A sedimentary evolvement record and its response to anthropogenic-driven pressure in the shallow Honghu Lake over a century

ABSTRACT

Lake sediments record the environment during the lake sedimentation whose characteristics can infer environmental changes and human activities. In this study, the ²¹⁰Pb chronologies and sedimentation rate of the sediment core in Honghu Lake were calculated by the Constant Rate of Supply model. The characteristics of the sedimentary environment were analysed by using physical and chemical indicators. Four stages were divided as follows: Stage A (before 1900): The relatively low sedimentation rate and nutrient content indicated an extremely stable sedimentary environment. Stage B (1900–1949): With the growth of population, the intensity of land use began to increase, with an averaged sedimentation rate of 0.252 g·cm⁻²·a⁻¹. Stage C (1949–1980): The sedimentation rate and nutrient content increased markedly. The intense human activity has damaged the surrounding vegetation leading to soil erosion and accelerated sedimentation rate. With the deterioration of the lake water environment, the organic matter source was mainly the internal source represented by algae and bacteria. Stage D (1980–2011): Influenced by the difference in land use types along the coast, the sedimentation rate of HH-A (0.570 g·cm⁻²·a⁻¹) is higher than that of HH-B (0.445 g·cm⁻²·a⁻¹). The results are of significance to the management of rural lakes and reservoirs.

KEYWORDS:

• ²¹⁰Pb
• sedimentation rate
• grain size of sediments
• Honghu Lake

1. Introduction

Sediment is at the natural intersection of the hydrosphere, lithosphere, soil sphere and biosphere, and plays an important role in the material and energy cycle of the water environment and the change in environmental quality (Fan et al., 2013). Lake sediments have become good indicators for studying environmental geochemical processes (Begy et al., 2011; Yusoff & Mohamed, 2019; Zhang & Xu, 2023). In recent years, scholars have carried out a lot of research on the sedimentary characteristics, material circulation and ecological effects of the water environment and sediments in key zones (Gao et al., 2021; Liu et al., 2022). As an important reservoir of nutrients such as nitrogen and phosphorus, sediment is not only a sink of exogenous pollution but also a potential source of the water environment, which reflects relatively stable during the sedimentation process (Chen et al., 2019; Mohammadi et al., 2022; Wang et al., 2009). Therefore, it is of great significance to study the spatial differentiation and sources of nitrogen and phosphorus nutrients in lake sediments for the protection and management of lake ecosystems.

As a geochronological method, ²¹⁰Pb dating is commonly used in the study of the Anthropocene, which has been widely used in various sedimentary environments and research fields (Li et al., 2021; Putyrskaya et al., 2022). Compared with the artificial radioisotopes ¹³⁷Cs and ^239,240Pu, ²¹⁰Pb has obvious advantages, which are easier to be adsorbed by fine particles in sediments, migrating uneasily (He & Walling, 1996). At the same time, it has an earlier dating range, up to 150 years ago, while ¹³⁷Cs and ^239,240Pu are dating through special time scales, the earliest observable years are 1948 and 1954 (Fontana et al., 2022; Guo et al., 2021). Through the specific activity of ²¹⁰Pb, the deposition rate and age of sediments can be calculated, and the history of the sedimentary environment can be explored (Wang et al., 2022; Xia et al., 2023). Therefore, ²¹⁰Pb is very suitable for century-scale sediment dating because there are no problems such as the failure to replenish after decay or the error increases, due to the increasing depth of the special time scale horizon from the surface.

Honghu Lake is a tributary of Poyang Lake in the Yangtze River Basin, a typical small and medium-sized shallow lake with multiple functions such as tourism and aquaculture (Yang et al., 2023). In recent decades, due to urbanisation and rapid development of industry and agriculture, human activities in the Honghu Lake have been more frequent. Some activities such as deforestation, reclamation, fishing, hunting, and construction of dikes and dams have a strong impact on lake sedimentation. Therefore, studying the relationship between changes in sedimentation characteristics and human activities in modern times of Honghu Lake not only helps to understand the history of environmental evolution but also has important ecological and environmental significance to improve the environmental conditions and service functions of the lake (Chen et al., 2020; Wu et al., 2021). This paper used the ²¹⁰Pb dating method to determine the sediment age, sedimentation rate, sediment nutrient distribution characteristics and their changes in Honghu Lake, and objectively evaluated and analysed the relationship between the changes of sedimentation characteristics and the evolution of the natural environment and human activities, providing a basis for the rational exploitation of lake resources and the formulation of regional ecological and environmental protection policies.

