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Review

The Source, Distribution, and Environmental Effects of Suspended Particulate Matter in the Yangtze River System

by
Jianxin Fan
1,2,*,
Jiaxin Yang
2,
Fulong Cheng
2 and
Shikuo Zhang
2
1
Engineering Laboratory of Environmental Hydraulic Engineering of Chongqing Municipal Development and Reform Commission, Chongqing Jiaotong University, Chongqing 400074, China
2
School of River and Ocean Engineering, Chongqing Jiaotong University, Chongqing 400074, China
*
Author to whom correspondence should be addressed.
Water 2023, 15(19), 3429; https://doi.org/10.3390/w15193429
Submission received: 12 August 2023 / Revised: 18 September 2023 / Accepted: 26 September 2023 / Published: 29 September 2023
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Abstract

:
Suspended particulate matter (SPM) plays a crucial role in biogeochemical cycles in the aquatic environment because of its ubiquitous presence, mobility, and physicochemical properties. This work gathered and summarized the published information about SPM in the Yangtze River system, including source, distribution, and environmental effects. Results indicated that the SPM concentration was high in the flood period and low in the dry period. Compared to midstream and downstream, the SPM concentration was the lowest in the upstream of the Yangtze River system. Furthermore, the level of SPM concentration was influenced by human activities, such as shipping, dredging, construction of water conservancy projects, and industrial/agricultural emissions, as well as natural conditions, such as wind, rainfall, and phytoplankton. Moreover, SPM would impact the element cycle in the water environment, including N, P, heavy metal, and persistent organic pollutants. In addition, SPM adsorbed these elemental ions or particles in water on its surface. Still, this adsorption was usually unstable, and when the properties of SPM itself or external environmental conditions changed, these elements would be released into the surrounding water body. With the flow of SPM in the water, these elements migrated upstream and downstream with the river. Thus, this work reveals the current status of SPM in the Yangtze River system, which provides an essential reference for further research on SPM in the water system.

1. Introduction

Suspended particulate matter (SPM) is all particulate matter suspended in rivers, lakes, and reservoirs, usually consisting of particles less than 62 μm in diameter [1]. SPM plays an important role in many biogeochemical cycles and serves multiple ecosystem functions [2]. Generally, SPM is divided into inorganic suspended particulate matter (ISPM) and organic suspended particulate matter (OSPM). In most cases, ISPM is partly derived from topsoil and sediments, with illite dominating, followed by chlorite, kaolinite, and montmorillonite [3]. The main ISPM also varied with catchment areas. Kaolinite was typically found in tropical river systems where weathering intensity was relatively high and tended to form ISPM from rivers in tropical or subtropical areas of intense chemical weathering [4]. OSPM is derived from plankton, aging or dead algae, microorganisms, and other biological particles [5]. Consequently, OSPM in SPM acts as an overlay layer for inorganic particles, dominating the interaction of SPM with the surrounding water environment. Typically, there is also particulate organic carbon (POC) in SPM, which is often part of inland water organic carbon stocks and is a substantial carbon pool [6], affecting both the dissolved and inorganic carbon cycles [7,8]. Most rivers carry suspended material enriched in organic carbon [9]. In addition, Liu et al. (2019) found that the POC content was generally higher in SPM in Asian rivers [10]. A high SPM content would decrease the water column’s transmittance, resulting in a decrease in the amount of submerged vegetation, which may further reduce the water quality [11]. SPM is dominated by material mobilized from the upstream drainage basin by erosion, such as allochthonous or autochthonous organic material that is then transported through the river basin [12]. Moreover, the degree of impact on water quality varies depending on the source of SPM [10]. In addition, the surface of SPM is covered with a bio-organic film composed of microorganisms and plankton, which adsorbs organic matter through the surface carbon and hydroxyl functional groups. This bio-organic film is the key to influencing the migration and transformation of pollutants and particles [13]. Moreover, SPM dominates the transport of related contaminants in the water system [14], which can reduce the transparency of the water body and change the underwater light field [15]. Many processes, such as movement, sedimentation, suspension, and resuspension, will cause pollutants to be fixed to the surface of the SPM. These pollutants can be transported downstream, significantly reducing water quality [13]. At the same time, it also controls the primary production by phytoplankton and bacterioplankton and, hence, secondary production (e.g., of zooplankton and fish) [16].
As the third-largest river in the world and the largest river in China, the Yangtze River has a total length of 6363 km and a basin area of 1.8 million square kilometers. An SPM study of 94 lakes in the Yangtze River system revealed that the concentration of SPM was 44.87 mg/L [11]. SPM with other substances adsorbed in these lakes might pose a risk due to the high resuspension rate [17]. What is more, in the Three Gorges reservoir area, a high SPM concentration level occurred during the flood period (average SPM concentration was 54.03 mg/L). Relatively low concentration in the dry period (average SPM concentration was 3.64 mg/L), it was also discovered that SPM had a significant effect on water quality by changing the concentration of the nutrient or blocking the light intensity in the water column [1]. Research on SPM in the Yangtze River system began in 1990, focusing on the concentration of trace metal particles on the SPM downstream of the Yangtze River and emphasizing the vital influence of natural weathering and erosion in the drainage basin on the SPM transport [18]. Subsequently, a study of elemental enrichment in the SPM of the Yangtze River was carried out in 1993. It was found that the content of heavy metals in SPM in the Yangtze River was lower than in other industrialized countries [19]. So far, research on SPM has mainly focused on particulate organic carbon (POC) [20], particulate phosphorus (PP) [21], particulate nitrogen (PN) [22], heavy metals [23], and persistent organic pollutants (POPs) [24] in SPM [14], among which the element cycle in water is also related to SPM in water systems [25].
In recent years, there have been more and more SPM studies on lakes and rivers in the Yangtze River system, and the effect of SPM concentration on pollutant adsorption in the water environment has also received more attention [20]. However, most studies about the SPM have focused on a lake or river of the Yangtze River system, and fewer studies have been conducted on the distribution, sources, and concentration levels of SPM in the entire Yangtze River Basin. Therefore, it is significant to study the current status of SPM in the Yangtze as a whole river system. This paper reviewed the sources, distribution characteristics, and environmental effects of SPM in the central water systems of the Yangtze River Basin in China, including the main tributaries of the Yangtze River and the lakes in the basin, to deeply understand SPM in the Yangtze River system and its possible potential risk. The main contributions of this paper are: (1) the main methods of collection and filtration of suspended particulate matter were summarized; (2) the spatial and temporal distribution characteristics and patterns of suspended particulate matter in the Yangtze River system were discussed; (3) the sources of suspended particulate matter in the Yangtze River system and the key factors affecting their levels were identified; (4) the environmental effects of suspended particulate matter and other elements and substances in the Yangtze River system were analyzed; and (5) the shortcomings of the current research in the Yangtze River system and the focus of the subsequent research on suspended particulate matter in the water system were pointed out.

