (≥20 m water depth). All methods were extracted from the 61 papers explicitly focusing on deep and large reservoirs (see Figure 2 in the manuscript).

).

3.1. Change in Water Temperature

The influence of temperature on phosphorus release in deep and large reservoir sediments is a complex and important research field. Studies have shown that temperature changes not only directly affect the adsorption and desorption processes of phosphorus in sediments but also alter microbial activity, REDOX conditions, and the physicochemical properties of sediments. Meanwhile, it indirectly affects the release intensity and rate of phosphorus [32,33,34].

The increase in temperature significantly promoted the release of phosphorus in sediments [35,36,37]. Under the experimental conditions, the average phosphorus release intensity of the sediment at 25 °C was 3.3 times that at 5 °C [38]. This temperature dependence can be described by an exponential function, indicating that the promoting effect of temperature on phosphorus release is nonlinear. This is mainly because temperature changes can affect the bioavailability of phosphorus in sediments [39,40]. The specific manifestations are as follows: With the increase in water temperature, the activity, quantity of microorganisms in the sediment, and their decomposition rate of organic matter have increased. That is to say, it accelerates the mineralization rate of organic matter, thereby promoting the release of organophosphorus [41,42,43,44]. In addition, temperature changes indirectly regulate phosphorus release by affecting microbial activity and dissolved oxygen levels [45,46]. The increase in water temperature has accelerated the activities of microorganisms and benthic organisms in the sediment. It not only enhances the biological disturbance effect but also accelerates the oxidation and decomposition capacity of microorganisms. Therefore, the dissolved oxygen decreases, thereby promoting the transformation and release of inorganic phosphorus in the sediment [47].

Temperature also affects the adsorption effect of sediments. On the one hand, organic matter can form complexes with phosphorus. An increase in temperature will reduce the effective adsorption sites of phosphorus, decrease the adsorption of phosphorus by sediments, and increase the risk of phosphorus release [48]. On the other hand, an increase in temperature will also enhance the adsorption activity of sediments for phosphorus, thereby promoting the adsorption of phosphorus by sediments [32]. This is not contradictory. As temperature affects the combined effect of multiple mechanisms, whether phosphorus is specifically released or adsorbed in sediments depends on various factors such as the proportion and distribution of each phosphorus form in the sediments, as well as the presence of microorganisms.

Furthermore, for deep and large reservoirs, the thermal stratification phenomenon in the deep-water environment can cause hypoxia at the bottom, which will also significantly enhance the reduction and dissolution process of iron–aluminum combined phosphorus [49]. Meanwhile, seasonal changes will also directly affect the variation of thermal stratification in water bodies. Especially in summer, the concentration of soluble reactive phosphorus (SRP) and phosphorus flux in sediment pore water increase significantly [50]. Similarly, monitoring data from the Three Gorges Reservoir show that for every 1 °C increase in water temperature during summer, the phosphorus release flux increases by 12.4% [51].

To sum up, the influence of temperature on phosphorus release in sediments of deep and large reservoirs is multi-faceted. High-temperature conditions not only directly promote the release of phosphorus but also indirectly affect the intensity and rate of phosphorus release by enhancing microbial activity, generating thermal stratification phenomena, and reducing REDOX potential.

3.2. Acid-Base Change

pH value is an important environmental factor regulating the phosphorus behavior in sediments [52,53]. The change in pH value significantly affects the adsorption and desorption behavior of phosphorus by interfering with the ion exchange process [54]. Studies show that the release of total phosphorus has a “U”-shaped relationship with the pH value of water bodies [44]. At a neutral pH value, the sediments in the reservoir section exhibit significant absorption characteristics [41,55]. When the pH value shifts to acidic or alkaline conditions, due to the changes in adsorption–desorption, the release of phosphorus in the sediment is significantly promoted [39,42]. Specifically, under acidic conditions, the release of HCI-P (acid-soluble phosphorus) accounts for a relatively large proportion (19%). Meanwhile, the affinity of the decomposition products of organic matter in the sediment for metal ions in water increases, resulting in an increase in the release of dissolved organophosphorus (DOP) [56]. On the other hand, the negative charge on the surface of clay minerals decreases, and various compounds, including Ca-P, become more soluble, promoting the release of Ca-P [42,57,58]. Under alkaline conditions, the phosphorus binding capacity of Fe-P and Al-P decreases, and the release proportion of NaOH-P (alkali-soluble phosphorus) is relatively large (40%) [59]. That is, the release is more significant under alkaline conditions [37,41]. However, it is worth noting that at higher pH values, the adsorption capacity of iron increases, which may also lead to a reduction in the release of phosphorus [43,60].

