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26 April 2025

Distribution Characteristics and Driving Mechanisms of Organic Matter in Sediment of Lakes in China: A Review

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1
National and Local Joint Engineering Research Center of Ecological Treatment Technology for Urban Water Pollution, Wenzhou University, Wenzhou 325000, China
2
Zhejiang Provincial Key Laboratory for Water Environment and Marine Biological Resources Protection, Wenzhou University, Wenzhou 325000, China
3
College of Life and Environmental Science, Wenzhou University, Wenzhou 325000, China
4
College of Water Conservancy and Hydropower Engineering, Hohai University, Nanjing 210098, China
Water2025, 17(9), 1294;https://doi.org/10.3390/w17091294
This article belongs to the Special Issue Sustainable Water Treatment Systems: Green Infrastructure and Bioremediation
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Abstract

Sediment is a core part of lake ecosystems, and its organic matter (OM) content is a key indicator of lake ecological health and regional carbon cycling. OM provides nutrients for phytoplankton and algae in water, thereby influencing the degree of lake eutrophication. However, excessively high OM content may trigger water eutrophication, alter sediment’s physical and chemical properties, and ultimately threaten the stability and health of ecosystems. This study innovatively selected Poyang Lake, Taihu Lake, Qinghai Lake, and Hulun Lake from China’s four major geographical regions to systematically investigate sediments’ OM content, sources, and distribution characteristics at different times. The results showed that the organic matter content of sediments in lakes from different regions varied significantly and was influenced by multiple factors, such as watershed characteristics, eutrophication levels, human activities, and climate change. Poyang Lake and Taihu Lake, characterized by high levels of agricultural activities and urbanization within their basins, exhibit significant fluctuations in organic matter content, with total organic carbon (TOC) levels ranging from 0.35% to 2.9% and 0.7% to 2.4%, respectively. In contrast, Qinghai Lake and Hulun Lake, influenced by natural conditions and ecological policies, show relatively stable TOC levels, ranging from 1.3% to 2.75% and 1.25% to 3.58%, respectively. By analyzing sediments’ OM content and combining methods such as organic carbon, nitrogen isotopes, and organic C/N ratios, it is possible to effectively assess the ecological health of lakes, provide critical data support for pollution control, and play a significant role in carbon cycle management.

1. Introduction

A large number of lakes exists in China. These lakes are diverse in type, widely distributed, and have different geological origins and evolutionary processes. The ecological environments of lakes in different regions also vary significantly. There are 2693 lakes in China with an area of 1.0 km2 or larger, covering 814,714.6 km2, accounting for 0.9% of the total land area of China. Therefore, lakes are important to China’s water resources and ecosystems, significantly influencing the environment, economy, human production, and daily life [1].
Sediment, an essential part of lake ecosystems, is the central hub of nutrient cycling in lakes and a dynamic interface where physical, chemical, and biological materials exchange between water and soil. Among these, organic matter, the primary carbon source in sediment, is a key indicator of lake productivity, ecological health, and pollution levels [2]. OM in lake sediments provides nutrients for phytoplankton and algae, promoting their growth and reproduction. However, excessive OM content can lead to water eutrophication and algal blooms, threatening ecosystem health [3]. For the ecological environment at the bottom of lakes, OM content influences the composition and properties of sediments, such as the composition and ecological functions of microbial communities and the types and properties of clay minerals in sediments [4]. TOC in sediments is an important part of OM in sediments. It is a fundamental parameter for describing the amount of OM in lakes. Therefore, analyzing the properties and spatial distribution characteristics of OM in lake sediments is of great significance for understanding the environmental effects of OM in lake sediments.
This study investigated the accumulation of OM in lake sediments across different geographical regions of China, which is important for lake management and environmental restoration. In particular, understanding the spatial distribution and sources of OM is crucial for lake ecosystem management and pollution control. Excess OM from agricultural runoff and industrial activities can lead to water quality degradation and biodiversity loss. This study provides valuable data for policymakers to design more effective strategies for reducing nutrient loads and restoring lakes. For example, targeted dredging and ecological restoration can remove excess OM, improve water quality, reduce harmful algal blooms, and restore aquatic biodiversity. In China, many lakes suffering from organic overloading, such as Taihu Lake, are undergoing new large-scale dredging projects to remove polluted sediments accumulated with excessive nutrients and OM. By integrating the data from this study with ongoing dredging plans, the long-term effectiveness of these measures in reducing OM levels and restoring ecological balance can be assessed. Additionally, this study can guide the optimization of dredging techniques by providing insights into sediment characteristics and OM sources, thereby enhancing the ecological benefits of such interventions.