2. Data Sources

This article was based on a search of Web of Science, ScienceDirect, SpringerLink, and other literature database websites. First, the search keyword “suspended particulate matter”, Until April 2023, 4329, 51,823, and 18,175 articles were retrieved from the Web of Science, ScienceDirect, and SpringerLink websites from 1990 to 2023, respectively. Then, the keyword “Yangtze River” was used to filter, and 170, 2313, and 1726 articles were obtained. Finally, a total of 87 papers having the highest correlation with SPM in the Yangtze River system were selected by hand from these papers. Through reading and investigating the literature chosen, it was obtained that the research on SPM in the Yangtze River system mainly focuses on (1) rivers: the mainstream and tributaries of the Yangtze River, the Three Gorges Reservoir area, and the estuary of the Yangtze River; (2) lakes: Dongting Lake, Poyang Lake, Taihu Lake, Chaohu Lake, etc.

3. Materials and Methods

3.1. Study Area

The Yangtze River Basin refers to the vast area through which the mainstream and tributaries of the Yangtze River flow, spanning the three major economic zones of eastern, central, and western China, with 19 provinces, municipalities, and autonomous regions. The upstream includes the Jinsha River Basin in Sichuan and the Three Gorges Reservoir, with the midstream starting from Yichang (100–2400 m above sea level) and the downstream at the estuary of the Poyang Lake (about 32 m above sea level). Except for some areas at the source, about 76% of the Yangtze River Basin belongs to the subtropical monsoon climate zone. The Yangtze River Basin has cold and dry winters and humid and hot summers, and precipitation throughout the basin is mainly concentrated in the rainy season, accounting for 70–90% of the annual rainfall [26]. The names of the sampling points and their abbreviations mentioned below are provided in Tables S1 and S2. Figure 1 shows the Yangtze River system with its main tributaries and the provinces through which it flows.

3.2. Sample Collection

Generally, there are two main ways to collect SPM water samples: a water sample collection machine and a manual collector, as shown in Table 1.
According to the summary of the water sample collection method of the Yangtze River system, there are many types of water sample collection machines. In Table 1, the Niskin sampler is the most used water sample collection machine, followed by the Rosette sampler. The Van Dorn sampler is the most practical because it can collect samples of specific water formations, making gathering SPM in water bodies easier. Then, the two discrete sampling systems in the table, the continuous-flow centrifuge (CFC) and hydrocyclone (HC) [14], and two water sample collection machines also have the function of water sample filtration. For manually collected water samples, in Table 1, the most common use is the polyethylene water sampler, followed by the plexiglass water sampler. The water sample is usually collected manually using a water sample collector and placed in a specific water sample preservation container.

3.3. Sample Filtration

There are two main filtration methods for SPM collected from the surface of the water body: filter filtration and filter membrane filtration, as shown in Table 2.
Two of the most commonly used filters for filter filtration are the glass ultrafiber filter and the cellulose acetate membrane filter. The membranes in the filters are available in different pore sizes and diameter sizes. In addition, there are also microporous membrane filters, fiber filters, and quartz filters. For membrane filtration, there are polytetrafluoroethylene (PTFE) membrane filtration and Millipore membrane filtration. The continuous-flow centrifuge and hydrocyclone mentioned in the previous water sample collection also extract SPM from the water sample. In situ continuous flow centrifugation relies on the size and density characteristics of the particles to facilitate their separation from water, similar to the sedimentation that occurs in natural systems, which is controlled by the pore size of the filter [44]. There is also a filtration unit consisting of a peristaltic pump (80EL005, Millipore Co., Burlington, MA, USA) and a 142 mm-diameter filter plate to filter water samples through 0.45 μm GFF [38].
Further filtration and extraction are required for the collected sample water to obtain the final SPM sample you want to study. As shown in Figure 2, taking the Whatman GF/F glass ultrafiber filter as an example, the SPM extraction process diagram is summarized according to the relevant references in this paper. Firstly, the water samples were collected by selecting the appropriate method and then using the water sampler chosen to collect the water samples. The water samples were kept in the designated containers and returned to the laboratory. Secondly, the collected water samples were filtered using Whatman GF/F glass microfiber filters. Thirdly, the filtered particulate matter samples were washed with purified water to remove the dissolved substances, and finally, the SPM sample was obtained.