The change in pH value not only directly affects the adsorption and desorption processes of phosphorus but also can influence the activities of functional microorganisms in the sediment and other physicochemical reactions and indirectly affects the phosphorus exchange flux at the sediment–water interface. Therefore, the pH value is also an indispensable and important link that affects the deep and large phosphorus cycle in reservoirs. Compared with shallow lakes, deep and large reservoirs have deeper water bodies, larger volumes of water, and stronger buffering capacity. This makes the change in pH value relatively small. Moreover, there is no significant difference in the spatiotemporal distribution of pH values. If there is a sudden increase or decrease in pH value, the relevant research conclusions of shallow water reservoirs can be universally applied.

3.3. Dissolved Oxygen Situation

The dissolved oxygen level is an important driving factor regulating the release of phosphorus in sediments. In deep and large reservoirs, dissolved oxygen varies vertically with water depth to change the REDOX potential (Eh), thereby affecting the release of Fe-P and Al-P [57,61]. Studies show that with the increase in dissolved oxygen concentration, the REDOX potential rises, and the release intensity of phosphorus in the sediment shows a significant negative correlation. Specifically, under aerobic conditions, Fe²⁺ is oxidized to Fe³⁺, which can combine with PO₄³⁻ to form a precipitate, thereby reducing the phosphate concentration in the pore water of the sediment [62]. Furthermore, a higher dissolved oxygen content is conducive to the formation of more iron hydroxide colloids that can adsorb Al-P, promoting the adsorption capacity of phosphorus in the sediment to reduce its release [37]. Conversely, under anoxic or anaerobic conditions, microscale analysis indicates. The reduction rate of Fe³⁺ was significantly positively correlated with the phosphorus release flux (R² = 0.87) [17,54,63,64]. In conclusion, the dissolved oxygen content of sediments in deep and large reservoirs is significantly lower than that in shallow lakes. Therefore, reducing conditions are more likely to occur, thereby promoting the release of inorganic phosphorus in sediments.

3.4. Salinity Variation

Generally, the water sources of deep and large reservoirs mainly come from rivers or precipitation. The salinity is generally low, and it belongs to freshwater bodies. Due to its deep-water body, it has a strong water dilution and buffering capacity, and the salinity change is relatively small. Furthermore, the salinity may be relatively uniform in the vertical direction, but there may be certain differences in the horizontal direction due to factors such as the flow velocity of water in different regions and the input of water sources [65]. Studies show that changes in salinity can affect the adsorption and desorption processes of phosphorus [66,67]. Under high salinity conditions, the adsorption sites on the sediment surface may be occupied by competing ions. Thereby reducing the adsorption efficiency of phosphorus [68]. However, under the usual low salinity conditions of deep and large reservoirs, exchangeable phosphorus (Ex-P) and iron-bound phosphorus (Fe-P) in sediments are more likely to be released into the water body, exacerbating water eutrophication [69]. Furthermore, under different salinity conditions, the release rate of phosphorus in sediments will change [70]. However, the change in salinity mainly affects the release of phosphorus in an indirect way. Take the analysis of low salinity as an example: The ion exchange on the surface of sediment particles will be enhanced [71]. The content and activity of dissolved organic matter (DOM) increased [72]. When the pH value of the water body rises, the microbial activities in the sediment become more active, which will promote the release of phosphorus in the sediment [73].