2. Overview of the Area of Investigation of Lake Sediments

Four major geographical regions in China, namely, the northern region, the southern region, the northwestern region, and the Qinghai–Tibetan region, were used in the present study, basing on zoning according to the characteristics of comprehensive geographical location, natural geography, and human geography. Four typical lakes (Figure 1) were selected as the research objects based on the following principles: First, the selected lakes should cover different climatic zones and topographic features to reflect the regional heterogeneity, e.g., Taihu Lake (South) represents shallow eutrophication lakes under a subtropical monsoon climate, Qinghai Lake (Qinghai–Tibet) reflects the low-disturbance characteristics of high alpine and deep-water lakes, and Hulun Lake (Northwest) reflects the unique environment of grassland lakes. Second, lakes with significant ecological problems or anthropogenic disturbances are prioritized, such as Poyang Lake due to prominent agricultural surface pollution in the basin and Taihu Lake due to long-term cyanobacterial outbreaks, in order to reveal the dynamic relationship between pollution and protection. In addition, the selection of research objects should be supported by long-term monitoring data, e.g., Taihu Lake and Lake Qinghai both have more than 20 years of sediment historical sequences and isotope traceability studies, to ensure the reliability and scientific validity of the analyses. Finally, the ecological service function and socio-economic value of the lakes should be taken into account, e.g., Hulun Lake (Northwest China) is the core area of grassland animal husbandry, to comprehensively assess the environmental and social impacts of OM accumulation. With the above criteria, this study selected Poyang Lake, Tai Lake, Qinghai Lake, and Hulun Lake as representative lakes, aiming to investigate and reveal the differences in the spatial distribution of OM in lake sediments, their sources, and their influencing factors.
Figure 1. Location maps of typical lakes (Poyang Lake, Taihu Lake, Qinghai Lake, Hulun Lake). This map is drawn based on the standard map (GS (2024) 0650) from the standard map service website of the Ministry of Natural Resources of China.
The southern region of China refers to the southern part of the monsoon zone in eastern China, south of the Qinling–Huai River line, with the Tibetan Plateau in the west and the East and South China Seas in the east and south, covering an area of about 2,518,000 square kilometers, or about 26.2 percent of the country’s total land area. The rivers in this region do not freeze in winter, the crops do not fail in winter, and the population in this region accounts for about 55% of the total population of the country, which is a relatively developed economic region in China, and also an area with a high density of lakes and population. Among them, Poyang Lake (115°50′ E~116°44′ E, 28°25′ N~29°45′ N) is located on the south bank of the middle and lower reaches of the Yangtze River and the northcentral part of Jiangxi Province, and it is the largest freshwater lake in China at present [5]. It receives water from five major water systems, namely, Gan River, Fu River, Xin River, Rao River, and Xiushui River, as well as Boyang River, Dong River, and Xijiang River, and injects water into the Yangtze River from the mouth of the lake, which is an overwatered, inverted, and seasonal lake, which provides good conditions for the development of the ecosystem of the lakeshore and the grassland wetland. It forms a unique natural geographic landscape of “high water lake phase and low water river phase”. During the flood season, the five rivers flood into the lake and then the lake water into the surrounding beach, the lake area expands, and the water flow is relatively gentle; winter and spring seasons, the lake falls through, the lake beach is exposed, the lake surface shrinks, and the water flow is rapid. Therefore, there is a great difference between the lake surface area in abundant and dry water, which is 1290 km2 in dry and 3900 km2 in abundant water [6]. Poyang Lake has a humid monsoon climate in the central subtropical zone, with a mild climate, abundant rainfall, abundant light, a long frost-free period, and four distinct seasons.
In addition, Taihu Lake is also strongly represented as the third largest freshwater lake in China [7]. Taihu Lake basin (119°08′ E–121°55′ E, 30°05′ N–32°08′ N) is located in China’s eastern Yangtze River Basin estuarine section to the south and the Qiantang River, Hangzhou Bay, in the middle of the total area of about 36,800 square kilometers. Basin boundaries in the west are according to the Tianmu Mountain, South Maoshan, and other mountainous areas, north of the Yangtze River estuary, east of the East China Sea, and south of Hangzhou Bay. The geomorphological characteristics of the terrain around the high, low in the middle, the overall presentation of the dish-shaped layout, the majority of alluvial plains, accounting for 4/6 of the total area of the Taihu Lake Basin; the waters accounted for about 1/6, and other mainly for the hills and mountains. The Taihu Lake basin belongs to the subtropical monsoon climate zone. The average annual temperature is about 16 °C, gradually increasing from north to south due to the influence of the monsoon wind strength and the weakness of the factors; the precipitation of the inter-annual difference is very large, and the distribution of perennial precipitation is uneven [8]. The Taihu Lake basin is located in one of China’s most prosperous economic development areas. The rapid development of agriculture and township industries has resulted in the discharge of large quantities of pollutants, and the fertilizers, pesticides, and sewage produced by agricultural production and daily life have also put great pressure on the environmental safety of Taihu Lake.
