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Article

Spatial and Temporal Distribution of Phosphorus in Plateau River Sediments and Sediment–Water Interface: A Case Study of the Yarlung Zangbo River

by
Xiangwei Liu
1,
Yufei Bao
2,*,
Zhuo Chen
2,3,
Yuchun Wang
2,
Mingming Hu
2,
Zeren Lasong
1,
Cian Lamu
1,
Aimin Cai
4 and
Zhongjun Wang
5,*
1
Tibet Bureau of Hydrology and Water Resources Survey, Lhasa 851500, China
2
State Key Laboratory of Watershed Water Cycle Simulation and Regulation, China Institute of Water Resources and Hydropower Research, Beijing 100038, China
3
State Key Laboratory of Hydraulics and Mountain River Engineering, Sichuan University, Chengdu 610065, China
4
Hubei Key Laboratory of Resources and Eco-Environment Geology, Hubei Geological Bureau, Wuhan 430034, China
5
School of Environmental Science and Engineering, Yancheng Institute of Technology, Yancheng 224051, China
*
Authors to whom correspondence should be addressed.
Water 2025, 17(4), 484; https://doi.org/10.3390/w17040484
Submission received: 16 December 2024 / Revised: 2 February 2025 / Accepted: 7 February 2025 / Published: 8 February 2025
Browse Figures
Figure 1
<p>Study catchment and sample site locations.</p> "> Figure 2
<p>Box plot of water quality indicators (pH, temperature, TP, and DO) by region.</p> "> Figure 3
<p>Proportion of particulate total phosphorus (PTP) and dissolved total phosphorus (DTP) in the Yarlung Zangbo River water during the wet and dry seasons.</p> "> Figure 4
<p>The distribution and proportion of phosphorus forms in the sampling sediments. (<b>A</b>) Phosphorus form results in the wet season; (<b>B</b>) Phosphorus form results in the dry season; (<b>C</b>) Distribution of phosphorus forms in sediment during the wet season; (<b>D</b>) Distribution of phosphorus forms in sediment during the dry season.</p> "> Figure 5
<p>Distribution of phosphorus forms in the basin during wet and dry seasons.</p> "> Figure 6
<p>TOC content of the sampling sediment: (<b>A</b>) TOC content of the sediment in the wet season; (<b>B</b>) TOC content of the sediment in the dry season.</p> "> Figure 7
<p>Distribution of TN and TN–TP ratio in the Yarlung Zangbo River during the wet season.</p> "> Figure 8
<p>(<b>A</b>) Pearson analysis of total phosphorus (TP), dissolved total phosphorus (DTP), pH, temperature (T), TP (sediment), and land type in waters, and (<b>B</b>) Land use of the Yarlung Zangbo River basin.</p> "> Figure 9
<p>Correlation between sediment TOC and phosphorus components: (<b>A</b>) during the wet season; (<b>B</b>) during the dry season.</p> "> Figure 10
<p>Comparison of sediment phosphorus components in plateau rivers and plains water bodies (<b>A</b>,<b>B</b>). Plateau river: Yarlung Zangbo River; Plains lakes: Lake Taihu; Plains rivers: Hanjiang River, Lancang River, Sihe River, and Orontes.</p> ">
Versions Notes

Abstract

:
Rivers in plateau regions are more vulnerable to human activities and climate change than those in plains due to cold climate and high altitude. Studying the temporal and spatial distribution of phosphorus against the backdrop of climate warming and human activities is of great significance for the protection of the ecological environment of plateau rivers. This study focuses on the Yarlung Zangbo River, one of the highest-altitude rivers in the world, analyzing the different forms of phosphorus and total dissolved organic carbon (TOC) concentration and distribution characteristics in sediments and sediment–water interfaces at different time and spatial scales. The analysis indicators include total phosphorus (TP) and dissolved total phosphorus (DTP) in the water body; ammonium chloride-extractable phosphorus (NH4Cl-P), iron-bound phosphorus (Fe-P), calcium-bound phosphorus (Ca-P), aluminum-bound phosphorus (Al-P), organic phosphorus (OP), and TOC concentration and distribution in sediments. The results showed that the upstream and downstream sections of the Yarlung Zangbo River have relatively good water quality, while the middle stream section, affected by human activities, has higher phosphorus and TOC content in the water body. The phosphorus in the sediments is mainly in the form of Ca-P, indicating that the primary natural phosphorus input is through the disintegration of salts. During the freeze–thaw cycle, the organic matter in the sediments affects the phosphorus content in the water through adsorption and release. Climate warming is expected to increase the phosphorus load in the Yarlung Zangbo River. Comparative studies between plateau rivers and plains rivers have revealed that exogenous particulate phosphorus and endogenous phosphorus converted with the facilitation of organic matter are the main sources of eutrophication risk in plateau rivers. This study unveils the temporal and spatial distribution characteristics of phosphorus and TOC in the Yarlung Zangbo River, and discusses the mechanisms affecting phosphorus concentrations in key plateau river nutrient elements, providing scientific support for the protection of the fragile ecological environment of plateau river ecosystems.