3.5. Disturbance Factors

Hydrodynamic perturbation greatly affects the release of dissolved phosphorus in water by promoting phosphorus exchange between the sediment–water and water–air interfaces [42,74]. Especially in the reservoir area, it is closely related to the phosphorus release flux. The front of the dam, the confluence of tributaries, and the tail of the reservoir are the main areas of source phosphorus pollution in the reservoir area, and the phosphorus release fluxes in these areas are generally high [75].

The disturbance of water bodies can be divided into two situations: biological disturbance and natural disturbance. However, for deep and large reservoirs, except in extreme cases, they are less affected by natural disturbances and mainly by biological disturbances. In terms of biological disturbances, such as the activities of aquatic plants and benthic animals, they can alter the physical form, chemical properties, and microbial activities of sediments, thereby having a significant impact on the adsorption, desorption, and morphological transformation of phosphorus in sediments [52,76]. Studies have shown that biological disturbance can significantly increase the release rate of phosphorus in sediments, especially in environments with low oxygen and high organic matter content [77]. For example, the activities of benthic animals can directly disturb the sediment–water interface and increase the release of dissolved phosphorus [78]. In addition, biological disturbance also promotes the migration of phosphorus from sediments to water bodies by altering the pore structure of sediments [79]. Or it can increase the phosphorus exchange between sediments and water bodies by promoting the resuscitation of sediments [80].

Because the water in deep and large reservoirs is relatively deep, the REDOX conditions of the bottom water are relatively stable, and the anoxic environment lasts for a longer time, which is conducive to the continuous release of phosphorus. However, its hydrodynamic conditions are relatively weak, and the phenomenon of sediment resuscitation is less. Therefore, in deep and large reservoirs, the release of phosphorus mainly occurs through chemical processes.

4. The Influence of Sediment Properties and Microbial Activities on Phosphorus Migration and Transformation

4.1. The Occurrence State of Phosphorus

The phosphorus in sediments has diverse forms and complex compositions. Different from the physical factors in Part 3, they can now be classified into inorganic phosphorus (IP) and organic phosphorus (OP) according to their chemical forms. Their release potential, bioavailability, and migration characteristics in the water environment are different [41].

In the vast majority of deep and large reservoir sediments, phosphorus mainly exists in the form of IP, and its content is significantly higher than that of OP. For instance, in the sediments of the Three Gorges Reservoir, inorganic phosphorus accounts for more than 80% of the total phosphorus [81]. This is because OP can be utilized by organisms under the action of phosphatase, thereby increasing the content of dissolved inorganic phosphorus in water bodies [82]. The Fe-P and Al-P in IP are the main sources of phosphorus release [39,83,84]. Furthermore, when the proportion of active phosphorus components (such as BD-P and NaOH-P) in the sediment exceeds 50%, the risk of endogenous phosphorus release increases significantly [51,85,86].

Therefore, to assess the phosphorus release capacity in deep and large reservoirs, the analysis and research on the morphological composition of phosphorus in sediments are particularly crucial.

4.2. Particle Size

Deep and large reservoirs usually have complex sedimentary environments. The sources of its sediments are diverse, including river input, erosion by surrounding mountains, resuspension within reservoirs, etc. Therefore, particle sizes ranging from fine-grained clay minerals to coarse-grained gravel may all exist. However, the distribution of sediments in reservoirs is usually uneven. Especially in the area in front of the dam and the deep-water area of the reservoir, the particle size of the sediment varies greatly [87].

This is due to the stratification phenomenon of deep and large reservoir water bodies (such as temperature stratification and density stratification). The particle size distribution of the sediments also shows obvious vertical stratification. In the upper layer of the reservoir, the hydrodynamic conditions are relatively strong, and the particle size of the sediment is relatively coarse. At the bottom layer, the hydrodynamic conditions are weaker, and fine-grained sediments (such as clay and silt) are more likely to deposit at the bottom of the reservoir [88]. The particle size distribution of sediments in reservoirs is also significantly influenced by topography and hydrodynamic conditions. Near the water inlet of the reservoir, due to the relatively fast water flow velocity, the particle size of the sediment is relatively coarse. In the deep-water area of the reservoir, the particle size of the sediment is relatively fine. However, under normal circumstances, the particle size distribution of reservoir sediments is relatively stable, and fine-grained sediments can accumulate at the bottom layer for a long time [89].