The northern region of China refers to the northern part of the monsoon zone in eastern China, mainly the area north of the Qinling–Huaihe River line, east of the Daxinganling and Wushengling Mountains, and east of the Bohai Sea and the Yellow Sea, with an area of about 2,131,000 square kilometers, or about 22.2 percent of the total land area of the country. The region has a temperate continental monsoon climate and a warm temperate continental monsoon climate, and many rivers in the region freeze in the winter. The region’s population is about 40% of the country’s total population, densely populated, well connected, and agriculturally developed. In the northern region, most of the lakes are located in the plains; among them, Baiyangdian is located in Hebei Province, China—it is the first largest inland lake in Hebei, with a total area of 366 square kilometers. Baiyangdian [9] gathered the water of nine rivers originating from the foothills of the Taihang Mountains in the upper reaches of the river, forming a group of 146 large and small lakes connected by more than 3700 ditches and rivers, and 36 villages are distributed among the islands in the group of lakes and along the shores of the lake. Chaohu Lake is located in Anhui Province in northern China [10]. It is one of the five largest freshwater lakes in China. Chaohu Lake is a shallow lake with a surface area of 780 square kilometers, an average depth of about 3.0 m, and a catchment area of 12,938 km2. The catchment area belongs to the subtropical-to-warm–temperate transitional monsoon climate. Over the past decades, the lake has suffered severe eutrophication due to the increasing lake basin and anthropogenic nutrient inputs.
China’s northwestern region is deeply inland, located north of the Kunlun-Altunshan-Qilian Mountains and the Great Wall, and west of the Daxing’anling and Wushengling Mountains, and the region covers a vast area, accounting for about 24.3 percent of the country’s total land area. The region’s eastern part is an undulating plateau, while the western part shows a surface pattern of intermittent distribution of mountains and basins. The district is about 4% of the country’s total population, making it a sparsely populated region. In this region, first and foremost is Hulun Lake, the fifth largest freshwater lake in China. The Hulun Lake basin (115.5° E–122.5° E, 46.9° N–50.3° N), located in the northeast of the Inner Mongolia Autonomous Region of China, is the largest inland lake in Inner Mongolia, with a vast area of water, known as the “Pearl of the Steppe” [11]. Hulun Lake is an irregularly inclined rectangular shape, long in the northeast-to-southwest direction and short in the southwest-to-southeast direction, with a length of 93 km, an average width of 32 km, and a circumference of 447 km, and with a water storage capacity of up to 13.85 billion cubic meters [12]. The Hulun Lake area has a temperate continental steppe climate, with cold winters, high wind speeds, and low precipitation in spring, warm and cool summers, and dramatic temperature changes in fall. Hulun Lake has been known as the “mother lake” of the Hulunbeier Grassland. The Hulunbeier Grassland and the Daxinganling Forest together constructed an important ecological barrier in northern China. The area plays an irreplaceable and important role in maintaining regional biodiversity, protecting the ecological security of northern China and even North China, and promoting sustainable economic and social development [13,14].
China’s Qinghai–Tibet region is located on the southwestern border of China, west of the Hengduan Mountains, north of the Himalayas, south of the Kunlun Mountains and the Arjinshan and Qilian Mountains, with a total area of about 2.6 million square kilometers, accounting for about 27 percent of the country. The region has a special highland climate, with year-round snow on the high mountains, glaciers, permafrost, and seasonal permafrost, which are also widely distributed. The region’s population is less than 1% of the country’s total population; the total area of lakes accounted for about half of the national lake area, and the most representative of the nature of the lake is Qinghai Lake. As China’s largest inland saltwater lake, Qinghai Lake is known as the sapphire of the Tibetan Plateau [15]. The basin (97°50′ E–101°20′ E, 36°15′ N–38°20′ N) is located in the northeastern part of the Qinghai–Tibetan Plateau, and the whole basin is a transition area controlled by both monsoon–humid and inland arid zones [16]. The altitude ranges from 3169 to 5268 m, and the terrain of the basin climbs gradually from southeast to northwest, surrounded by high mountains, forming a closed-plateau inland basin surrounded by mountains [17]. There are eight sub-basins in the basin, of which the Buha River basin is the largest, accounting for about 1/2 of the total area of the Qinghai Lake basin. Buha River and Shaliu River are the two largest rivers in the Qinghai Lake basin, and the runoff of the two rivers accounts for 64% of the total runoff [16]. The vegetation types in the basin include scrub vegetation, grassland vegetation, alpine rhyolite vegetation, meadow and swamp vegetation, cultivated vegetation, and sandy vegetation [18]. The Qinghai Lake basin has an arid climate, strong winds, large temperature differences between day and night, a short vegetation growing season, abundant light, strong solar radiation, windy and sandstorms in spring, and cool summers and cold winters [19]. The natural environment and ecosystem of Qinghai Lake are closely related to climate change, and the regional response is very significant.