2. Methods and materials

2.1. Overview of the study area

Honghu Lake is located in Yujiang County, Yingtan City, Jiangxi Province. It is situated in the red soil low hill basin, surrounded by low hills and mountains on its east, west, and south sides, and located at longitude 116°40’−117°20’ E and latitude 27°40’−28°20’ N. Honghu Lake has a total storage capacity of 13.26 million m³, a maximum surface area of 4,000 acres, an effective storage capacity of 11.86 million m³, a rain catchment area of 7.24 km², a diversion catchment area of 1,495 km², and an average water depth of 5.7 m (http://www.yujiang.gov.cn/). The average water exchange in the lake is 25 days, the lake area has long light hours, with an annual average of 1,952 h of sunshine. The climate is mild, with an annual mean temperature of 17.6°C and an annual average frost-free period of 269 d. The rainfall is abundant, with an annual average rainfall of 1693.6 mm. It is a subtropical monsoonal humid climate, with hot and sunny summers, less rain and snow in autumn and winter, and very short cold periods. Honghu has functions such as domestic drinking, fishery breeding, and industry and agriculture water use. In recent years, there has been rapid development of industry and agriculture in the lake basin, and the industrial layout is dominated by the production of phosphorus chemicals, food, copper smelting, iron and steel, machinery, non-ferrous metals and so on (http://www.yujiang.gov.cn/). A large amount of industrial wastewater, agricultural runoff, and domestic sewage flow into the lake, the water quality of the lake is polluted, and Honghu Lake has faced the threat of eutrophication.

2.2. Experimental analysis

2.2.1. Sample collection

In July 2012, a gravity sediment sampler (4.4 cm internal diameter) produced in Austria was used to collect sediment cores from Honghu Lake at a water depth of 4.5 m (Figure 1). Two sediment cores named HH-A and HH-B were collected. The length of the sediment cores collected was between 50 and 55 cm, with approximately 5 cm of overlying water. The suspension layer was undisturbed, and the interface water was clear. The upper part of the core appeared grey-black with a relatively clear texture, while the lower part appeared black-brown peaty sand. After collection, the cores were taken back to the laboratory and frozen in an ultra-low temperature refrigerator (set at −50°C for 48 h), cut with a homemade sample cutter, and then the cut samples were air-dried, ground, and dried to a constant weight at 50°C.

Figure 1. The Map of Hong Lake and the sampling site.

2.2.2. Test for ²¹⁰Pb_ex of sediments

About 10 g of samples were weighed and sealed in the same size centrifuge tube for one month until ²²⁶Ra and ²¹⁰Pb reached the state of decay equilibrium. The activities of ²¹⁰Pb and ²²⁶Ra were determined by a high-purity germanium γ-spectrometry system (GWL-120-15, Ortec, U.S.A.). The test time for each sample is over 40,000 s. The excess specific activity of ²¹⁰Pb (expressed as ²¹⁰Pb_ex) in the sample is the difference between the specific activity of ²¹⁰Pb and the specific activity of ²²⁶Ra. The specific activity of ²¹⁰Pb is calculated from the peak area of the 4615KevC-ray spectrum, and the specific activity of ²²⁶Ra is calculated from the peak area of ²¹⁰Pb spectrum (γ-ray spectrum peak at 351.9 Kev), and the ²¹⁰Pb with very short half-life is the decay product of ²²⁶Ra. Among them, ²²⁶Ra and ²¹⁰Pb standard samples were provided by the China Institute of Atomic Energy.

2.2.3. Test for nutrients of sediments

Sediment samples were weighed freeze-dried and milled 200 mesh sediment samples for measurement of total nitrogen (TN), total phosphorus (TP), and total organic carbon (TOC) content, respectively. A 0.02 g of the sample into a 50 mL stoppered colorimeter tube, 25 mL of oxidiser solution, heated to 120°C and kept for 30 min, 25 mL and 10 mL of the supernatant and tested TN and TP, respectively. Among them, TN was determined on a UV spectrophotometer with wavelengths of 220 nm and 275 nm; TP was determined using a 751-type spectrophotometer after adding a molybdenum antimony mixed chromatograph. The TOC was determined by the TOC analyser (SSM-5000A, Shimadzu, Japan).