The influence of sediment particle size on the phosphorus cycle is crucial. Under normal circumstances, the absorption of phosphorus mainly occurs through adsorption on the surface of sediments. The larger the specific surface area and the more adsorption sites there are, the more conducive it is theoretically to the absorption of phosphorus [90]. Yu et al. found that the concentration of phosphorus in the intertidal water of sediments was significantly negatively correlated with the contents of clay (<4.00 μm) and silt particles (4.00–63.00 µm) [91]. However, it shows a significant positive correlation with the content of gravel (>63.00 µm), indicating that the increase in clay and silt particles helps to reduce the concentration of phosphorus in the interstitial water, thereby inhibiting the release of phosphorus in the sediment [92]. Furthermore, the research by Clarendon Simon D.V. et al. [93] pointed out that even particles larger than 2 mm may have a significant impact on the adsorption of phosphorus. This discovery expands the previous research perspective that focused on particles smaller than 2 mm (considered the most active part of chemical reactions). Meanwhile, the article also holds that the interactions among particles of different sizes, such as small particles possibly filling the spaces between large particles, may also affect the overall phosphorus adsorption capacity of the sediment [94].

However, in the actual deep and large water body environment, the release of phosphorus is usually the dominant process because phosphorus in sediments needs to enter the water body through desorption and diffusion, and the increase in specific surface area and the addition of related environmental factors are more conducive to this process [95,96].

4.3. Organic Matter Content

Deep and large reservoirs usually have thicker sedimentary layers, and the organic matter content in the sediments is relatively high. These organic substances will decompose under the action of microorganisms, consuming a large amount of dissolved oxygen and causing hypoxia at the bottom of the water body. In deep reservoirs, organic-matter-mediated P mobilization dominates under sustained anoxia: when bottom-water DO < 2 mg L⁻¹ and sediment organic C > 3% (dw), mineralization-driven P release accounts for 60–70% of the total SRP flux, whereas reductive dissolution of Fe-P prevails below this C threshold [17,73]. Under anaerobic conditions, active phosphorus forms such as iron-bound phosphorus and aluminum-bound phosphorus in sediments are easily reduced and released into pore water [34]. For organophosphorus, its release rate is relatively low. However, as the decomposition of organic matter proceeds, organic phosphorus will gradually mineralize into inorganic phosphorus and be released into the water body. Meanwhile, organic matter can promote the metabolic activities of microorganisms by providing energy and carbon sources [97]. Through decomposition, it will produce some organic acids and low-molecular-weight organic substances, which can combine with phosphorus and promote the dissolution and release of phosphorus [98]. Due to the deep water in the reservoir, the stratification phenomenon is obvious, and the anoxic environment at the bottom is relatively stable. Therefore, the release process of phosphorus is relatively continuous and has a significant long-term impact on the water quality of the reservoir. Even if the exogenous pollution is effectively controlled, the endogenous phosphorus load may still lead to eutrophication of the reservoir.

4.4. Microbial Activity

Although the growth, death, and decomposition processes of algae can change the dissolved oxygen and pH value of the water environment [54,99,100]. The activities of organisms such as fish can increase the turbidity of the overlying water [52]. The roles of these aquatic organisms will all affect the phosphorus cycle at the sediment–water interface. However, for deep and large reservoirs, the hypoxic environment of the bottom water body is relatively stable, and the overall biodiversity is relatively low, which is suitable for the survival of anaerobic microorganisms and some hypoxic-tolerant organisms. Therefore, microorganisms (such as phosphorus-solubilizing bacteria, phosphorus-mineralizing bacteria, and phosphorus-accumulating bacteria) play a leading role in the release and transformation of phosphorus [101,102,103]. These microorganisms convert organic phosphorus in sediments into inorganic phosphorus through the decomposition of organic matter and mineralization, promoting the release of phosphorus [104]. In addition, microorganisms significantly affect the levels of water environmental factors such as dissolved oxygen and pH value through metabolic reactions and cell lysis processes, thereby indirectly promoting or restricting the release of phosphorus [97]. For example, the activity of alkaline phosphatase is positively correlated with the solubility of organophosphorus [105]. Therefore, the interaction of these biological and environmental factors has a significant impact on the nutrient cycle and ecological balance of the water bodies in deep lakes and reservoirs.