3. Status and Trends of OM Content in Lake Sediments

3.1. Poyang Lake

As illustrated in Figure 2, the TOC content of Poyang Lake sediments has remained within the range of 0.5% to 1.2% over the past three decades [20,21,22,23,24,25,26,27,28,29], exhibiting a gradual upward trend. This increase may be attributed to the significant economic development and urbanization within the watershed, which have contributed to the deterioration of lake water quality and, consequently, a rise in OM concentration in the sediments. Among them, the TOC content in the sediments of Poyang Lake was significantly lower in 1992, which may be related to the fact that the period of collecting sediment samples in 1992 was a dry water period. Poyang Lake has abundant water from April to September yearly and dry water from October to March. Significant differences exist in water quality, area, recharge–runoff–discharge, and storage between abundant and dry water [30,31]. Some studies have shown that the concentration of OM in the sediments of the lake area of Poyang Lake is significantly higher in the abundant water than in the dry water period [32]. During the abundant water period in the area in the late rice transplanting season, the farmland applies a large amount of organic fertilizers and chemical fertilizers, and the surface source pollution load caused by surface runoff increases. Pollutants enter the lake body with surface runoff during the abundant water period, thus promoting sediment disturbance. During the dry season, the water from the “Five Rivers” decreases, and the pollutants in the northern lake area are constantly diluted by the water from the middle and upper reaches of the Yangtze River, which leads to a decrease in the OM content of the sediments.
Figure 2. TOC content changes in the sediment of Poyang Lake.
Compared with the lowest value in 1992, the concentration of TOC in the sediment of Poyang Lake in 2012 increased by 96.67 times. The dramatic increase in TOC content was mainly because the samples were collected during the abundant water period and mainly from the vicinity of villages. In the vicinity of villages near the lakeshore or far away from the river, the relatively smooth water flow during the abundant water period is favorable for the deposition of organic carbon. At the same time, the smooth water flow is also favorable for the propagation of aquatic bacteria, algae, and other aquatic plants. The growth of these organisms fixes the atmospheric CO2 and carbonate carbon in the water, which ultimately exist in the form of organic carbon in the sediment, resulting in high TOC contents in the sediments in the vicinity of the villages.
On the other hand, the preservation of sediment TOC is related to the fact that the sediment TOC is not the same as that in the water column, which is the main source of OM. On the other hand, preserving sediment TOC is closely related to the elemental composition of OM. OM from algae has a high H/C atomic ratio, which is easy to oxidize and decompose and not easy to preserve; herbaceous species have a low H/C atomic ratio, which is relatively more difficult to oxidize and decompose; and woody species and charcoal are the carbonaceous residue, which is the most stable component [33,34]. The water depth near the village is relatively shallow, and the OM in the sediment not only comes from bacteria and algae but also from many submerged and aquatic plants. These aquatic plants contribute to the OM with longer carbon chains and lower H/C atomic ratios, and the OM is relatively more difficult to decompose, so the amount of preservation is correspondingly increased, so the organic carbon content near the village is increased [28].

3.2. Taihu Lake

Taihu Lake is a typical shallow lake in China with the development of the basin economy at the same time; accompanying eutrophication is becoming increasingly severe, especially the irrational development of Taihu Lake resources and pollutants discharged, exacerbating the degree of eutrophication of Taihu Lake [35,36,37]. Along with the eutrophication of shallow lakes, the entire lake ecosystem may appear relatively stable: “grass-type lakes” and “algae-type lakes”.
The north of Taihu Lake has been severely “algalised” and has now become a typical “algal” lake [38]. According to Figure 3, the TOC content in Taihu Lake’s northern lake area is between 0.7% and 1.2% [39,40,41,42,43]. From 1949 to 1975, although the values were below the average, there was an overall gradual upward trend. Before the 1950s, the lake area was in a relatively natural state, with minimal human impact, and OM primarily originated from the decomposition and deposition of aquatic plants, with lake-derived OM being the main component. After 1949, under the policies of the Great Leap Forward and the People’s Commune movement, agriculture and rural industries developed rapidly, and human activities intensified the input of exogenous substances, leading to an increase in TOC content [39]. In 2003, TOC content in the northern lake area surged to 2.08%. After the 1990s, with the implementation of national afforestation and grassland restoration projects, as well as the Taihu Lake “Zero Point” Action and other pollution control measures [44], the overall trend of water quality deterioration in Taihu Lake was brought under control, with some short-term improvements in water quality. However, water pollution remained unchecked. After the year 2000, with the development of the economy and society, wastewater from the surrounding areas continued to infiltrate Taihu Lake, gradually deteriorating water quality. The water quality in the northern lake deteriorated to below Class V throughout the year, and eutrophication exacerbated frequent blue–green algae blooms in Taihu Lake. In 2018, the TOC content reached as high as 2.5%, indicating that external control measures were unable to curb the damage caused by human activities to the ecological environment of Taihu Lake. Additionally, the massive accumulation of algae also had a significant impact on OM. Data from Taihu Lake phytoplankton monitoring show that during the 2003 survey period, the total phytoplankton concentration was approximately 100,000 cells/L, while in 2018, it reached approximately 90 million cells/L [45].
Figure 3. TOC content changes in the sediment of Taihu Lake.
In addition, algal-type ecosystems are developing in some lake areas, such as the lake’s center and the southwest lake area [46]. According to Figure 3, the TOC content in the west of Taihu Lake is in the range of 0.8–1.2% [42,45,46,47,48], with considerable variation. The Taihu Lake basin is located in one of the most prosperous areas of China’s economic development, and under the interference of the accelerated urbanization process, the lake water ecological security has been seriously threatened, and the water quality of the lake has deteriorated more rapidly, with the highest value of TOC reaching 2.42%. Since the “water crisis” in Wuxi City in 2007, implementing ecological restoration projects and governmental environmental protection policies may have impacted the accumulation of OM in sediments [49,50]. Exogenous inputs have been controlled to a certain extent, such as the implementation of ecological dredging projects since 2008, which has led to an obvious trend of reducing the nutrient content of the surface sediments. It had a decreasing trend. However, since 2017, the TOC content has had an increasing trend, probably because the ecological dredging project can remove most of the bottom sediment pollutants, which is conducive to improving water quality and the ecological environment. However, 99.7% of the persistent harmful pollutants will be adsorbed on the fine particles with a diameter of less than 74 μm, and the strong disturbance generated by dredging easily causes the resuspension of fine particles of sediment, which also increases the probability of the release of contaminants from the bottom mud [51,52,53,54].
The east of Taihu Lake is a typical grass-type lake area, with OM in its sediments primarily derived from submerged plants and diatoms [55,56]. Figure 3 shows that the TOC content in the east of Taihu Lake has remained relatively stable at around 1.23% [42,47,57]. The higher TOC content in 2010 may be attributed to the intensive net pen aquaculture in Dongtai Lake and the vigorous growth of aquatic plants. The east of Taihu Lake includes Xu Lake and Dongtai Lake. Although they belong to the same lake district, there are significant differences in OM content between the two. In 2010, the TOC content in Dongtai Lake reached as high as 3.29%. Although the large-scale reduction of purse seine was completed at the beginning of 2009, and the area of purse seine culture in East Taihu Lake was about 2600 hm2 in 2010 [58], Yang et al. suggested that the area of purse seine culture in Dongtai Lake should be controlled at least to less than 1000 hm2 to ensure the ecological sustainable development of Dongtai Lake [59]. Therefore, the deposition of fish and crab bait and excreta caused by purse seine, and problems such as the narrowing of the lake surface, the reduction of the blowing range, and the weakening of the wind and waves still exist, coupled with the intensification of swamping. The composite index of swamping in Dongtai Lake from 1959 to 1997 increased from 1.47 to 2.41, which led to the massive growth of aquatic plants and floating-leafed plants, and the aquatic plant cover reached 97% in 2009, which is the best aquatic plant development in the whole lake. Dongtai Lake was the best aquatic plant development in the whole lake [60], and the deposition of a large amount of aquatic plant residues may be the main reason for the dramatic increase in OM in Dongtai Lake.