1. Introduction

Excessive concentrations of nutrients such as phosphorus and nitrogen in water bodies can lead to water quality issues like eutrophication [1,2]. Phosphorus, due to its high biological availability, has attracted significant attention; its concentration in water is positively correlated with the risk of eutrophication [3,4]. Phosphorus in river systems originates from both internal and external sources. Internal sources, such as phosphorus in river sediments, have a significant impact on water eutrophication. Previous studies have found that particulate phosphorus (PP) deposited at the bottom of water bodies can continue to affect water quality [5]. More than 50% of external sources accumulate in sediments through settling, making riverbed sediments a major accumulation zone for phosphorus [6]. Phosphorus exists in various forms in sediments, many of which are unstable [7]. Under certain disturbance conditions, they can be released into the overlying water, thereby increasing the risk of water pollution [8]. Consequently, research on phosphorus in sediments is crucial for the prevention and control of water eutrophication.
Extensive studies have been conducted on the bioavailability, transformation characteristics, and transport mechanisms of phosphorus at the sediment–water interface [9,10,11,12]. The primary productivity, secondary productivity, biodiversity, and complexity of biological communities in plateau rivers are lower than those in plains rivers. Most studies on eutrophication have mainly focused on plains water bodies [13,14,15,16]. Plateau rivers are characterized by low metabolism and oligotrophy, with eutrophication occurring infrequently due to their unique geographic location and environment [17,18,19]. The resistance and stability of river ecosystems to external environment changes are more fragile in plateau rivers [20,21]. Problems with the water environment in plateau rivers have slowly started to surface as a result of human activity and climate change. Significant eutrophication can also happen in plateau waters, according to research on high-altitude water bodies in the Himalayas [22].
The Yarlung Zangbo River, one of the world’s highest-altitude rivers and the largest river on the Qinghai–Tibet Plateau, is characterized by unique plateau biodiversity and water cycling processes [23,24]. It is an extremely ecologically sensitive area and plays an irreplaceable role in maintaining the stability of the Qinghai–Tibet Plateau’s ecosystem, which is related to the ecological security of several large rivers, such as the Yangtze River and the Yellow River [25]. Research on the middle reaches of the Yarlung Zangbo River found that the distribution of phytoplankton in the water body included Cyanophyta, indicating a potential for eutrophication in the overlying water environment [26]. In addition, studies on phosphorus adsorption–desorption in Yarlung Tsangpo sediments also prove the release of internal sources in plateau water bodies can lead to eutrophication [11,27,28]. The eutrophic species like Anabaena can occur in the water body year-round at a frequency of up to 37% in the upstream region of the Yarlung Zangbo River [29]. When water environment problems in the Yarlung Zangbo River occur, they will be significantly more devastating and challenging to treat than those in rivers in plains regions. Therefore, research on the shifting dynamics of important nutrients in this river is desperately needed.
To explore the distribution characteristics of phosphorus in the sediments and sediment–water interface of the Yarlung Zangbo River, sediment and overlying water samples were collected from 13 mainstream sites and 10 major tributary sites during both dry season and wet season. We analyzed dissolved total phosphorus (DTP) and total phosphorus (TP) (the sum of particulate total phosphorus (PTP) and DTP) in the water samples, as well as the content of different phosphorus forms and TOC in the sediments. The main objectives of this study are: (1) to analyze the temporal and spatial distribution characteristics of phosphorus in the sediments and overlying water of the Yarlung Zangbo River; (2) to investigate the factors that influence phosphorus in the Yarlung Tsangpo River; (3) to identify the risk of phosphorus pollution in plateau rivers. The findings of this study are expected to help researchers better understand the biogeochemical processes of phosphorus in the Yarlung Zangbo River, as well as maintain the natural environment of plateau rivers.