2.2.4. Test for grain size of sediments

About 0.05–0.10 g of dried samples were weighed and placed in a 100 mL small beaker. Then, 10 mL of 10% hydrogen peroxide (H₂O₂) was added and heated to remove the organic matter in the samples. After reacting sufficiently until excess hydrogen peroxide was completely decomposed, 5 mL of 10% hydrochloric acid (HCl) was added to remove carbonate and organic matter aggregates (mainly calcium aggregates). After the complete reaction, distilled water was added to 100 mL, and the mixture was left to stand for 12 h. The supernatant was extracted and repeated 3–4 times to wash off excess hydrochloric acid and make the solution neutral. Finally, 10 mL of 2% sodium hexametaphosphate dispersant was added, shaken well, and then placed in ultrasonic oscillation for 15 min to form a highly dispersed grain suspension for measurements. The measurements were conducted using the Mastersizer 2000 laser grain size metre produced by Mal-ven Corporation in the UK. The measurement range was 0.02–2000 μm, with a resolution of 0.1 μm at the grain level and a relative error of <1% for repeated measurements.

All samples were tested at the Jiangsu Provincial Key Laboratory of Environmental Evolution and Ecological Construction.

2.3. The principle of the ²¹⁰Pb dating method

The natural radioactive lead nuclide ²¹⁰Pb is the alpha decay daughter of ²²²Rn (half-life of 1,622 a), which in turn is the decay intermediate of ²²⁶Ra (half-life of 1,622 a) in the ²³⁸U series. Atmospheric ²¹⁰Pb enters lakes through dry and wet deposition and accumulates in sediments. Some of the accumulated ²¹⁰Pb in sediments becomes excess ²¹⁰Pb (²¹⁰Pb_ex) because it does not coexist and equilibrate with its parent, ²²⁶Ra. By analysing the specific activity of ²¹⁰Pb_ex in different layers of sediment core samples, the sedimentation rate and the age of certain layers can be calculated (Abril, 2022; Abril-Hernández, 2023).

The ²¹⁰Pb dating method is currently based on the Constant Rate of Supply model (CRS). This model is suitable for calculating the sedimentation rate when the ²¹⁰Pb_ex input flux remains constant and the sedimentation rate of sediments may vary over time (Appleby, 2002; Figols & Bonotto, 2022). As shown in Figure 2, the distribution of ²¹⁰Pb_ex content in Honghu Lake presents an irregular serrated distribution, indicating that the sedimentation rate of Honghu Lake varies over time. Therefore, the CRS model should be used to calculate the sedimentation age of sediment cores and establish the corresponding sedimentation age sequence. The formula for calculating the sedimentation age of a certain layer of sediments can be expressed as:

Figure 2. Vertical profile distribution of ²¹⁰Pb_ex in sediment cores HH-A and HH-B in Honghu Lake: (a) HH-A (b) HH-B.

(1) $\mathrm{t}={\lambda }^{-1}ln\left(A_0/A_h\right)$(1)

Where A_h is the cumulative total amount of ²¹⁰Pb_ex in each layer of sediments below a certain depth h (Bq·cm⁻²); A₀ is the total cumulative input of ²¹⁰Pb_ex in the sediment core (Bq·cm⁻²); and λ is the radioactive decay constant of ²¹⁰Pb (λ = 0.03114 a⁻¹). The sedimentation rate can be obtained as S = Z/t, where Z is the mass depth, the cumulative value of sediments above a given depth Z, and is corrected by porosity (g·cm⁻²); and S is the sedimentation rate (g·cm⁻²·a⁻¹).