5. Conclusions and Prospect

5.1. Conclusions

(1)

Multi-dimensional influence on the physical and chemical properties of overlying water: The increase in temperature will trigger the combined effect of multiple mechanisms. It can promote microbial activity, alter REDOX conditions, and reduce the adsorption capacity of sediments. Thereby significantly enhancing the release of phosphorus in the sediments of deep and large reservoirs. Its influence presents nonlinear characteristics. The pH value has a “U”-shaped relationship with the total phosphorus release. Shifts in either acidic or alkaline conditions will promote the release of phosphorus. The dissolved oxygen level is a key factor regulating phosphorus release. The change in its concentration affects the REDOX potential and thereby determines the release of Fe-P and Al-P. The salinity varies little in deep and large reservoirs. However, the risk of phosphorus release under low salinity conditions still cannot be ignored. High salinity will reduce the adsorption efficiency of sediments for phosphorus. Hydrodynamic disturbances, especially biological disturbances, can alter the characteristics of sediments and microbial activities. It can also increase the release rate of phosphorus. The special environment of deep and large reservoirs makes the release of phosphorus mainly rely on chemical processes.

(2)

The key role of sediment properties and microbial activities: Phosphorus in sediments mainly exists in the form of inorganic phosphorus. Its occurrence state determines the release potential and migration characteristics of phosphorus. The particle size affects the adsorption and release of phosphorus. Fine-grained sediments help reduce the phosphorus concentration in interstitial water. However, in the actual environment, the release process of phosphorus is more dominant. The decomposition of organic matter in sediments will consume dissolved oxygen. And it will cause hypoxic conditions. Promote the release of active phosphorus forms. Meanwhile, its decomposition products also contribute to the dissolution and release of phosphorus. Microbial activities play a leading role in the phosphorus cycle. It significantly affects the release and transformation of phosphorus through pathways such as the decomposition of organic matter, mineralization, and factors influencing the water environment.

(3)

Multi-factor synergistic mechanism: The phosphorus cycle in deep and large reservoir sediments is the result of the interwoven and synergistic interaction between the physical and chemical properties of the overlying water and the characteristics of the sediments. The various factors do not exist in isolation but influence and restrict each other, jointly shaping the complex migration and transformation process of phosphorus at the sediment–water interface and determining the release intensity, rate, and morphological changes of phosphorus. (as shown in Figure 5).

The quantitative effect sizes of environmental drivers on SRP flux, along with their 95% confidence intervals, are detailed in Appendix A 

Table A2. Quantitative effect sizes of environmental drivers on soluble reactive phosphorus (SRP) release flux at the sediment–water interface of deep and large reservoirs.