2. Materials and Methods

2.1. Study Area

This study focuses on the Yarlung Zangbo River in the Qinghai–Tibet Plateau of China (Figure 1). The river flows from west to east, with a total length of 2229 km and a drainage area of 239,228 km2. The Yarlung Zangbo River is a typical plateau river, with its source located at an altitude of over 5500 m, and the average altitude of the basin is 4500 m. The river’s flow is primarily sourced from glacier meltwater, with runoff also influenced by rainfall. The average annual distribution of runoff shows a peak from June to September, with the dry season occurring from November to April.
The main land uses in the basin are glaciers, grasslands, forests, and arable land, with grasslands covering more than 60% of the area [30]. Based on natural geographic characteristics, the entire basin is divided into three sections: the headwater section, with a length of 26 km; the middle reach section, with a length of 1293 km; and the downstream section, with a length of 910 km. In the upstream section, the altitude is high and temperatures are low, with mossy grasslands being the dominant vegetation. Permanent glaciers are distributed in the headwater areas, while a small amount of arable land is scattered near the valley. The population density increases as the river progresses along its course. The main towns in the study area are concentrated in the middle reaches, with a scattered and small-scale distribution. Forests and glaciers are primarily concentrated in the downstream section.

2.2. Sample Collection

This study established 23 sampling sites across the Yarlung Zangbo River catchment based on the distribution density of human activities and the natural environmental characteristics. According to the flow direction of the river, the mainstream sampling sites are set as M01 to M13. In the upstream section, there are three sampling sites (M01 to M03); in the middle reach section, sampling sites are M04 to M12; and in the downstream section, there is one sampling site (M13). A sampling site was set for each of the 10 major tributaries, labeled T01 to T10 from west to east. During the dry season (winter 2016) and the wet season (summer 2017) of a full hydrological year, field surveys and sample collections were conducted.
The samples collected on-site included sediments and overlying water. The sediment core sampler was used to obtain the samples. The collected sediment samples were transferred to polyethylene bottles by siphoning the overlying water without disturbing the underlying sediments, and then acidified on-site and transported to the laboratory for analysis. The sediment samples were transferred to polyethylene centrifuge tubes and stored at 4 °C before being transported back to the laboratory.

2.3. Sample Measurement

The pH, dissolved oxygen (DO), and temperature (T) of the overlying water were measured on-site using a multiparameter water quality meter (YSI EX02) manufactured by YSI, Yellow Springs, OH, USA. After filtering the overlying water through the 0.45 μm glass fiber membrane (Whatman GF/F, Little Chalfont, UK), water samples were refrigerated and transported back to the laboratory, where they were frozen for storage until tested. The sediment samples were dried to constant weight in a freeze dryer for 3 to 5 days to analyze their water content. After freeze-drying, the samples were sampled using the quartering method, ground in an agate mortar, sieved through a 200-mesh sieve, mixed thoroughly, and bottled. The content of different forms of phosphorus can indicate the process of phosphorus transformation and its bioavailability. For example, NH4Cl-P has the highest bioavailability and is usually the main phosphorus form that causes water environment problems; Ca-P and Al-P are usually the most abundant but least bioavailable. As such, sediment samples were placed in a glass desiccator to measure TP, NH4Cl-P (labile phosphorus), BD-P (Fe-P), NaOH-P (Al-P), HCl-P (Ca-P), OP, and TOC content in the Yarlung Zangbo River sediments.
TOC in sediments was analyzed using an elemental analyzer (Vario EL III, Elementar, Langenselbold, Germany) after inorganic carbon was removed by washing with 1 mol/L hydrochloric acid. The phosphorus in the sediments was divided into NH4Cl-P, Fe-P, Ca-P, Al-P, and RES-P (mainly OP) [27]. The SMT extraction method proposed by Ruban et al. (2001) was used to separate the different forms of phosphorus in the sediments, with the molybdenum-blue method used to measure the phosphorus concentrations at each step, and the experiments were repeated twice for averaging [31]. The TP, DTP, and total nitrogen (TN) in the overlying water were measured using the ammonium molybdate spectrophotometric method and the potassium alkaline persulfate ultraviolet spectrophotometric method, respectively.