3.3. Qinghai Lake

Figure 4 shows that the TOC content of Qinghai Lake exhibited a trend of an initially slight increase, followed by a decrease and eventual stabilization. The TOC content was maintained at about 2.5% before 2000 but decreased to 1.4% after 2000 [61,62,63,64]. According to a previous study [61], before 1958, the TOC content of Qinghai Lake was stable at about 1.2%, which may be related to the low agricultural activities in the basin at that time, and the OM was mainly autochthonous in the lake. However, the TOC content gradually increased after 1960 and reached its highest value in 2001. On the one hand, due to natural and anthropogenic causes such as overgrazing, grassland rodent and insect pests, and land desertification in the Qinghai Lake Basin, the vegetation development has been continuously harmed, and the ecological environment has been deteriorating; on the other hand, it may be related to the drought year of 2001. In 2000, the year experienced the most severe drought year, and the precipitation in that year fell to the lowest level in history [65]. The dry climate and low precipitation in the basin resulted in a correspondingly low lake overflow or even the termination of the outflow process, which favored the enrichment of OM in the lake, thus leading to high OM content. The main reason for the decrease in TOC content after 2001 was the implementation of ecological protection and restoration measures such as the “returning farmland to forests and grasslands” program in Qinghai Lake Basin after that year [66,67]. These measures promoted the benign cycle of woodlands, grasslands, and wetlands in the basin; gradually increased the vegetation cover; and improved the ecological environment.
Figure 4. TOC content changes in the sediment of Qinghai Lake.

3.4. Hulun Lake

According to Figure 5, the TOC content in the sediments of Hulun Lake remains stable, ranging from 1.7% to 2.5%. Over time, the lake’s primary source of OM has shifted from aquatic plants to terrestrial sources [68,69,70,71]. This change signifies a progressive increase in the influence of human activities on Hulun Lake. The lower TOC content in 2008 may be mainly related to the temperature distribution of Hulun Lake and the climatic characteristics of the lake area. The OM that year was collected by drilling holes through the ice at each sampling point on the frozen lake surface in winter, and the OM content of Hulun Lake sediments has seasonal differences. Due to the local drought and windy spring, cool and short summer, sharp cooling and early frost in the fall, and long cold winter, such climate characteristics determine the lake body has scarce water plants and a short growth period of algal plankton, which inhibit the type and quantity of plankton, resulting in low OM content [71]. The TOC content was high in 2009 because, on the one hand, many impacts of grazing and global warming were experienced. The average grazing intensity of the whole grassland in 2006 was about 1.7 livestock units per hectare, which is lower than the maximum stocking rate in Inner Mongolia, because herders prefer to graze their animals near lakes or rivers, which leads to overgrazing in these areas. In addition, the mean annual temperature of Hulun Lake increased at about 0.05 °C per year, and the water level decreased from 544.8 m in 1991 to 540.2 m in 2009. The other aspect is related to the location of the present sampling, which is close to the northwestern part of the lake. In the northwestern part of Hulun Lake, there is a large area of pasture, tourist attractions, and fisheries, while the eastern and southern parts of the lake are mainly sandy. Therefore, under the transport of the prevailing winds from the northwest and the surface runoff, the grass clippings, soil, or livestock feces from the northwestern part of the lake enter the lake body, and some of them are deposited into the sediment after a series of physical, chemical, and biological effects, resulting in the accumulation of OM; moreover, the water level of this sampling location is close to the lake level. The accumulation of OM: In addition, the lake is deeper on the northwestern shore of Hulun Lake, and the reducing environment is favorable for preserving OM in the sediments. At the same time, compared with the sandy substrate of larger particle sizes distributed in the southeast, the silt of smaller particle sizes is mainly distributed in the northwest and, therefore, can adsorb more OM [71].
Figure 5. TOC content changes in the sediment of Hulun Lake.