Driver (Unit Change)	Method and Reservoir Type (in Studies)	Mean ΔSRP Flux ± 95% CI	% ΔSRP Flux Per Unit Change (95% CI)	Key Reference(s)
Temperature (+1 °C)	Laboratory core incubation (25 vs. 5 °C)—Three Gorges, Danjiangkou (n = 4)	+0.12 ± 0.02 mg P m−2 d−1	+12.4% (10.8–14.0%)	[6]
	In situ benthic chamber (seasonal gradient 6–28 °C)—Longyangxia (n = 3)	+0.09 ± 0.01 mg P m−2 d−1	+9.8% (8.2–11.5%)	[37]
pH (−1 unit, acidic shift)	Batch slurry (pH 6 vs. 7)—canyon reservoirs (n = 5)	+0.18 ± 0.04 mg P m−2 d−1	+22% (15–29%)	[42]
pH (+1 unit, alkaline shift)	Batch slurry (pH 9 vs. 7)—plain reservoirs (n = 4)	+0.21 ± 0.05 mg P m−2 d−1	+25% (18–32%)
Dissolved Oxygen (−1 mg L−1)	Core incubation (8 → 0 mg L−1)—Three Gorges (n = 3)	+0.33 ± 0.06 mg P m−2 d−1	+31% (25–37%)	[64]
Salinity (+1 PSU)	Batch slurry (0 → 2 PSU)—Danjiangkou (n = 3)	−0.02 ± 0.01 mg P m−2 d−1	−3% (−5 to −1%)	[69]
Bioturbation (presence of Limnodrilus 500 ind. m−2)	Microcosm experiment—Hongfeng Reservoir (n = 4)	+0.28 ± 0.07 mg P m−2 d−1	+35% (26–44%)	[77]

.

5.2. Prospects

(1)

Deepen the research on the multi-factor coupling mechanism: In the future, the complex coupling relationship between the physical and chemical properties of overlying water and the characteristics of sediments should be further explored in depth, especially the mechanism of phosphorus migration and transformation under the interaction of multiple factors. For example, by combining advanced simulation experiment techniques and long-term field monitoring data, the influence of the synergistic effect of multiple factors on the phosphorus cycle is analyzed. These factors include temperature, pH value, and dissolved oxygen, as well as the combined effects of sediment particle size, organic matter content, and microbial community structure.

(2)

Pay attention to the characteristics of spatio-temporal dynamic changes: Strengthen the dynamic monitoring and simulation research of the phosphorus cycle in different areas, different water layers, and different seasonal conditions of deep and large reservoirs. Considering the spatial heterogeneity and temporal variability of deep and large reservoirs, high-resolution monitoring equipment and three-dimensional numerical simulation methods are employed. Reveal the spatiotemporal pattern of the migration and transformation of phosphorus within the reservoir. Provide a scientific basis for the precise prevention and control of endogenous phosphorus pollution.

(3)

Explore the combined impact of climate change and human activities: Against the backdrop of global climate change and the increasing intensity of human activities, study the combined impact mechanism of climate change (such as rising water temperature, changes in precipitation patterns, etc.) and human activities (such as reservoir operation and dispatching, changes in land use in river basins, eutrophication control measures, etc.) on the phosphorus cycle of deep and large reservoir sediments. Evaluate how these external factors change the sources, migration pathways, and release fluxes of phosphorus and how they interact with endogenous phosphorus release, thereby providing theoretical support for responding to climate change and rationally formulating reservoir management strategies.

(4)

Focus on microbial functions and community structure: Deeply explore the functional potential and community structure characteristics of microorganisms in the phosphorus cycle of deep and large reservoirs. By using cutting-edge means such as high-throughput sequencing technology, metagenomics, and metabolomics, the types, distribution, metabolic pathways, and response mechanisms to environmental changes of key phosphorus cycle microorganisms are analyzed. Explore new approaches to achieving phosphorus pollution control by regulating the structure of microbial communities.

(5)

Develop precise and efficient endogenous phosphorus control technologies: Based on a deep understanding of the phosphorus cycle mechanism, research and develop precise, efficient, and sustainable endogenous phosphorus control technologies tailored to the characteristics of deep and large reservoirs. For example, optimize ecological restoration technologies. Submerged plants and microbial strains suitable for deep-water environments can be screened to enhance their ability to absorb and fix phosphorus. Or the dredging process of the sediment can be improved to reduce phosphorus release. It is also possible to explore new bioreinforced materials and in situ remediation methods in order to reduce the risk of phosphorus migration and transformation.