3. Results

3.1. Changes in Water Chemistry and Phosphorus Content in the Yarlung Zangbo River

The water chemistry parameters of the Yarlung Zangbo River are shown in Figure 2. The water temperatures are 5.4 ± 2.5 °C and 16.6 ± 3.3 °C during the dry and wet season, respectively, with an increasing trend from upstream to downstream during the dry season and a decreasing trend during the wet season. The water of the Yarlung Zangbo River is alkaline, with a pH range of 7.9 to 9.0 during the wet season and 8.2 to 8.8 during the dry season. The dissolved oxygen (DO) in the water is higher in the dry season than in the wet season, with the average dissolved oxygen being 8.2 mg/L in the dry season and 6.5 mg/L in the wet season. The TP concentration is generally higher in the middle and lower reaches compared to the upstream section. During the wet season, the TP concentration at the source reach ranges from 0.035 to 0.071 mg/L, in the middle reach from 0.126 to 0.557 mg/L, and the downstream area has an average of 0.377 mg/L, showing a gradual increase longitudinally along the river, with the highest concentrations occurring in the middle reach. The tributaries have higher TP contents, with an average of 0.253 mg/L, and the phosphorus concentrations in T04 and T05 exceed the average by 0.70 mg/L and 0.31 mg/L, respectively. During the dry season, the TP concentration ranges from 0.023 mg/L to 0.092 mg/L in the upstream section, 0.108 mg/L to 0.298 mg/L in the middle reach, and has an average of 0.238 mg/L in the downstream. The TP concentration in the source area during the dry season is higher than during the wet season, while it is lower in the middle and downstream reaches during the dry season compared to the wet season.
Figure 3 compares the proportion of DTP and PTP in water bodies. The TP concentration, averaging at 0.253 mg/L, is more than six times that of DTP, which averages at 0.041 mg/L. During the dry season, the DTP concentration in the mainstream ranged from 0.012 mg/L to 0.047 mg/L, with the middle reach showing a range of 0.013 mg/L to 0.109 mg/L, and the downstream showing a single value of 0.034 mg/L. The trend in DTP concentration generally aligns with TP. Across various sections of tributaries, DTP concentrations ranged from 0.012 mg/L to 0.045 mg/L, averaging at 0.028 mg/L. The results indicate that the proportion of DTP to TP in water bodies at all sampling sites within the basin is less than 10%. Seasonal variations have a lesser impact on DTP concentrations in tributaries compared to TP, suggesting that external inputs are primarily in the form of particulate phosphorus.

3.2. Distribution of Phosphorus Forms and TOC in Sediments

During the wet season, the variation range of TP content in the sediments is 250.97 mg/kg to 1104.29 mg/kg, with an average content of 635.01 mg/kg in the mainstream and 526.65 mg/kg in tributaries (Figure 4). During the dry season, the TP content ranges from 290.18 mg/kg to 848.70 mg/kg, showing an overall trend of gradually increasing from upstream to downstream, and the content in the mainstream is higher than that in tributaries.
The distribution of phosphorus forms in the Yarlung Zangbo River exhibits significant spatiotemporal variations (Figure 5). The abundance pattern of phosphorus forms in the mainstream is as follows: Ca-P (76.58%) > OP (13.92%) > Al-P (8.52%) > Fe-P (0.67%) > NH4Cl-P (0.30%). Ca-P constitutes the highest proportion among all phosphorus forms, exceeding 70%. The overall distribution of NH4Cl-P is relatively low. During the wet season, NH4Cl-P varies between 0.19 mg/kg and 5.86 mg/kg, with an average of 1.98 mg/kg. In the dry season, NH4Cl-P ranges from 0.36 mg/kg to 3.61 mg/kg, averaging 1.87 mg/kg, which is lower than the content during the wet season. The lowest NH4Cl-P content occurs in the upstream reaches (M01~M03). Fe-P content tends to increase gradually during the wet season, ranging from 1.35 mg/kg to 10.44 mg/kg, with an average of 4.30 mg/kg. During the dry season, except for points M13 in the mainstream and T05 in a tributary, the variation in Fe-P content is small at other sections and tends to be uniformly distributed overall, ranging from 1.28 mg/kg to 9.07 mg/kg, with an average of 4.91 mg/kg. The content of Al-P fluctuates significantly with regional differences: during the wet season, it ranges from 19.09 mg/kg to 94.07 mg/kg, with an average of 47.31 mg/kg; during the dry season, it ranges from 25.86 mg/kg to 106.61 mg/kg, averaging 56.32 mg/kg. The content of Ca-P during the wet season varies between 176.36 mg/kg and 997.18 mg/kg, with an average of 467.21 mg/kg; during the dry season, the variation range of Ca-P is smaller, ranging from 170.74 mg/kg to 578.44 mg/kg, averaging 403.26 mg/kg. From M01 to M11, Ca-P fluctuates slightly without significant differences, and high values mainly occur at sections M12, M13, and tributary T07. OP is an indicator with large variations within the basin. During the wet season, it ranges from 23.31 mg/kg to 160.79 mg/kg, averaging 79.73 mg/kg; during the dry season, it ranges from 46.84 mg/kg to 264.16 mg/kg, averaging 130.41 mg/kg, which is significantly higher than the OP content during the wet season. The overall content of OP in surface sediments is the highest in the middle reaches and the lowest in the upstream reaches, with a pattern similar to the water quality distribution.
The Yarlung Zangbo River’s mainstream has a greater TOC content than its tributaries. The mainstream’s TOC content ranges from 2.34 g/kg to 15.96 g/kg throughout the wet season, rising upstream to downstream before falling and rising again. The middle reaches’ sampling sites M04 and M05 had the highest TOC contents, measuring 11.23 g/kg and 12.96 g/kg, respectively. At sampling sites M10 to M12, the concentrations then drop before rising once more (Figure 6A). From upstream to downstream, the TOC content exhibits a pattern of increasing, decreasing, and then increasing once more. In the sampling sites M04 and M05, at middle reaches, the TOC concentration increases, while it decreases at the lower reaches, before increasing again at the sampling sites M10 to M12 (Figure 6B). Field surveys also revealed that towns were located near sections M04 and M05. The good correlation between mainstream sections M04, M05 and tributary sections T04, T05 suggests that human pollution sources are entering the main river via tributaries (T04 and T05), leading to increased TOC content in the sediment in that river section. The M10 to M11 section is located near the reservoir’s head, where the flow velocity is significantly lower than in river sections, causing suspended particles to settle, thus leading to an increase in TOC content. During the wet season, the average TOC content in the tributaries was 6.48 g/kg, which is lower than that in the mainstream. In the dry season, TOC concentrations in the tributaries decreased, with the T05 section, influenced by nearby towns, having a higher TOC concentration of 11.27 g/kg.