4. Spatiotemporal Distribution of SPM

4.1. Temporal Distribution

There is a temporal distribution of SPM in the Yangtze River Basin system, which has apparent seasonality during the year, and the difference mainly occurs during the flood period (May–October) and dry period (November–April).
As shown in Figure 3, the concentration of SPM in the flood period of rivers in the Yangtze River Basin was higher than that in the dry period because high rainfall during the flood period caused a large amount of particulate matter to be washed into the river and led to a high SPM concentration in the flood period. In the previous study of the Yangtze River in the reservoir section of the Three Gorges Dam, it was found that when the local water flow rate in the Three Gorges reservoir area was too large during the flood period, the bottom sediment would also be suspended again, thereby increasing the concentration of SPM [26]. However, a few studies of SPM in the Yangtze River system showed that the SPM concentration in the dry period was higher than that in the flood period, as shown in Figure 3 and Table S1. Notably, in the Houliu River (a tributary of Aha Lake) and the Xiaoche River (a tributary of Hongfeng Lake) upstream of the Yangtze River system, the concentration of SPM in the dry period was slightly more significant than that in the flood period. But at the Wanzhou Hydrological Station upstream of the Three Gorges Dam, and especially near the estuary of the Yangtze River, the SPM concentration in the dry and flood periods varied greatly. The concentration of SPM in the dry period was much higher than that in the flood period. SPM studies conducted downstream of the Yangtze River and the Yangtze River estuary showed that the river transported less sand during the dry period, and the sediment from the bottom of the river was resuspended because of the wind, resulting in a higher SPM concentration in the dry period [45].
The concentration of SPM in lakes in the Yangtze River Basin is shown in Figure 3 and Table S2. This paper summarizes the study of the lakes in the Yangtze River system. Much research was conducted on the great lakes, and some concentrated on small lakes, including Aha Lake and Hongfeng Lake in Guizhou Province upstream of the Yangtze River. It was generally agreed that the concentration of SPM in the flood period was slightly higher than in the dry period. However, the study of Poyang Lake found the opposite conclusion, showing the characteristics of a higher concentration of SPM in the dry period and a lower concentration of SPM in the flood period [46]. Liu et al. (2019) found that the temporal changes in the shallow lake SPM were mainly human induced. In previous studies of Poyang Lake, significant dredging activity had been reported in these waters, accompanied by very high concentrations of SPM, which might have led to high SPM concentrations during the dry period [47]. For instance, Shuai et al. (2021) found that the periods strongly influenced changes in lake SPM concentration. Additionally, the reason for the negative correlation between SPM concentration and precipitation was that precipitation and river flow increased during the flood period, which reduced the concentration of suspended solids [48].
In general, under normal circumstances, SPM in tributaries and lakes of the water system is temporally characterized by high SPM concentrations during the flood period when rainfall is high and low SPM concentrations during the dry period when rainfall is scarce. This phenomenon is because rainwater during the flood period washes a large amount of particulate matter into the water body, resulting in a high SPM concentration. However, in some exceptional cases, such as external factors, including anthropogenic activities, the inflow of tributaries, or the backfilling of seawater, it is possible to cause the opposite phenomenon of SPM in time distribution.
For inter-annual changes, in this paper, the interannual variation of SPM concentration in the upstream, midstream, and downstream of the Yangtze River system, the Wujiang River, the downstream of the Three Gorges Dam, and the estuary of the Yangtze River were selected, respectively, as shown in Figure 4 and Table S3. From Figure 4, the SPM concentration showed a downward trend as the years increased. It could also be seen that there was a significant decrease in SPM concentration in 2008, followed by a slight recovery in subsequent years, possibly due to the extreme rainstorm weather in 2008 affecting the SPM concentration data for that year. For the reduction of SPM concentration in the upstream of the Yangtze River, because of artificial environmental protection, and for the midstream and downstream of the Yangtze River, a large number of SPM was intercepted in the reservoir area due to the construction of the Three Gorges Dam, which significantly reduced the SPM concentration in the midstream and downstream of the Yangtze River [49]. In a previous study of SPM downstream of the Three Gorges Dam, the concentration of SPM also generally declined between 2002 and 2012 [50].