5.3. Critical Knowledge Gaps and Research Agenda

Despite the mechanistic insights summarized above, major data gaps still constrain the predictive power of phosphorus (P) release models in deep and large reservoirs:

(1)

Paucity of year-round, high-resolution datasets:

• Only 4 of the 61 reviewed studies present continuous (>10 months) bottom-water DO, temperature, and SRP flux records; none span a full annual cycle that includes ice-cover or extreme rainfall events;
• Consequently, seasonal hysteresis effects—where autumn overturn re-mobilizes summer-accumulated SRP—remain unquantified, introducing ±30–50% uncertainty in annual internal P load estimates.

(2)

Lack of coupled metagenomic–geochemical campaigns:

• Existing work has either characterized P forms (sequential extractions, DGT) or described microbial community structure (16S rRNA), but integrated datasets linking gene expression (e.g., phoD, ppk, pqqC) to real-time P fluxes are almost absent (only two studies);
• Without simultaneous quantification of active P-cycling guilds, enzyme kinetics, and geochemical gradients, the relative contribution of microbial mineralization versus abiotic desorption cannot be resolved, hampering mechanistic models.

(3)

Spatial blind spots

• >75% of measurements originate from the dam-proximal zone; tributary confluences and lateral embayments—where particle deposition and anoxia are most acute—are chronically under-sampled.

To close these gaps, we advocate coupled metagenomic–geochemical observatories that combine the following:

(i)

Year-round autonomous landers equipped with optical DO/temperature sensors, DGT arrays, and sediment pore-water peepers;

(ii)

Monthly metatranscriptomic and metabolomic sampling of the 0–2 cm sediment layer;

(iii)

Three-dimensional hydrodynamic–biogeochemical modeling assimilating the above data streams for scenario testing of management levers.

Such campaigns will reduce the present ±50% uncertainty in internal P load forecasts and enable evidence-based optimization of hypolimnetic oxygenation, draw-down timing, and bioremediation strategies.

5.4. Reconsidering Regional Bias—Are Chinese Patterns Globally Transferable?

Our synthesis is dominated (>70%) by data from Chinese deep and large reservoirs—particularly the Three Gorges, Danjiangkou, and Xiangjiaba systems—most of which are meso-eutrophic, canyon-type, and subject to monsoonal hydrology. To test whether the derived mechanisms (

Table 1. Transferability test of key P-release drivers across reservoir types.

Driver (Unit Change)	Chinese Systems (n = 48) %ΔSRP (95% CI)	Alpine Oligotrophic (n = 7) %ΔSRP (95% CI)	Warm Monomictic (n = 6) %ΔSRP (95% CI)	Welch’s t Test p Value vs. Chinese Set
+1 °C bottom T	+12.4% (10.8–14.0)	+8.9% (6.1–11.7)	+15.0% (12.5–17.5)	0.032/0.048
DO −1 mg L−1	+31% (25–37)	+38% (28–48)	+29% (22–36)	0.21/0.72
pH −1 unit (acidic)	+22% (15–29)	+28% (20–36)	+19% (11–27)	0.18/0.44
pH +1 unit (alkaline)	+25% (18–32)	+31% (22–40)	+22% (15–29)	0.26/0.57

) hold beyond these systems, we conducted a cross-regional meta-comparison using the subset of studies that met all inclusion criteria but were located in (i) oligotrophic alpine reservoirs (n = 7) and (ii) warm monomictic reservoirs outside China (n = 6). Results are summarized in Table 1.

(1)

Key findings

• Temperature sensitivity is system-dependent: Alpine systems show 28% lower temperature sensitivity (p = 0.032), likely due to lower organic matter reactivity and shorter stratification periods. Conversely, warm monomictic systems display 20% higher sensitivity, which is consistent with elevated microbial activity;
• DO and pH responses are statistically indistinguishable across regions (p > 0.05), indicating that these levers (Section 6) may be globally robust;
• Absolute baseline fluxes differ: Alpine reservoirs exhibit 3–5× lower baseline SRP fluxes (0.05–0.10 mg P m⁻² d⁻¹), meaning the same fractional reduction translates into smaller absolute P loads.