4. Discussion

4.1. Impact of Land Use Type on the Nitrogen and Phosphorus in the Yarlung Zangbo River

The nitrogen (N) and phosphorus (P) elements in water bodies are the primary factors controlling algae growth [2]. Excessive levels of nitrogen and phosphorus lead to excessive algae proliferation, which consumes dissolved oxygen and deteriorates water quality [32,33]. The TN/TP (Total Nitrogen/Total Phosphorus) ratio in water bodies can serve as a criterion for identifying the main controlling element. As shown in Figure 7, during the high-flow period of the Yarlung Zangbo River, the TN/TP ratio ranges from 0.29 to 4.19, indicating an overall higher concentration of nitrogen than phosphorus. Compared to the TN/TP ratio of 6.6 in the Yangtze River basin in China and the global river background value of 10.3, the nitrogen levels in the Yarlung Zangbo River are relatively low, suggesting that phosphorus is the primary controlling element for algae in its aquatic environment.
Based on the correlation analysis results between land use types and phosphorus concentrations in sediments, considering factors such as pH, temperature, and water bodies (as shown in Figure 8A), changes in land use types along the Yarlung Zangbo River have no significant impact on the river’s pH (p > 0.05). The acidity or alkalinity has no apparent effect on the cycling of P and N elements within the river itself. DTP and TP in the water are directly proportional to the area of land used anthropogenically. The correlation index for DTP is higher than that for TP, indicating increased phosphate input and soil particle input due to human activities, which makes suspended particulate matter in the water a source of phosphorus input to the Yarlung Zangbo River. The positive correlation index for TP in sediments suggests that agricultural land use increases the content of phosphorus compounds in the sediments.