3. Results and discussions

3.1. Characteristics of ²¹⁰Pb vertical distribution in sediment core

Figure 2 shows the vertical profile distribution of ²¹⁰Pb_ex from sediment cores HH-A and HH-B in Honghu Lake. According to the above ²¹⁰Pb_ex dating method, the serrated distribution of the sediment cores in Honghu Lake is suitable for the ²¹⁰Pb CRS model, and the sedimentation ages corresponding to the sediment cores from the surface to the bottom are calculated as HH-A (2011–1864) and HH-B (2011–1862), each sediment core has undergone nearly 150 years of sedimentation. From the vertical profile distribution of ²¹⁰Pb_ex (Figure 2), the specific activity of ²¹⁰Pb_ex in each sediment core in Honghu Lake generally shows a serrated fluctuating distribution characteristic from the surface to the bottom. In particular, it is shown that the specific activity of ²¹⁰Pb_ex in HH-A reached a maximum value (274.96 Bq·kg⁻¹) at a depth of 2 cm from the surface. Below 8 cm from the surface, occurs reduced volatility, with a minimum value of 41.02 Bq·kg⁻¹ distributed at a depth of 45 cm. From Figure 2(a), it can be concluded that the sediment core HH-A was well preserved with relatively little disturbance, and its vertical profile of ²¹⁰Pb_ex can be divided into three parts: (1) From the surface to the depth of 8 cm, the specific activity of ²¹⁰Pb_ex has a relatively large value (178.57–274.96 Bq·kg⁻¹); (2) Between the depth of 9 cm and 32 cm, the specific activity of ²¹⁰Pb_ex has a fluctuating trend which is in little variation, ranging from 87.8 Bq·kg⁻¹ to 179.26 Bq·kg⁻¹; (3) Below 32 cm, the specific activity layer of ²¹⁰Pb_ex decreased gradually, ranging from 41.02 Bq·kg⁻¹ to 147.63 Bq·kg⁻¹, with the minimum value at 45 cm. The results show that the maximum specific activity of ²¹⁰Pb_ex in the sediment core of HH-B does not appear at the surface, but at a depth of 9 cm, with a value of 292.72 Bq·kg⁻¹. The reason why it happens may be influenced by the mixing effect in the surface of sediments or the loss of ²²²Rn. Below 9 cm, the specific activity of ²¹⁰Pb_ex had a similar trend to that of HH-A, the specific activity of ²¹⁰Pb_ex shows a serrated decreasing trend, and the minimum value occurs at 44 cm, with a value of 68.59 Bq·kg⁻¹.

3.2. Characteristics of sediment grain size change

In this study, the sediment cores of Honghu Lake are classified into three levels according to the grain size: clay (<4 μm), fine silt (4–16 μm), and sand (16–63 μm). Figure 3 shows the sediment content of each grain size level in the sediment cores of Honghu Lake, which is dominated by fine silt in Honghu Lake, and then followed by clay and sand. It is shown that the content of clay, fine silt, and sand in HH-A is 15.28%–47.65%, 35.46%–49.49%, and 11.23%–39.34% respectively, with a mean of 29.12%, 43.95%, 24.87% in turn. The median grain size ranges from 4.42 μm to 13.96 μm. The median grain size of the sediments in HH-A shows a predominance of fine silt. It is shown that the content of clay, fine silt, and sand in HH-A is 36.96%–47.68%, 17.58%–33.74%, and 17.58%–33.74% respectively, with a mean of 29.12%, 43.80% and 25.20% in turn. The median grain size ranges from 4.42 μm to 13.96 μm. The median grain size of the sediments in HH-A shows a predominance of fine silt. The median grain size is between 5.69 μm and 13.25 μm in HH-B. The results show that the grain size of sediment core HH-B from the bottom upwards irregularly fluctuated, the content of sand is always at a low level, and the whole core is dominated by clay and powder. In terms of the grain size, there is little change in the grain size of each group in both cores; in terms of the median grain size, the range of change in HH-A is slightly larger than that in HH-B. From the hydrodynamic condition of the lake, the sediment content of each grade particle size of the two column cores has little change, indicating that the hydrodynamic force of the whole lake is relatively stable. However, the sediment core HH-A is close to the upper stream of the whole watershed and closer to the lakeshore, more strongly influenced by hydrodynamic conditions and shows a large change in median grain size. On the contrary, HH-B is in the middle of the watershed and farther away from the lakeshore than HH-A, so the hydrodynamic environment of the sediment core HH-B is relatively more stable, and the change in the median grain size of sediments is smaller.

Figure 3. (Grain size or content) change of each group in sediment cores HH-A and HH-B in Honghu Lake: (a) HH-A (b) HH-B.