4.2. Spatial Distribution

For the spatial distribution of SPM, the upstream, midstream, downstream of the Yangtze River system, tributary, and mainstream of the Yangtze River system were investigated and analyzed, as shown in Figure 5 and Table S4.
In the studies upstream of the Yangtze River, including the Aha Lake and Hongfeng Lake SPM in Guizhou Province and the Wulong Station, they found that the minimum concentration of SPM was generally small, reaching 1.19 mg/L, 2.59 mg/L, and 2.00 mg/L in these places [51]. In the Yangtze River system, the SPM concentration increased from upstream to downstream [35]. It was suggested that SPM tended to increase with the direction of the flow, possibly due to the introduction of more artificial particulate matter further downstream than in the source area [35]. For example, at Fuling Station and Cuntan Station, it was reported that the concentration of SPM reached 226.80 mg/L and 227.00 mg/L at their highest [51], probably because more SPM was artificially imported through the city of Chongqing. Fan et al. (2021) also found lower concentrations in the upstream samples and higher concentrations in the downstream samples of the Minjiang River, a tributary of the upper Yangtze River [52]. However, the trend was not noticeable. What’s more, it was discovered that the concentration of SPM tended to decrease due to the retention effect of the reservoir area on the river water. The concentration of SPM in the Three Gorges reservoir area dropped to 65.30 mg/L [50]. For instance, Liu et al. (2022) reported that after the Three Gorges Dam’s completion, the SPM downstream of the Three Gorges Reservoir Area of the Yangtze River decreased significantly [53]. In the inner area of the Three Gorges Reservoir, the SPM also showed a decreasing trend from the tail area to the upstream due to the deposition of particles along the mainstream of the Three Gorges Reservoir [51].
In the midstream of the Yangtze River, the study of the Dongjing River that detected the Yangtze River section found that the SPM concentration reaching 147.24 mg/L was higher than that in the Three Gorges reservoir area. Because of the diversion, transfer, and connection of rivers and lakes in the Yangtze River Basin, water flow slowed down, increasing SPM concentration. These factors resulted in the water quality upstream of the Three Gorges Yangtze River being better than that of the midstream of the Yangtze River [54]. Moreover, the concentration of SPM in the midstream of the Yangtze River increased continuously due to the increase in riverbed erosion and the inflow of water [26]. Especially in the Hanjiang River, a tributary of the midstream of the Yangtze River, and Nangang, the minimum concentration of SPM had reached 203.20 mg/L and 270.30 mg/L [55].
In the downstream part of the Yangtze River, the change in downstream SPM concentration was less evident than that in the midstream. For example, in Chaohu Lake, a typical large lake downstream of the Yangtze River, its SPM was 61.90 mg/L at its maximum and 31.60 mg/L at its minimum. To the SPM downstream of the Yangtze River, the mainstream of the Yangtze River was diluted to a certain extent due to the diversion of downstream lakes and the inflow of a large number of tributaries, making the downstream human input less significant [27]. The maximum SPM concentration in Taihu Lake was reduced to 39.11 mg/L. Moreover, the concentration of SPM in the mainstream of the Yangtze River was also affected by the combination of water flow, slope, and channel width. In contrast, the spatial variability of the average SPM concentration in the Yanghuai River Basin downstream of the Yangtze River was relatively small [11]. However, the SPM concentration near the Yangtze River estuary increased to 155.40 mg/L. This phenomenon might be due to the large number of ships navigable at the estuary, which caused sediment resuspension at the bottom of the river, thereby increasing the concentration of SPM. At the Yangtze River estuary, the SPM concentration began to drop to 92.00 mg/L [55]. The previous study of SPM concentration in the Yangtze River estuary showed a general decreasing trend from land to sea [56]. It was most likely that the concentration of suspended solids was reduced due to seawater’s dilution at the Yangtze River estuary.
The study of mainstream and tributaries showed that the concentration of SPM in the mainstream was significantly higher than that in the tributary, and the distribution of different tributaries was also different [1]. As shown in Figure 5 and Table S4, the SPM concentration of the Hanjiang River and the Dongjing River, tributaries of the midstream of the Yangtze River, was significantly higher than that of the mainstream of the Yangtze River. Because the flow of the mainstream was small, while most of the mainstream of the Yangtze River had a large water flow due to the inflow of water from the tributaries, the SPM was diluted, and the SPM concentration of the mainstream of the Yangtze River was small.
For the spatial distribution, the SPM concentration in the mainstream of the Yangtze River gradually increases from the source to the upstream. The upstream of the Three Gorges Reservoir area retains river water due to the construction of water conservancy projects, resulting in the settlement of SPM. Then, SPM concentration temporarily shows a trend of decreasing downstream of the Three Gorges Reservoir Area. However, due to the divergence of riverbeds and lakes in the midstream and downstream of the river, the concentration of SPM increases, but the spatial heterogeneity of SPM in the midstream and downstream of the Yangtze River system is not high. However, the SPM concentration is significantly higher than that in the upstream. When it is close to the estuary of the Yangtze River, due to the influence of seawater irrigation at the estuary and other factors, the SPM in the river is diluted, showing an increased trend of SPM concentration in the first stage and then a decreased trend from the midstream and downstream to the estuary of the Yangtze River. Previous studies found that upstream water quality was better than midstream and downstream [54]. For tributaries and mainstream, the SPM concentration of tributaries is higher than that of the mainstream, and the concentration of SPM in lakes is significantly lower than that of rivers [57].

5. The Source of SPM and the Factors That Affect Its Concentration Level

5.1. Sources of SPM

5.1.1. Autochthonous Production

Autochthonous SPM production is imported from the resuspension of aquatic organisms (such as phytoplankton, zooplankton, and microorganisms) and sediments [58,59]. Moreover, SPM in the aqueous system is greatly affected by autochthonous production [60].
SPM in the Yangtze River system’s lakes is mainly primary production, and phytoplankton is the main component of lake SPM. The abundance of algae-derived SPM can also be changed by increasing the number of phytoplankton [61]. In shallow lakes of eutrophication in the midstream and downstream of the Yangtze River, phytoplankton production and sediment resuspension have also been found to be two essential sources of SPM [10]. In most cases, algal particles contribute more to the SPM, and the corresponding algal particles are larger than those of mineral particles [48,62]. In studies of eutrophic lake bodies such as Taihu Lake, it was found that most of the SPMs in the lake were cyanobacterial aggregates [63,64]. Large macrophytes and plankton production become the primary sources of SPM in rivers in the Yangtze River system. A previous study conducted in Qingjiang, Enshi, north of the Yangtze River, discovered that the entry of aquatic plants, including macroplants and phytoplankton, into adjacent river ecosystems might lead to high values of SPM organic matter [65]. In addition, it was found that plankton can also seriously affect the source and transformation of SPM in the mainstream of the Yangtze River [35]. A study about SPM in the Three Gorges reservoir section expressed that the tributaries with relatively low SPM concentrations contained a higher percentage of algae-derived SPM [1].

5.1.2. Allochthonous Input

Allochthonous SPM inputs are mainly from rainfall runoff, tributary inflows, and anthropogenic emissions, and essential sources of allochthonous inputs are runoff from land-based plant biomass present in watersheds, topsoil, and sediments eroded from organic-rich bedrock [66].
The SPM of lakes in the Yangtze River system is mainly input from tributaries. The Goxi River, the largest inflow river into Taihu Lake, significantly increased SPM concentrations when large amounts of particulate matter flowed into the open area south of Taihu Lake during rainfall [5]. In the study of Chaohu Lake, a shallow turbid lake, it was also shown that the SPM concentration of Chaohu Lake was higher, which was also derived from the input of tributary rivers [42]. The SPM of rivers in the Yangtze River system is mainly input from rainfall runoff and anthropogenic discharge. Heavy rainfall can affect changes in SPM in river systems through rainfall runoff [20]. For instance, it had been found that the northwest Qianxi region, where Caohai Lake was located, was rich in mineral resources. Due to the imperfect use of equipment, backward technology, and the random dumping and piling of smelting wastewater and slag, the source of SPM in the tributaries here was mainly the input of human discharge [23].

5.2. Affecting Factors

5.2.1. Anthropogenic Factors

Shipping

In rivers, high concentrations of SPM are caused by sandy suspension caused by shipping [11]. The study of shipping in Poyang Lake revealed that shipping led to sediment disturbance and increased concentrations of suspended solids [46]. Lake shipping can also change the spatial distribution of SPM [11]. Shipping activities affect the distribution of smaller particles to a greater extent [67]. And due to the perturbation of the ship’s engine, ships related to underwater detection have a greater degree of influence on the size of the SPM [67].