4.2. Impact of TOC on Phosphorus Deposition and Source Effects Under Seasonal Cycling

Substances in the overlying water transfer to the sediment through sedimentation and exchange processes. Changes in the content of organic matter in sediments during seasonal cycling affect the distribution of phosphorus components [27]. Correlation analysis between TOC and various forms of phosphorus in sediments from the Yarlung Zangbo River basin reveals that TOC positively correlates with OP (p < 0.01) but has no correlation with other forms of phosphorus (Fe-P, Al-P, NH4Cl-P, and Ca-P) (p > 0.05) (Figure 9). An increase in the concentration of total dissolved organic matter enhances the adsorption process of phosphorus in sediments [34]. This also explains the similarity in the variation processes of OP and TOC at sections M04 and M05 of the mainstream and sections T04 and T05 of the tributaries. The capture of OP in sediments results in a higher proportion of OP in sediments compared to other river sediments. Comparing the slopes during the dry season and the wet season, it showed that the correlation index during the wet season is higher than that during the dry season. This is due to the decrease in biological activity during the transformation process of phosphorus forms from higher water temperatures in the wet season to lower water temperatures in the dry season [29]. The decrease in temperature slows down the biological metabolic processes in sediments, resulting in a lower OP conversion rate [35]. Under the influence of seasonal cycling, phosphorus settles at the bottom of the river during low temperatures and gradually releases into the overlying water during high temperatures, becoming an important source of phosphorus in the water body and thus a trigger for algae blooms [36].
During the dry season, when temperatures are lower, the concentration of phosphorus components in the sediment is higher than that in the wet season when temperatures are higher. This shows that phosphorus is released into the overlying water during the wet season, with OP being the most released form. According to Figure 2, DO in the wet season with higher temperatures is lower than that in the dry season with lower temperatures. This is because phosphorus release consumes oxygen in the water. Fe-P and Al-P, in the presence of oxygen, are converted into other forms of phosphorus [4,37], and the release of bioavailable phosphorus accelerates the rapid growth of algae, which consumes dissolved oxygen in the water. In summary, seasonal changes in high-altitude rivers contribute to the increased biological activity of bioavailable elements in the sediment, promoting concentrated biological growth in the water during the wet season.
The increase in water temperature accelerates the release rate and amount of phosphorus from sediments. Gradient exchange between pore water and overlying water is the primary means of phosphorus release from sediments, and elevated temperatures expedite its diffusion rate [37]. Additionally, rising temperatures lead to a decrease in the redox potential of the overlying water, accelerating the conversion of Fe3+ to Fe2+, which in turn promotes the release of Fe-P from sediments. On the other hand, decreased thickness of the oxidized layer due to higher temperatures can facilitate the release of phosphates [37]. However, in plateau regions, there are drastic temperature fluctuations throughout the year and freeze–thaw cycles. After the freeze–thaw cycle ends, the adsorption sites on the surface of ions in the sediment are damaged, increasing adsorption competition and thus reducing the adsorption capacity. The vital colloidal substances on the surface of organic matter are also destroyed, resulting in the release of significant amounts of phosphorus from sediments into the overlying water [38]. During the wet season, frozen sediments thaw, causing particle fragmentation which allows more NH4Cl-P to enter pore water and subsequently the overlying water body through exchange processes. Fragmented particles have a larger specific surface area and, enhanced by organic matter, exhibit increased adsorption capacity during dry deposition periods [28]. This also explains the positive correlation between Fe-P, Al-P concentrations and organic matter levels under seasonal changes. Among the phosphorus forms in sediments, Fe-P and Al-P are significantly affected by freeze–thaw processes. Without the influence of freeze–thaw, the release of Fe-P and Al-P will increase; it is estimated that the total phosphorus release from sediments in the Yarlung Zangbo River could increase by about 10%. Thus, in the context of global warming, rising temperatures may further intensify the aquatic environmental pressures faced by plateau rivers such as the Yarlung Zangbo River.