According to Figure 3(a), it is known that the sediment core HH-A can be divided into two parts, 0–30 cm as well as below 30 cm. Obviously, the particles in the upper part of HH-A are coarser than those in the lower part. The upper part is dominated by fine silt, and the lower part is dominated by clay fine silt. Some studies have pointed out that not only do natural factors such as an annual change in rainfall and the effects of flooding cause fluctuations in grain size characteristics of sediments but also the modification and destruction of lake watershed environments by human activities may lead to a continuous increase in the mean grain size of sediments (Gao et al., 2015; Tang et al., 2020). According to the results of the ²¹⁰Pb dating method and historical information, the 0–30 cm part of the column core HH-A was dated to the period after 1970, which coincided with the Cultural Revolution and the period of ‘food for the program’ in China. Besides, the ecological environment around Honghu Lake was damaged by excessive exploitation of the land and over-farming. Therefore, the grain size of the sediment core gradually becomes coarser from the bottom upwards, indicating that the influence on lake sedimentation gradually shifted from a process dominated by natural factors to a process dominated by human activities. The sediment core HH-B located in the place where an interception dam near the upper tributary of the incoming water direction, resulting in the longitudinal profile grain size of HH-B not changing significantly than HH-A.

3.3. Analysis of sedimentation rate change in Hong Lake

Based on the ²¹⁰Pb dating method, the corresponding sedimentation rate can be calculated by combining the depth and sedimentation age sequence of each sediment core in Honghu Lake, and the mean sedimentation rate of Honghu Lake from 1862 to 2011 ranged from 0.407 to 0.467 g·cm⁻²·a⁻¹ (Figure 4). This is similar to the dating results of other shallow lakes in the middle and lower Yangtze River (Shi & Qin, 2008; Yi et al., 2006), which indicates that the radionuclide dating results of sediment cores in Honghu Lake are relatively reliable. As shown in Figure 4, the sedimentation rate of the two column cores has a similar change trend. The sedimentation rate of the Honghu sediment column core has changed greatly, showing a trend of first increasing and then decreasing on the whole in the past 150 years. The environmental significance of the sediment cores in Honghu Lake can be roughly divided into four stages (A, B, C and D) according to the characteristics of the sedimentation rate change.

• (1) Stage A (before 1900): The sedimentation rate was overall in the low range. The mean sedimentation rate of sediment core HH-A was 0.085 g·cm⁻²·a⁻¹ and that of sediment core HH-B was 0.097 g·cm⁻²·a⁻¹. During this period, the ecology of Honghu Lake was in a primitive natural state, with little interference from human activities, slight soil erosion, and mainly natural sedimentation.

• (2) Stage B (1900–1949): The sedimentation rate increased slowly with a small range of fluctuation, and the sedimentation rate was overall higher than that in Stage A. The mean sedimentation rate of HH-A and HH-B was 0.269 g·cm⁻²·a⁻¹ and 0.234 g·cm⁻²·a⁻¹ respectively. It is shown that (Yuan et al., 2009), the national population was 245 million in 1851, and the population reached 542 million at the end of 1949, with a mean population growth of 0.55% during the 100 years from 1851 to 1949. The trend of population growth in the Honghu region was similar to that of the whole country, with a slow rise in population, enhanced demand for land and a slight increase in soil erosion. However, natural sedimentation was still dominant during this period.

• (3) Stage C (1949–1980): The sedimentation rate grew rapidly and fluctuated widely. The mean sedimentation rate of HH-A was 0.487 g·cm⁻²·a⁻¹ and that of HH-B was 0.517 g·cm⁻²·a⁻¹. The sedimentation rate of Honghu Lake showed a large fluctuation during this period, which was related to the increase in population around the lake and the disturbance of human activities. The population of Jiangxi Province increased from 13.14 million in 1949 to 32.70 million in 1980 (http://tjj.jiangxi.gov.cn/). It is known that there were two fertility peaks in China from 1949 to 1980, and the natural population growth rate was kept above 20‰ (Feng et al., 2005). With the increasing population, the demand for farmland increased, and the phenomenon of reclaiming land for food cultivation around the lake area was prominent, resulting in the destruction of land resources in the area and the increase of soil erosion. From 1957 to 1979, due to the iron and steel refining and other reasons, a large number of forests were cut down, and the destruction of land and vegetation was obvious, as well as a large amount of sediment and other materials flowed into the lake, which caused an increase in the rate of sedimentation. As shown in Figure 4, the sedimentation rate of Honghu Lake reached its first peak in the 1960s, which may be related to the peak of population growth of the lake area in the late 1950s and the extensive reclamation activities in the 1960s and 1970s. During the second peak period in the late 1970s, the maximum sedimentation rate reached 0.843 g·cm⁻²·a⁻¹, which resulted from the Cultural Revolution and the policy of food for the programme (Dai et al., 2009; Xiang et al., 2002). The excessive exploitation of the land and over-farming caused another destruction of the ecological environment around Honghu Lake and variations in the sedimentation environment.