Dredging

Resuspended substances and residues are generated during dredging [68]. Liu et al. (2016) found that dredging increased the phosphorus content and phosphorus flux of SPM accumulated in the river water body. Moreover, under the influence of lake currents, the SPM generated by dredging activities spread to other river and lake areas, thereby increasing the SPM concentration in other areas [32]. Sand dredging activities also significantly impact the content of river sand overhangs. For example, the sand dredger will cause high water turbidity during sand mining operations, and the SPM content in the water body will increase significantly [69]. Since there is no fixed time for dredging and dredging activity, this may change the seasonal distribution of SPM in the channel [70].

Construction of Water Reservoirs

Dam construction can change the SPM concentration in the water body during the construction and operation of cascade reservoirs [71]. Especially the impact of dam construction on the SPM concentration upstream of the Yangtze River. In the study of cascade reservoirs in the Jinsha River Basin, it was discovered that the development of water elevator steps helped to reduce the concentration of suspended solids. Meanwhile, significant monthly changes in SPM concentration were also found in the Panzhihua, Luning, Huatan/Baihetan, and Xiangjiaba areas. These results indicated that the SPM in these areas was greatly affected by cascade reservoirs, and the decrease in SPM concentration mainly occurred in reservoirs and their vicinity. Reservoirs propagated their effects upstream and downstream, which affected river biodiversity and river ecosystems by constructing the movement and exchange of SPM, organic matter, and nutrients. In addition, cascade reservoirs might cause discontinuous fluctuations in SPM properties, thereby changing the SPM dynamics of rivers [20]. After the construction of the Three Gorges Dam (TGD) in the midstream of the Yangtze River, it impounded water from a natural state to an artificially controlled state, and the water flow rate was significantly reduced, which was conducive to the deposition of SPM [53] and the reduction in the SPM concentration [35]. During the water storage process of the Three Gorges Reservoir, the concentration and particle size of SPM in the Three Gorges Reservoir tended to decrease due to the deposition of large particle-sized particles under the long water retention time [51]. Therefore, the impact of hydropower development on SPM deserves detailed investigation.

Industrial and Agricultural Pollution

In recent years, with the population’s rapid growth and industry development, a large amount of domestic sewage and industrial wastewater (such as wastewater from paper mills and chloralkali plants) have been discharged into rivers and lakes. When studying SPM in Chaohu Lake, it was discovered that chemical wastewater might be the primary source of organic pollutants, such as polychlorinated phenyl ether, in SPM in the lake [72]. The effluent debris (untreated sewage) generated by the plant contributed 16.8–26.6% to the SPM input of the Yangtze River [35].

5.2.2. Natural Factors

Wind Stress

The resuspension and sedimentation of particulate matter are mainly affected by waves generated mainly by wind [73]. Wind is also an essential factor affecting the distribution of SPM in the surface water layer by enhancing vertical mixing. Wind speed has little effect on the change in average concentration of SPM across the lake, but in shallow lakes, wind stress plays an important role in changing the SPM’s concentrations [10]. Strong winds can cause dramatic changes in the concentration of SPM in a short time [11]. Numerous studies showed that wind-driven sediment resuspension was the main force controlling short-term changes in SPM concentration [74], and this phenomenon was also found in other large inland shallow lakes such as Poyang Lake, Dongting Lake, and Taihu Lake [32]. Due to the shallow water and frequent wind speeds, the sediment resuspension rate of Taihu Lake was higher than that of many other lakes in the world [75]. In addition, a very high positive correlation between wind speed and SPM concentration was found in Taihu Lake [76], and the change in SPM concentration in spring and winter was mainly driven by wind speed changes [5].

Rainfall

SPM in water systems can reach high levels within hours to days after a heavy rainfall, and a large number of suspended solids carried by the river are transported to the lake in a short time. River emissions associated with heavy rainfall also affect changes in SPM by transporting sediment, nutrients, and pollutants [77]. Under the influence of heavy rainfall levels, the basin transfers a large number of suspended particles into the lake through runoff, resulting in a rapid increase in the SPM concentration at the corresponding inlet and a significant expansion of the turbid plume area. However, in lake waters, the river plume area and spatio-temporal expansion are primarily controlled by rainfall and river discharge. The amount of rainfall over the watershed was the main factor regulating the area of the river plume. In Taihu Lake, both the maximal and average river plume areas significantly increased with increasing amounts of rainfall [32]. Xu et al. (2019) discovered that when heavy precipitation occurs, the flow of the Goxi River, a tributary of Taihu Lake, increases significantly [69]. A large amount of sediment would flow into the river channel with rainwater, resulting in a sharp increase in water and SPM flowing into Taihu Lake, which further affected the distribution of SPM in the water body.

Phytoplankton

The production of phytoplankton is an important source and influencing factor of SPM in lakes [10]. In the tributaries with lower SPM concentrations, the proportion of algae-derived SPM is higher, and the main category of particulate matter reservoirs is positively correlated with chlorophyll (Chla), indicating that live algae contribute more to SPM [78]. Schmitt et al. (2014) found that in the past 15 years, algal blooms frequently occurred in summer in Taihu Lake, and the accumulation of algal blooms increased the concentration of SPM in the lake [74]. In summer, algae-derived SPM was also shown to dominate in Taihu Lake and Sanwan (Gongshan Bay, Meiliang Bay, and Zhushan Bay) [34], and the occurrence of phytoplankton and algae had a significant impact on the stability of SPM [60]. In addition, a gradual temperature increase can result in rapid phytoplankton growth and an increase in autochthonous SPM in the water column as algal biomass increases [58].
The affecting factors mentioned above are shown in Figure 6.