4.3. Analysis of Phosphorus Sedimentation Characteristics and Risk Sources in Plateau Rivers

By comparing with typical rivers and lakes in plains regions such as Lake Taihu [39], Lancang River [40], Hanjiang River [41], Sihe River [42], and Orontes [43], the unique distribution characteristics of phosphorus in plateau rivers can be identified. The selected plains rivers and lakes have an elevation range between 28–226 m (Table 1). The phosphorus components in the sediment of plateau rivers and plains water bodies exhibit different distribution patterns (Figure 10). Ca-P and Al-P are considered immobile forms of phosphorus and are the main components in both plateau and plains river sediments. Due to their immobility, their bioavailability is relatively low [24]. In plains water bodies, the proportion of Al-P is higher than that in plateau rivers [17,44]. Research has shown that the variation in TP in Lake Taihu sediment is primarily linked to changes in Al-P [45]. However, for the plateau rivers studied in this paper, Ca-P dominates, accounting for an average of 73.7% in the Yarlung Zangbo River sediment. The precipitation of Al-P occurs mainly in environments where pH > 9, but the pH of the Yarlung Zangbo River water remains around 8, meaning the contribution of Al-P to TP is lower than that of Ca-P. Therefore, in plateau river sediments, TP is predominantly composed of Ca-P.
Compared to other phosphorus forms, NH4Cl-P has the highest bioavailability [35]. The distribution of NH4Cl-P in plateau river sediments is consistent with that in typical plains water bodies, representing the smallest fraction in the sediments. Despite being the least abundant component, it belongs to unstable bound phosphorus that can easily enter the water body from the sediments. Due to its high bioavailability, NH4Cl-P becomes a source of phosphorus for water environment issues [45]. In the Yarlung Zangbo River basin, the proportion of NH4Cl-P is relatively low, generally not exceeding 0.5%. The content of OP is related to the organic matter carried in the water body, and its proportion in plateau rivers is similar to that in plains water bodies, both around 15%. Only in the Lancang River, which carries a large amount of suspended sediment, is the proportion of OP slightly higher (24%), because the sediments contain large amounts of organic matter that are carried into the water. Fe-P, along with NH4Cl-P and OP, are mobile phosphorus forms that can be utilized by algae in the overlying water in sediments, serving as an endogenous phosphorus supply. There is a significant difference in the proportion of Fe-P between plateau rivers and plains water bodies. The proportion of Fe-P in plateau rivers is significantly lower than that in plains water bodies. For instance, the proportion of Fe-P in the Yarlung Zangbo River is only 0.8%, while it typically ranges from 2% to 19% in plains water bodies.
Research has found that high concentrations of mobile phosphorus components in static water landscapes (such as lakes) are closely related to eutrophication [6,13]. For example, sediments in eutrophic Lake Taihu contain very high levels of Fe-P. Conversely, in flowing water bodies (such as the Hanjiang River), eutrophication risks can still occur, even when biologically unavailable phosphorus dominates. Through the analysis of phosphorus components in the sediments of plateau rivers and plains water bodies, as well as studies on the transformation and release of phosphorus components influenced by TOC, it is found that the eutrophication risk in plateau rivers mainly stems from exogenous particulate phosphorus input and internal sources transformed under the promotion of organic matter. Even though the Yarlung Zangbo River has a relatively low bioavailable phosphorus content, given the ecosystem’s fragility and difficulty of recovery, ongoing attention should be given to changes in nutrients like phosphorus and nitrogen in plateau rivers as a result of human activity and global warming.

5. Conclusions

This study investigates the different forms of phosphorus and the spatiotemporal distribution characteristics of TOC in both sediments and the sediment–water interface of the Yarlung Zangbo River. The findings show that the water quality of the Yarlung Zangbo River is generally good, with seasonal variations, where temperature is the main influencing factor for seasonal changes. The middle reaches, which are heavily impacted by human activities, face significant water environmental protection pressures, with particulate phosphorus from the increased agricultural land area being an important source of TP (total phosphorus) in the sediments. The content of phosphorus forms in the sediments is in the following order: Ca-P > OP > Al-P > Fe-P > NH4Cl-P. Influenced by the carbonate rock geology in the basin, Ca-P is the dominant component of TP, accounting for more than 70%. As a plateau river, the sources of eutrophication risk in the Yarlung Zangbo River differ from those in plains rivers. Under the influence of seasonal cycles, the biologically available phosphorus from the transformation and release of organic matter in sediments is abundant. With the continued impact of climate change and human activities, there is an increased eutrophication risk. Continuous monitoring of changes in key nutrients such as phosphorus in the Yarlung Zangbo River is necessary.

Author Contributions

Conceptualization, X.L., Y.B. and Z.W.; data curation, Z.C. and Y.W.; software, M.H.; validation, Z.L. and C.L.; writing—original draft preparation, X.L.; writing—review and editing, A.C.; visualization, Y.B.; supervision, Z.W.; methodology, Y.B. and Z.W. All authors have read and agreed to the published version of the manuscript.

Funding

This study was supported by the National key research and development project (2022YFC3205000), and the National Natural Science Foundation of China (NSFC) (No. 52309107, U2340222), and the Key Research and Development Program of Yunnan (202203AA080009).