3.5. Comparative Analysis of OM in Lake Sediments

There are significant differences in the content and trend of OM in lake sediments in different regions, which are closely related to the watershed characteristics of lakes, the degree of eutrophication, the intensity of human activities, and climate change, as shown in Figure 1. The TOC content of Poyang Lake is 0.35–2.9%, the TOC content of Taihu Lake is 0.7–2.4%, the TOC content of Qinghai Lake is 1.3–2.75%, and the TOC content of Hulun Lake is 1.25–3.58%. Generally speaking, Poyang Lake and Taihu Lake are located in economically developed regions, and the TOC content is dominated by industrial and agricultural pollution and human activities, with more considerable fluctuation. Qinghai Lake and Hulun Lake are more affected by the combination of natural conditions and regional ecological policies, and phases characterize the change in TOC content.
The TOC content of Poyang Lake has the largest span among the four lakes, and its change is mainly related to the pollution input from industry and agriculture in the basin and hydrological conditions. Because Poyang Lake has a subtropical monsoon climate and uneven distribution of precipitation all year round, the surface pollution from farmland and aquatic plant reproduction further aggravate the deposition of OM in the abundant water period. In contrast, the dilution effect of the incoming water from the Yangtze River inhibits its accumulation in the dry water period. Therefore, the differences in OM content between the abundant and dry water periods are caused.
Taihu Lake and Poyang Lake belong to the same subtropical monsoon climate zone. However, Taihu Lake is in one of the most prosperous areas of China’s economic development, with Suzhou, Wuxi, Changzhou, and other economically developed cities along its shores, with a dense distribution of industries and a high degree of agricultural intensification, and it is therefore subject to a greater influence of human activities and a higher degree of eutrophication. The different ecological types of Taihu Lake have led to significant differences in TOC changes, with exogenous pollution (e.g., urbanization and industrial wastewater) dominating TOC fluctuations in the North Lake due to severe algal phenotypes and a localized drastic increase in TOC due to disturbances caused by purse seine farming and the sedimentation of aquatic plant residues in the East Lake, which has been subjected to numerous ecological management measures since 2008 but has still resulted in difficulty eradicating the problem of endogenous releases.
The TOC content of Qinghai Lake shows a trend of “rising and then falling”, and the fluctuation amplitude is relatively small. The lake area is mainly dominated by algal autochthonous OM, with relatively few human activities. Grazing pressure, grassland degradation, and precipitation fluctuations are the key factors affecting OM content. After 2001, policies such as “returning pasture to grass” and “returning farmland to wetland” have significantly suppressed the input of land sources, effectively improved the ecological situation, and made Qinghai Lake a typical lake area with more effective ecological restoration.
The TOC content of Hulun Lake has significant seasonal variations, so the variation is large. The source of its OM has gradually shifted from aquatic plants to land-based input dominance, reflecting the enhanced impact of human activities. The rich pasture and intensive pastoralism along the lake’s northwestern shore, coupled with wind erosion, feces, and soil inputs into the lake, accelerated the accumulation of OM under smooth water flow and silt deposition conditions. In addition, the input of OM from land-based sources is further exacerbated by the decline in water levels and pasture degradation caused by global warming.

4. Sources of OM in Lake Sediments

4.1. OM Concepts and Groupings

Lake sediments contain a large amount of living and non-living OM. There are three forms of non-living organic matter, namely, particulate organic matter (POM), colloidal organic matter, and dissolved organic matter (DOM), of which DOM is the most active part of the OM component of the sediment. In lakes, the content of non-living OM usually exceeds that of living OM by a factor of ten to several hundred. Non-living OM has always played an important role in the structure and function of water ecosystems. The main component of OM is carbon; we usually use carbon value to express the OM content.
Dissolved OM includes two types of components: One is produced in the body of water with easily decomposed components, including sugars, proteins, peptides, chlorinated acids, fatty acids, and other low molecular compounds; the carbon and nitrogen ratio of these substances is about 12:1. The other is mainly foreign humus and other difficult-to-decompose stable components of the carbon and nitrogen ratio as high as 45–50, so that the water is brown. The former has a large turnover rate and low instantaneous concentration, and it is the main contributor to the material cycle and energy flow; the latter has a low turnover rate and a high present amount, and the amount entering the lake is equal to the slow decomposition of microorganisms.