• (4) Stage D (1980–2011): The sedimentation has generally slowed down as a whole. Since the 1990s, with the implementation of returning farmland to forests and grasses, people no longer carried out extensive cultivation of the Honghu Lake, the ecological environment of the Honghu Lake watershed was improved, and the soil erosion condition was eased. The mean sedimentation rates of HH-A and HH-B were 0.570 g·cm⁻²·a⁻¹ and 0.445 g·cm⁻²·a⁻¹, respectively. The difference between the mean sedimentation rates of the two sediment cores was large at this stage. In 1991, the sedimentation rate of HH-A reached the highest point of 1.397 g·cm⁻²·a⁻¹, and that of HH-B was 0.406 g·cm⁻²·a⁻¹. The sediment core HH-A was close to the lake shore, and the land use type along the shore was dominated by farmland. The lakeshore close to the HH-B was dominated by forest land, with up to 90% forest cover and a dam at the upper branch. Therefore, under the same precipitation conditions, due to the difference in land use types, the sediment content in Honghu Lake with surface runoff is different, and the sedimentation rate is greatly different. As a whole, despite there was an overall decrease in sedimentation rate, the environmental impact of human activities was still so great that the sedimentation rate in Honghu Lake remained high.

Figure 4. Sedimentation rate changes in sediment cores HH-A and HH-B in Honghu Lake: (a) HH-A (b) HH-B.

Compared with other water bodies in the Yangtze River basin (Table 1), the sedimentation rates of Honghu Lake were similar to that of Taihu (0.050–0.113 g·cm⁻²·a⁻¹), which took place suddenly around 1980. The whole of Taihu Lake suddenly changed around 1980, because of the rapid development of industry and agriculture (Di et al., 2015). According to government statistics, the total registered population of Yujiang District was 379,370 in 2011, of which 61% were rural. HH-B was closer to the centre of the lake, and the sedimentation rate of HH-B has decreased significantly since 1980 due to the interception effect of the dam, while HH-A has fluctuated due to frequent disturbance of human activities around the lake shore.

Table 1. Comparison of sedimentation rate among typical lakes in the Yangtze River Basin.

Lake	Sedimentation rate (g·cm^−2·a^−1)	Dating method	Reference
Taihu Lake	0.050–0.113	^210Pb (CRS)	Di et al., 2015
Chaohu Lake	0.224–0.242	^210Pb (CFCS)	Zan et al., 2011
Poyang Lake	0.270–0.350	^210Pb	Mei et al., 2018
Dainchi Lake	0.057–0.300	^137Cs	Zhang et al., 2009
Wujiangdu Reservoir	0.155	^210Pb (CIC)	Yang et al., 2017

3.4. Characteristics of nutrients distribution in sediments

Lake sediments are important reservoirs of carbon, nitrogen, phosphorus and other nutrient salts in lakes, and they are essential to the carbon, nitrogen, and phosphorus cycles of the entire lake system as well as other material cycles of the lake ecosystem (Wang et al., 2023). The TOC in lakes, as an important component of carbon in sediments, is mainly derived from lake aquatic organisms and terrestrial plant debris brought about by watershed erosion, and the large amount of siltation of TOC is a direct result of the eutrophication effect of lakes and a sign of accelerated degradation of lakes. Enrichment of TN comes mainly from the excessive application of domestic sewage and agricultural fertilisers as well as the uptake of atmospheric nitrogen by a large number of harmful algae. In general, the TP is treated as a beneficial component in agricultural soils but is considered harmful in environmental quality standards in lakes, where elevated TP in the water bodies can lead to eutrophication and deterioration of water quality (Li et al., 2022; Shin et al., 2022; Wang et al., 2022). The TOC content ranged from 0.99% to 2.5%, with a mean value of 1.71%; the TN content ranged from 0.12% to 0.42%, with a mean value of 0.25%; and the TP content ranged from 0.03% to 0.13%, with a mean value of 0.07% (Figure 5). The overall trend of TOC, TN, and TP content in the sediments showed a gradual increase from bottom to surface. In the depth of 5–47 cm, the vertical change trends of TOC, TN, and TP in the sediment were consistent (1863–2008): they were all relatively stable below 35 cm (before 1949); in the 16–35 cm (1953–1992) there was a rapidly increasing trend, with an increase of 70.61%, 48.83%, and 79.14%; the increasing trend slowed down in 5–16 cm (1992–2008), which was a small increase. In this stage, TN concentration reached its maximum value at 5 cm depth with a maximum value of 0.42%; TOC reached its maximum value at 6 cm depth with a maximum value of 2.50%.