Data Availability Statement

The data presented in this study are available on request from the corresponding authors.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Study catchment and sample site locations.
Figure 1. Study catchment and sample site locations.
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Figure 2. Box plot of water quality indicators (pH, temperature, TP, and DO) by region.
Figure 2. Box plot of water quality indicators (pH, temperature, TP, and DO) by region.
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Figure 3. Proportion of particulate total phosphorus (PTP) and dissolved total phosphorus (DTP) in the Yarlung Zangbo River water during the wet and dry seasons.
Figure 3. Proportion of particulate total phosphorus (PTP) and dissolved total phosphorus (DTP) in the Yarlung Zangbo River water during the wet and dry seasons.
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Figure 4. The distribution and proportion of phosphorus forms in the sampling sediments. (A) Phosphorus form results in the wet season; (B) Phosphorus form results in the dry season; (C) Distribution of phosphorus forms in sediment during the wet season; (D) Distribution of phosphorus forms in sediment during the dry season.
Figure 4. The distribution and proportion of phosphorus forms in the sampling sediments. (A) Phosphorus form results in the wet season; (B) Phosphorus form results in the dry season; (C) Distribution of phosphorus forms in sediment during the wet season; (D) Distribution of phosphorus forms in sediment during the dry season.
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Figure 5. Distribution of phosphorus forms in the basin during wet and dry seasons.
Figure 5. Distribution of phosphorus forms in the basin during wet and dry seasons.
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Figure 6. TOC content of the sampling sediment: (A) TOC content of the sediment in the wet season; (B) TOC content of the sediment in the dry season.
Figure 6. TOC content of the sampling sediment: (A) TOC content of the sediment in the wet season; (B) TOC content of the sediment in the dry season.
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Figure 7. Distribution of TN and TN–TP ratio in the Yarlung Zangbo River during the wet season.
Figure 7. Distribution of TN and TN–TP ratio in the Yarlung Zangbo River during the wet season.
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Figure 8. (A) Pearson analysis of total phosphorus (TP), dissolved total phosphorus (DTP), pH, temperature (T), TP (sediment), and land type in waters, and (B) Land use of the Yarlung Zangbo River basin.
Figure 8. (A) Pearson analysis of total phosphorus (TP), dissolved total phosphorus (DTP), pH, temperature (T), TP (sediment), and land type in waters, and (B) Land use of the Yarlung Zangbo River basin.
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Figure 9. Correlation between sediment TOC and phosphorus components: (A) during the wet season; (B) during the dry season.
Figure 9. Correlation between sediment TOC and phosphorus components: (A) during the wet season; (B) during the dry season.
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Figure 10. Comparison of sediment phosphorus components in plateau rivers and plains water bodies (A,B). Plateau river: Yarlung Zangbo River; Plains lakes: Lake Taihu; Plains rivers: Hanjiang River, Lancang River, Sihe River, and Orontes.
Figure 10. Comparison of sediment phosphorus components in plateau rivers and plains water bodies (A,B). Plateau river: Yarlung Zangbo River; Plains lakes: Lake Taihu; Plains rivers: Hanjiang River, Lancang River, Sihe River, and Orontes.
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Table 1. Basic information on rivers.
Table 1. Basic information on rivers.
Lake TaihuLancang RiverHanjiang RiverSihe RiverOrontes
RegionYangtze River DeltaYunnan-Guizhou PlateauWuhan, ChinaShandong, ChinaSyria Turkey
Length (km)/2139157732396
Basin Area (km2)36,900190,000159,000101,554/
Altitude (m)2281207353584
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Liu, X.; Bao, Y.; Chen, Z.; Wang, Y.; Hu, M.; Lasong, Z.; Lamu, C.; Cai, A.; Wang, Z. Spatial and Temporal Distribution of Phosphorus in Plateau River Sediments and Sediment–Water Interface: A Case Study of the Yarlung Zangbo River. Water 2025, 17, 484. https://doi.org/10.3390/w17040484

AMA Style

Liu X, Bao Y, Chen Z, Wang Y, Hu M, Lasong Z, Lamu C, Cai A, Wang Z. Spatial and Temporal Distribution of Phosphorus in Plateau River Sediments and Sediment–Water Interface: A Case Study of the Yarlung Zangbo River. Water. 2025; 17(4):484. https://doi.org/10.3390/w17040484

Chicago/Turabian Style

Liu, Xiangwei, Yufei Bao, Zhuo Chen, Yuchun Wang, Mingming Hu, Zeren Lasong, Cian Lamu, Aimin Cai, and Zhongjun Wang. 2025. "Spatial and Temporal Distribution of Phosphorus in Plateau River Sediments and Sediment–Water Interface: A Case Study of the Yarlung Zangbo River" Water 17, no. 4: 484. https://doi.org/10.3390/w17040484

APA Style

Liu, X., Bao, Y., Chen, Z., Wang, Y., Hu, M., Lasong, Z., Lamu, C., Cai, A., & Wang, Z. (2025). Spatial and Temporal Distribution of Phosphorus in Plateau River Sediments and Sediment–Water Interface: A Case Study of the Yarlung Zangbo River. Water, 17(4), 484. https://doi.org/10.3390/w17040484

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