4.2. OM Source Classification

The sources of OM in lake sediments are more complex, and the interactions and influences between different types of sources together determine the formation and distribution of OM in lake sediments [72]. Overall, the sources of OM [73] in lake sediments can be divided into four major segments (Figure 6). One of them is plant residues. Various factors, such as lake hydrology, climate, and nutrients, influence the content of plant residues in lake sediments. For example, lake depth, eutrophication status, and watershed coverage affect the quality and quantity of plant debris in lake sediments. In addition, vegetation type, growing season, etc., also affect the composition and proportion of OM from plant sources. The second is animal remains. The death and excreta of algae, zooplankton ben, these animals, etc., in lake sediments are also important sources of OM. The amount and nature of their release are closely related to the characteristics of lake water bodies, the ecological environment, and animal species.
Figure 6. Potential sources of OM. Reproduced with permission from [74]. Copyright 2023, MDPI.
For example, the structure and life cycle of algal communities have an important influence on their mortality rates and, thus, on changes in algal-sourced OM in lake sediments. The third is microbial metabolism. The type and content of OM released from microbial metabolic activities in lake sediments are influenced by environmental factors, microbial community structure, metabolic pathways, and morphology. For example, environmental conditions such as oxygen supply status, pH value, and substrate temperature will affect microbial activity and metabolic pathways, impacting the composition and content of microbial-sourced OM in the lake substrate. Final is anthropogenic sources. Anthropogenic sources mean that the OM in lake sediments mainly comes from pollutants and wastes caused by human activities. These pollutants and wastes mainly come from chemical fertilizers, pesticides, industrial wastewater, municipal sewage, landfills, etc., which significantly impact lake ecosystems and human health.

4.3. OM Source Analysis

OM in lake sediments can preserve important historical information, such as the succession of lake plant species, the level of primary productivity, the evolution of the water body nutrient status, and the natural factors controlling the water quality evolution. This information provides a crucial basis for lake development, utilization, and maintenance. Therefore, identifying the sources of sediment OM has been a central focus of sediment OM research. Currently, the primary methods for studying sediment OM sources include measuring organic carbon and nitrogen content, organic C/N ratios, lignin, and lipid molecular markers. Combining stable carbon and nitrogen isotopes with organic C/N ratios is an essential tool for tracing the migration and transformation of sediment OM within systems and analyzing its sources [75]. A compilation of data where elemental or isotopic composition was used to determine contributions of different sources of lake sediments OM is presented in Table 1.
Table 1. Examples of the relative contributions of different source materials to lake sediments OM in China, determined using stable C and N isotopic signatures and C/N ratios.
Organic carbon isotopes provide more direct evidence for photosynthesis, carbon assimilation, and the isotopic [82] compositional characteristics of carbon sources in plants. Terrestrial plants grow by fixing atmospheric CO2 through photosynthesis in order to synthesize their material composition and can be classified according to their photosynthesis mechanism as C3 plants, C4 plants, and CAM plants. C3 plants include the vast majority of trees, shrubs, and grasses growing in cold and wet environments, etc.; C4 plants are grasses growing in drier and hotter environments; and Sedums are mainly succulent plants such as cacti, etc., whose stomata can close on their own in dry and hot weather. The variability of the photosynthesis mechanism in different types of plants leads to differences in carbon isotope fractionation, causing significant differences in the δ13C signals of OM. According to previous studies, the range of δ13C values generally varied from −37‰ to −24‰ for C3-type plants, from −19‰ to −9‰ for C4-type plants, and from −30‰ to −10‰ for CAM plants. For lakes dominated by autochthonous OM, aquatic plants can be divided into floating plants and submerged plants, with floating plants having a closer range of δ13C content than terrestrial C3 plants and submerged plants having higher carbon isotope values than phytoplankton. Since organic nitrogen occurs preferentially in plant proteins and nucleic acids, the protoxin content of lower plants, such as lake algae and macroalgae (~24%), is much higher than the protoxin content of Salicaceae (~6%). Therefore, the C/N ratio of the former is usually small, between 4 and 10, whereas that of terrestrial plants is usually >20 and can be even as high as 45–50, and thus, by the size of the OM C/N of the sediments, the source of OM can be roughly judged whether it is autochthonous or externally imported from the lake. According to Meyers’ study [83], OM can be classified into four types based on the organic carbon content and C/N ratio: marine algae, lake algae, C3 plants, and C4 plants (Figure 7).
Figure 7. The δ13C value and C/N ratio of major source plants in lake sediment OM. Reproduced with permission from [83]. Copyright 1994, Elsevier.