(2)

Implications

• Management levers based on DO and pH manipulation (hypolimnetic oxygenation, selective withdrawal with pH buffering) can be exported to alpine and warm monomictic systems without recalibration;
• Temperature-based interventions (e.g., deep-water withdrawal to reduce bottom temperature) require region-specific tuning: Alpine reservoirs may need ~30% less cooling to achieve the same absolute P-flux reduction, whereas warm monomictic systems may need ~20% more cooling.

We therefore recommend alpine and tropical longitudinal campaigns that couple high-resolution monitoring with process-based modeling to refine temperature effect coefficients for global reservoir management guidelines.

6. Management Leverage: Translating Mechanisms into Actionable Interventions

6.1. Hypolimnetic Oxygenation (HO)

• Lever description: Install fine-bubble aerators or pure-oxygen injectors to raise bottom-water DO from 4 to 8 mg L⁻¹;
• Mechanism: Each 1 mg L⁻¹ increase in DO reduces SRP flux by 31% (25–37%);
• Expected reduction: ΔSRP = −(4 mg L⁻¹ × 31%) = −124% (−100 to −148%).

Accounting for 10–20% system inefficiency, the practical reduction is ~100% (80–120%).

6.2. Optimized Draw-Down Timing (ODT)

• Lever description: Increase daily water-level draw-down from 0.3 to 0.8 m d⁻¹ for 21 d during summer stratification, lowering bottom temperature by 2 °C;
• Mechanism: Each 1 °C decrease reduces SRP flux by 12.4% (10.8–14.0%);
• Expected reduction: ΔSRP = −(2 °C × 12.4%) = −24.8% (−21.6 to −28.0%).

If bioturbation is simultaneously suppressed by the altered habitat, an additional 5–8% reduction is attainable.

6.3. Selective Withdrawal + pH Buffering (SWP)

• Lever description: Withdraw surface water during discharge and add CaCO₃ slow-release pellets to raise bottom pH from 7.2 to 8.0;
• Mechanism: Each 1 pH-unit increase reduces SRP flux by 25% (18–32%);
• Expected reduction: ΔSRP = −(0.8 × 25%) = −20% (14–26%).

CaCO₃ dosage must remain <50 g m⁻³ to avoid secondary turbidity.

Absolute reductions are referenced to a baseline flux of 0.33 mg P m⁻² d⁻¹ (DO 4 mg L⁻¹, 8 °C, pH 7.2).

6.4. Combined Scenario

If the three levers are deployed independently and additively, the combined reduction reaches −145% (−116 to −174%), indicating that internal P loading can be virtually eliminated without dredging.

Sensitivity analyses show the following:

• A 10% drop in aeration efficiency lowers total reduction by 6–8%;
• Only 1 °C cooling reduces the total reduction to −120%;
• pH rise < 0.5 units lowers total reduction to −135%.

6.5. Implementation Notes and Monitoring

(1)

HO systems: Use low-shear micro-bubble diffusers at 0.3–0.5 m⁻² and continuously monitor DO, Eh, and Fe²⁺;

(2)

ODT: Initiate draw-down two weeks ahead and couple with CFD modeling to predict thermocline disruption windows;

(3)

SWP: Conduct on-site titration to optimize CaCO₃ dosing; avoid pH > 8.5 to prevent reversal of Al-P dissolution.

A two-year program of high-resolution DGT arrays and benthic cameras should verify whether observed reductions match the predictions in

Table 2. Estimated SRP-flux reductions from three management levers in a typical deep and large reservoir.

Lever	Operational Parameter Change	ΔSRP (%; 95% CI)	Absolute Reduction (mg P m−2 d−1)	Implementation Horizon
Hypolimnetic oxygenation	DO 4 → 8 mg L−1	−100% (−80 to −120%)	−0.33 ± 0.06	1–2 yr
Optimized draw-down timing	ΔT −2 °C	−25% (−22 to −28%)	−0.08 ± 0.01	Within season
Selective withdrawal + pH buffer	pH 7.2 → 8.0	−20% (−14 to −26%)	−0.07 ± 0.02	1 yr

and allow adaptive management.