6. The Effects of SPM on Contamination

6.1. The Sorption of N and P on SPM

Particulate nitrogen (PN) is an essential contributor to nutrient load and a potential source of dissolved nutrients [22]. Furthermore, PN is a vital source of nitrogen in rivers [79]. Gao et al. (2019) discovered that the concentration of PN in the lake was increased by three kinds of rivers flowing into Taihu Lake [22]. SPM from polluted rivers typically contained high levels of nitrogen [80], with agriculturally polluted rivers having the most significant impact on TN, DTN, NO3−, and dissolved inorganic nitrogen (DIN) concentrations in Taihu Lake and NH4+ concentrations in industrially polluted rivers [22]. In SPM studies upstream of the Yangtze River, phytoplankton were found to be the primary source of PN in SPM [35]. In the aquatic environment of Chaohu Lake downstream of the Yangtze River, most of the excess N from external sources was usually concentrated in the SPM due to plankton debris [80]. In addition, PN content in SPM in rivers in southwest China ranged from 0.17–0.93%, with an average of 0.52 ± 0.24% [66].
In the lake system of the Yangtze River system, P tends to bind to SPM [25], and at the same time, P in SPM is mainly found in eutrophicated lake water bodies [58,81]. In the Taihu Lake Basin, it was also discovered that the phosphorus in SPM was mainly particle phosphorus [22]. Particulate phosphorus (PP) is the proportion of phosphorus attached to the SPM and can be produced by algal particles, bacterial cells, and resuspended deposits [58]. Phosphorus sources in shallow eutrophication lakes include labile phosphorus in SPM [21], where high phosphorus concentrations were mainly derived from redox-sensitive P (Fe-P), aluminum-bound P (Al-P), and organic P (Org-P) [68]. Additionally, the phosphorus in SPM could be divided into inorganic phosphorus and organic phosphorus. Particulate inorganic phosphorus mainly appeared in the mineral form of SPM, such as orthophosphate (Ortho-P), pyrophosphate (Pryo-P), polyphosphate (Poly-P), etc. [82]. Particulate organic phosphorus was a combination of phosphorus input in life and detrital organic molecules, such as monophosphate (Single-p), diester (Diester-P), and phosphate [83].

6.2. The Sorption of Heavy Metals on SPM

Heavy metals refer to metals with a density greater than 4.5 g/cm3, including gold, silver, copper, iron, mercury, lead, cadmium, etc. SPM has a high specific surface area, and reactivity and dissolved heavy metals are easily adsorbed by SPM [23]. Therefore, heavy metals in water tend to accumulate in SPM [84]. Similarly, SPM has a strong adsorption capacity for heavy metals due to reactions at the solid-water interface [23]. Fan et al. (2021) found that SPM and sediments carry more than 90% of the heavy metal load in aquatic systems [52].
For lakes in the Yangtze River system, a wide variety of heavy metals of SPM were found in karst lakes in Guizhou, and their abundance order was Fe > Mn > Zn > Pb > Cr > Ni > Cu > Cd [23]. SPM also had the largest adsorption capacity for Fe. In addition, previous studies also found in Aha Lake and Hongfeng Lake that Zn was present in SPM in the form of sphalerite (ZnS) [85]. SPM concentration in water was also an important factor affecting the partition coefficient of heavy metals in SPM [86]. Studies on rivers in the Yangtze River system showed that the δ56Fe value in the SPM of the rivers and lakes in southern China varied between −1.36 and 0.07‰, with a highly linear relationship between δ56Fe and Fe/Al, which proved that the terrestrial source SPM carried by the main tributaries of lakes had an essential effect on Fe in the lakes [39]. Cd, As, Pb, and Se in the Yangtze River were highly enriched in SPM, and SPM concentration and water pH controlled the adsorption of metals by particles [40]. In the analysis of the results of dredging in the Chaohu Bay area, it was discovered that after dredging, high levels of metals from contaminated inflows into rivers were adsorbed on the surface of SPM, which might adversely affect the sediment–water interface (SWI) [84]. Li et al. (2022) found that SPM diffused throughout the aquatic environment, and almost all heavy metal concentrations in rivers increased with the flow direction, indicating that heavy metals were adsorbed on the surface of SPM and migrated with it [13].