5. Factors Affecting OM Content in Lake Sediments

The degree of eutrophication of water bodies is one of the important factors affecting the content of OM in lake sediments (Figure 8). The higher the degree of eutrophication of the water body, the higher the number of algae and other phytoplankton in the lake, and the OM produced by the decomposition of this phytoplankton after their deaths will be deposited at the bottom of the lake, leading to an increase in the OM content in the lake substrate [84]. Lake water depth and hydrodynamic conditions also affect the OM content in the lake substrate [85]. Lakes with shallow water depths have poorer hydrodynamic conditions, and the OM in the substrate is not easily stirred up, oxidized, and decomposed. Thus, the OM content is higher. On the contrary, lakes with deeper water depths have better hydrodynamic conditions, and the OM in the sediments is easily oxidized and decomposed. Thus, the OM content is lower. The land use around the lake also affects the OM content in the lake sediments. Human activities such as agriculture and urbanization around lakes can lead to many pollutants entering lakes, such as pesticides, fertilizers, domestic sewage, industrial wastewater, etc. These pollutants usually contain much OM, which can significantly increase lake sediments’ OM content. Changes in climate and precipitation will also affect the growth of vegetation and the hydrological environment around the lake, affecting the OM content in the lake sediment. The geological conditions of lakes also affect the OM content of lake sediments. The OM content of lake sediments varies in different geological conditions, e.g., the OM content of lake sediments is usually higher in sandstone and mudstone areas.
Figure 8. OM transport and transformation pathways in eutrophic lakes (blue arrows represent cycles affecting eutrophication, red arrows represent cycles affecting climate warming). Reproduced with permission from [74]. Copyright 2023, MDPI.
In addition, the OM content in lake sediments is closely related to water quality conditions [86]. When the water quality, which includes total phosphorus (TP), total nitrogen (TN), biochemical oxygen demand (BOD5), the permanganate index (CODMn), and chlorophyll-a (CHl-a), of a lake is good, the OM content in the lake is relatively low. This is because when the water is clear and rich in oxygen, aerobic microorganisms quickly decompose OM, reducing its accumulation. In addition, good water quality is also conducive to the growth of aquatic plants, which can absorb nutrients in the water and reduce the accumulation of OM. On the contrary, when the water quality of a lake is poor, the OM content in the lake will increase. This is because when water turbidity is high and oxygen levels are low, the rate of microbial decomposition of OM slows down, leading to the accumulation of OM in the water (Figure 9). In addition, poor water quality can lead to the death and decay of aquatic plants, increasing the amount of OM in lakes.
Figure 9. Schematic diagram of algal accumulation pathways. Reproduced with permission from [74]. Copyright 2023, MDPI.
Dredging projects, as the core measure for controlling internal pollution in current lake sediment management in China, exhibit significant dual effects on OM dynamics in sediments. On the one hand, they rapidly reduce internal organic loads by directly removing eutrophic sediments, thereby improving water quality. Taking Taihu Lake as an example, the ecological dredging projects implemented since 2008 have shown a decreasing trend in nutrient content in sediments in some lake areas [46]. By removing large amounts of organic-rich sediment, ecological dredging projects have significantly reduced nutrient levels in surface sediments, improved water quality in the short term, increased water transparency, reduced chlorophyll concentrations, and effectively suppressed excessive algal growth [53]. On the other hand, mechanical disturbance during dredging may cause fine particles (<74 μm) of sediment to resuspend, releasing adsorbed organic pollutants (such as polycyclic aromatic hydrocarbons), thereby posing risks of secondary pollution. In recent years, total organic carbon levels have shown a noticeable upward trend in the North Lake and West Lake areas of Taihu Lake. Similar issues have been observed in managing the Caohai area of Dianchi Lake. Although the dredging project implemented in 1998 removed some bottom sediments, the water quality did not achieve the expected improvement [80]. During dredging, pollutants may be continuously released from bottom sediments into the water body, leading to adequate replenishment and maintenance of pollutant concentrations at high levels, weakening the effectiveness of external remediation measures [52].

6. Conclusions and Outlook

This study systematically investigated typical lakes in China’s four major geographical regions (Northern, Southern, Northwestern, and Qinghai–Tibet), including Poyang Lake, Taihu Lake, Qinghai Lake, and Hulun Lake, revealing spatial heterogeneity in bottom sediment OM content across different regions. The study found that Poyang Lake and Taihu Lake, characterized by humid climates, high levels of eutrophication, and intensive human activities, have poorer water quality but lower average TOC values than Qinghai Lake and Hulun Lake. Poyang Lake and Taihu Lake are located in developed regions where human activities have significant impacts. However, government-implemented ecological restoration projects and environmental protection policies have partially altered the lakes’ ecological environments, influencing OM accumulation in sediments. Qinghai Lake and Hulun Lake, on the other hand, have low temperatures and weak water dynamics, leading to slow OM decomposition and long-term accumulation. OM sources are primarily endogenous (algae, aquatic plants) and terrestrial (agricultural runoff, human pollution). Terrestrial inputs significantly influence Poyang Lake, Taihu Lake, and Hulun Lake, while Qinghai Lake is primarily self-sourced.
Future research should focus on the impact of climate change (such as temperature or water temperature changes predicted by the CMIP6 model) on OM in lake sediments. According to a relevant study [87], global climate change-induced water temperature increases may significantly alter lakes’ physical, chemical, and biological characteristics. For example, OM decomposition in shallow lake sediments may accelerate, releasing more greenhouse gases. In contrast, enhanced water temperature stratification in high-altitude, deep lakes may inhibit the mineralization of bottom OM [88]. Additionally, regional ecological policies and the unique characteristics of the lakes should be considered to optimize management measures such as dredging further, thereby enhancing the precision of lake ecological protection and carbon cycle management. Furthermore, since this study concludes based on only a few lakes, future research should expand the sample size and coverage to more comprehensively reflect the current status and trends of OM in lake sediments across China.

Author Contributions

Conceptualization, C.Z. and F.R.; methodology, C.Z. and F.R.; software, C.Z. and F.R.; validation, S.L.; formal analysis, C.Z. and F.R.; investigation, C.Z. and F.R.; resources, C.Z. and F.R.; data curation, C.Z. and F.R.; writing—original draft preparation, C.Z. and F.R.; writing—review and editing, L.W. and C.F.; visualization, S.L.; supervision, L.W. and C.F.; project administration, C.Z. and F.R.; funding acquisition, L.W. and C.F. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the National Key R&D Program of China (No. 2022YFE0105000).

Data Availability Statement

The data presented in this study are available on request from the corresponding author. The data are not publicly available due to funder restrictions.

Acknowledgments

The authors express their sincere gratitude for the work of the editor and the anonymous reviewers.

Conflicts of Interest

The authors declare no conflicts of interest.

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