6.3. The Sorption of Persistent Organic Pollutants (POPs) on SPM

Compared with large particles, POPs are more likely to bind to SPM with a smaller particle size, a relatively large specific surface area, more adsorption sites, and a relatively high organic matter content [24]. Because the flow rate of lake water is slower than that of rivers, SPM retention time is longer, and the amount of organic pollutants detected in lake SPM is also higher than that in rivers [31]. At present, POPs research in SPM mainly focuses on lake systems, such as the discovery that SPM contains phenolic endocrine disruptors (EDCs) [87], phthalate esters (PAEs) [88], polycyclic aromatic hydrocarbons (PAHs) [89], organochlorine pesticides (OCPs) [24], polybrominated diphenyl ethers (PBDEs) [24], polychlorinated biphenyls (PCBs) [24], organophosphate esters (OPEs) [33], polychlorinated diphenyl ethers (PCDEs) [72], polyfluoroalkyl substances (PFAS) [90], pentachlorophenol (PCP) [91], and so on.
For the lakes in the Yangtze River system, previous studies showed that the PFAS concentration found in the SPM of Poyang Lake is 0.0013–0.0098 μg/kg in the SPM of the Yangtze River, and there were mainly long-chain PFCAs and PFSAs in the SPM [90]. Phenolic EDCs were detected in all water samples in the northern part of Taihu Lake at concentrations of 1.52–32.6 μg/kg, indicating that they were ubiquitous in the northern aquatic environment of the Taihu Lake Basin [92]. The organic matter and minerals in SPM were the main components of adsorbing EDCs, and the high organic carbon content in SPM made it easy to adsorb EDCs in water [87]. After studying 6 PAEs in the SPM system of Chaohu Lake, it was shown that PAEs were ubiquitous pollutants in Chaohu Lake, and the detection range of Σ6 PAEs in SPM was 14.4–7129 μg/kg [93]. PCDEs were also found in Chaohu Lake. PCDEs were typical halogenated aromatic pollutants with a variety of toxicological effects on organisms, and the concentration of PCDEs in the SPM of Chaohu Lake was between 0.33 and 2.013 μg/kg [72]. The normalized adsorption coefficient (logKoc) of soil organic carbon content can explain the distribution of PCDEs in Chaohu SPM to a certain extent, and in general, compounds with a higher logKoc value were more easily absorbed by SPM [72]. There were also studies to detect Σ12 PAEs and Σ20 PAHs in the SPM of 46 lakes in China. PAHs were dominant hypocyclic polycyclic aromatic hydrocarbons; the content of Σ12 PAEs in the SPM phase was 0.175–10.921 μg/kg [45], and the concentration range of Σ20 PAHs was 0.334–38.427 μg/kg [89]. PCP can be detected in all water environments of most rivers or lakes in China, among which PCP pollution was the most serious in the Yangtze River Basin, with a concentration range of 0.0011–0.594 μg/kg, especially the highest residual amount in Dongting Lake [91].
For rivers in the Yangtze River system, the average concentration of Σ6 OPEs in SPM was found in the Minjiang River water body as 38,463.79–45,641.89 μg/kg [41], and the results showed that the concentration of OPEs in SPM samples was negatively correlated with SPM content. SPM content was one of the main factors affecting the concentration of OPEs in SPM [41]. OPEs were found in the Zijiang, the main tributary of the midstream of the Yangtze River, and the concentration of OPEs in SPM ranged from 1.4–19.1 μg/kg [33], and the OCPs, PBDEs, and PCBs of SPM in Nanfei River, a tributary of Chaohu in the downstream of the Yangtze River, discovered that the contents of the three POPs in SPM were 57.38 μg/kg, 26.7 μg/kg, and 1.336 μg/kg, respectively [24].
In general, the environmental behaviors of SPM can be summarized in Figure 7. SPM enters the water body through sediment resuspension and the erosion of the soil on the riverbank slope, then migrates from upstream to downstream with the current in the river. SPM in the water body will adsorb some substances on the surface, such as N, P, POPs, and heavy metals. N and P elements, especially nitrate and phosphate, can be adsorbed on SPM surfaces. While these adsorption processes are usually unstable, some changes would occur due to changing environmental conditions. Additionally, heavy metals and POPs entering the water body can be adsorbed by fine SPM, and the sorption process of SPM for contaminants varies with composition. Thus, a variety of SPM is widely distributed in the Yangtze River system and migrates in river and lake water environments.

7. Conclusions and Perspectives

This paper discovered that SPM is ubiquitous in the Yangtze River system in China. SPM within the water system has an apparent seasonal distribution and spatial heterogeneity. Under normal circumstances, the concentration of SPM in the flood period is significantly higher than that in the dry period. Moreover, from the overall perspective of the Yangtze River system, the SPM concentration is lower than that in the midstream and downstream, and the water quality upstream is better than that in the midstream and downstream. Additionally, the SPM concentration in the tributaries of the Yangtze River system is also higher than that in the mainstream of the Yangtze River. The input of SPM in the water system is divided into allochthonous input and autochthonous production, and human factors and natural factors are the key factors that affect the concentration level of SPM in the water system. Moreover, different environmental effects are generated by SPM with other substances in the water system, such as P, N, heavy metals, and POPs. In recent years, due to the protection and attention given to the Yangtze River Basin system, research on SPM in the Yangtze River Basin has increased yearly, but some problems still need to be solved. In the future, the following issues should be addressed: (1) a set of standard water sampling and treatment methods should be established for the treatment of SPM water samples to avoid differences in experimental results caused by different sampling and filtration methods; (2) researches should be focused on the distribution and sources of SPM in the large-scale water system of the Yangtze River; (3) researches on the SPM in the upstream of the Yangtze River should be further strengthened; (4) relevant models should be established to further study the adsorption mechanism and pollutant transport path between SPM and other substances in the basin water system; and (5) relevant ecological or physical means should be developed to manage with the water pollution of SPM in the water system.

Supplementary Materials

The following are available online at https://www.mdpi.com/article/10.3390/w15193429/s1, Table S1: Distribution of SPM concentration in dry period and flood period of rivers in the river system, Table S2: Distribution of SPM concentration in dry period and flood period of lakes in the river system, Table S3: Interannual variation of SPM concentration of the Yangtze River, Table S4: Spatial distribution of SPM concentration in Yangtze River system.

Author Contributions

J.F. conceived and designed research methodology; J.Y. and S.Z. performed data curation; J.F., J.Y. and F.C. completed the writing—original draft and writing—review and editing; J.F. implemented the supervision; J.F. carried out the project administration and funding acquisition. All the authors approved the final manuscript. All authors have read and agreed to the published version of the manuscript.

Funding

This work was financially supported by the National Natural Science Foundation of China (41977337), the Natural Science Foundation of Chongqing of China (CSTB2022NSCQ-MSX0394), the Science and Technology Research Program of Chongqing Municipal Education Commission (No. KJZD-K20230071), the Graduate Education Innovative Found Program of Chongqing Jiaotong University (CYB21220).

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest.

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Water 15 03429 g001
Figure 1. Yangtze River system with its main tributaries and the provinces through which it flows.
Figure 1. Yangtze River system with its main tributaries and the provinces through which it flows.
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Figure 2. Schematic diagram of the SPM extraction process in the Yangtze River system.
Figure 2. Schematic diagram of the SPM extraction process in the Yangtze River system.
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Figure 3. Distribution of SPM concentration in the dry and flood period in the river system (note: explanation of abbreviations mentioned are provided in Tables S1 and S2).
Figure 3. Distribution of SPM concentration in the dry and flood period in the river system (note: explanation of abbreviations mentioned are provided in Tables S1 and S2).
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Figure 4. Interannual variation of SPM concentration in the Yangtze River.
Figure 4. Interannual variation of SPM concentration in the Yangtze River.
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Figure 5. Spatial distribution of SPM concentration in the Yangtze River system (note: 1 explanation of these abbreviations mentioned is provided in Table S4. 2. The above figure shows the sampling points in Hubei Province. 3. The figure below shows the sampling points in Jiangxi Province and Anhui Province).
Figure 5. Spatial distribution of SPM concentration in the Yangtze River system (note: 1 explanation of these abbreviations mentioned is provided in Table S4. 2. The above figure shows the sampling points in Hubei Province. 3. The figure below shows the sampling points in Jiangxi Province and Anhui Province).
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Figure 6. Sources and influencing factors of SPM in the Yangtze River system.
Figure 6. Sources and influencing factors of SPM in the Yangtze River system.
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Figure 7. Diagram of the environmental effects of SPM.
Figure 7. Diagram of the environmental effects of SPM.
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Table 1. Water sample collection methods.
Table 1. Water sample collection methods.
Sampling Machine
NamePictureFeaturesReferences
Niskin sampler
(General Oceanics Inc., Miami, FL, USA)		Water 15 03429 i001It features a “smooth flow” structure of the sampling tube, and there are no cone valves or ball valves at either end of the sampling tube to obstruct the flow of water through the sampling tube.[27]
Binnensammler floating collector (BS)Water 15 03429 i002Two vertical fins at the bottom and rear ensure a stable position parallel to the flow direction, and the funnel-shaped interior of the BS acts as a sedimentation disc as the SPM-water mixture moves from horizontal flow mode to circulating flow mode.[14]
Self-constructed Phillips sampler (PS)Water 15 03429 i003PS is characterized by reducing the flow rate by expanding the diameter from 4 mm (inlet) to 100 mm (central cavity).
Continuous-flow centrifuge (CFC)Water 15 03429 i004CFC is characterized by the fact that sampling sites are limited to flat sidewalks by the river, SPM is collected on the surface of polytetrafluoroethylene (PTFE)-coated CFC bowl, and wet sediment samples can be taken directly from the CFC bowl.
Hydrocyclone (HC)Water 15 03429 i005HC is characterized by the fact that water enters the cylinder at a high flow rate and is forced to form a downward vortex, which, due to the action of centrifugal force, the particles are pressed against the wall, where they are deposited, and then collected in a tank.
Rosette samplerWater 15 03429 i006Rosette multichannel water sample collector with a maximum sampling depth of 1500 m is suitable for nearshore and river water collection.[28]
Hydro’s integrated water sampler Water 15 03429 i007It is used for columnar integral sampling of the water body, and the sampling depth can be freely set. Columnar water samples of different depths can be obtained.[29]
Van Dorn samplerWater 15 03429 i008It is divided into horizontal and vertical samplers. Horizontal samplers collect water samples of specific water formations, especially on the demarcation layer or sediment surface. Vertical samplers are used in weakly corroded water bodies and are suitable for plankton and floating sediment collection.[1]
Sediment trapWater 15 03429 i009Multichannel sedimentation traps are designed primarily for the automated collection of sediments from lakes, continental shelves, and aquatic environments with relatively vertical particle flows.[24]
Water pumpWater 15 03429 i010The water pump rotates through the impeller at high speed. The liquid rotates with the blades and, under the action of centrifugal force, flows out from the nozzle. The submersible pump is unsuitable for VOCs sampling, and its disturbance is significant.[30]
Manual sampler
NamePictureFeaturesReferences
Plexiglass water samplerWater 15 03429 i011It collects water samples within 0~30 m depth of rivers, lakes, reservoirs, and oceans. The top two semi-circle upper caps can be easily opened and closed, and the bottom is equipped with round holes and floating plates to ensure that when the bottle body sinks underwater, the water flow can freely enter and exit the body.[31]
Polyethylene water samplerWater 15 03429 i012When the components to be measured of the collected sample are divided into the main parts of glass, it is best to use a polyethylene water sample collector. [32]
Stainless steel water samplerWater 15 03429 i013It is suitable for water sample collection for analyzing organic matter, microorganisms (bacteria), and other indicators and for collecting corrosive samples containing acid and alkali.[33]
Table 2. Methods of water sample filtration.
Table 2. Methods of water sample filtration.
Filter Filtration
NameSizeReferences
Whatman GF/F glass ultrafiber filter
(Whatman, UK)		0.7 μm pore size, 47 mm diameter[34]
0.45 μm pore size, 47 mm diameter[35]
0.7 μm pore size, 25 mm diameter[36]
0.45 μm pore size, 50 mm diameter[37]
0.45 μm pore size[38]
Cellulose acetate filter
(Sartorius, Germany)		0.45 μm pore size, 47 mm diameter[27]
Microporous HA membrane filter Millipore HA (MilliporeSigma, Burlington, MA, USA)0.45 μm pore size,47 mm diameter[39]
Fiber filters0.7 μm pore size[29]
Normal filter0.45 μm pore size[40]
0.20 μm pore size
Quartz filter0.45 mm[41]
Membrane filtration
NameSizeReferences
PTFE membrane0.45 μm[42]
Millipore membrane0.45 μm[43]
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Fan, J.; Yang, J.; Cheng, F.; Zhang, S. The Source, Distribution, and Environmental Effects of Suspended Particulate Matter in the Yangtze River System. Water 2023, 15, 3429. https://doi.org/10.3390/w15193429

AMA Style

Fan J, Yang J, Cheng F, Zhang S. The Source, Distribution, and Environmental Effects of Suspended Particulate Matter in the Yangtze River System. Water. 2023; 15(19):3429. https://doi.org/10.3390/w15193429

Chicago/Turabian Style

Fan, Jianxin, Jiaxin Yang, Fulong Cheng, and Shikuo Zhang. 2023. "The Source, Distribution, and Environmental Effects of Suspended Particulate Matter in the Yangtze River System" Water 15, no. 19: 3429. https://doi.org/10.3390/w15193429

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