Figure 5. Variation of TN, TP, TOC percentage and the value of C/N, N/P along with the HH-B depth.

Sediment organic matter C/N can determine whether the source of organic matter is autochthonous to the lake or exogenous input. In general, the C/N of non-vascular plants and phytoplankton organic matter ranges from 4 to 10, while the C/N of terrestrial vascular plants is over 20. The C/N of organic matter in sediments is greater than 8, which is often considered to be influenced by the two sources of matter, with a higher proportion of terrestrial-sourced organic matter in the sediments resulting in a greater C/N (Douglas et al., 2022; Khan et al., 2015). The C/N of the bottom mud of Honghu Lake was between 5 and 9, and mainly concentrated in the range of 6–8, indicating that the endogenous organic matter of Honghu Lake occupied a higher proportion. The C/N showed a gradual decrease in the trend of change from the bottom to the surface, among which, the C/N ratio was greater than 8 between 1863 and 1935 and reached a maximum value of 9.4 in 1899, which indicated that at this time, the endogenous and exogenous organic matter in the organic matter of the lake each occupied a certain proportion. The C/N ratio was less than 8 above 39 cm to the surface from 1935 to 2011, with a minimum value of 5.4 in 2008, indicating that the lake began to be dominated by endogenous organic matter.

The N and P content and ratio in sediments are usually a comprehensive reflection of the two dynamic processes of N and P deposition and sediment dissolution in water, and to some extent, N/P can reflect the eutrophic status of lakes (Lin et al., 2023; Rahhou et al., 2022). Figure 5 showed that the N/P of the HH-B core had an irregular trend from the bottom upwards, and its ratio ranged from 3.0 to 4.8, with a mean value of 3.9, which was similar to Chaohu Lake (Colman et al., 1995). The reason for this may be due to the intense biochemistry at the sediment–water interface of shallow lakes, where N was converted to gas and P was enriched in the sediments through degradation and other actions.

3.5. Correlation analysis of sediment core HH-B

Correlation analysis refers to the analysis of two (or more) variable elements that are correlated in order to measure the closeness of the correlation between two variable elements (Afzal et al., 2017; Denis, 2019). The correlation analysis between nutrient content, sedimentation rate, and grain size of the HH-B sediments was carried out using SPSS statistical software, and the results are shown in Table 2. It can be seen that TN showed a positive correlation with sand. In this study, TP and TOC content showed a positive correlation with clay, indicating that TP and TOC were mainly adsorbed in clay, which is similar to the results of previous studies. All the elements showed a strong correlation with correlation coefficients in the range of 0.9 above, which means that their sedimentation behaviour is similar and from the same source (Jia et al., 2009), indicating that they are homologous. Meanwhile, there was also a correlation between all the elements and the sedimentation rate, with a correlation of around 0.5.

Table 2. Correlation efficiency of HH-B sediments.

	<4μm	4–16μm	16–64μm	The sedimentation rate	TN	TP	TOC
TN	−0.01^1)	0.19^1)	0.02^1)	0.509^1)	1
TP	0.03^1)	−0.11^1)	−0.01^1)	0.478^1)	0.96^1)	1
TOC	0.01^1)	−0.20^1)	−0.02^1)	0.497^1)	0.97^1)	0.92^1)	1

1) Sig < 0.01.

4. Conclusion

1. The mean sedimentation rate of Honghu Lake from 1862 to 2011 was 0.407-0.467 g·cm⁻²·a⁻¹ by using the ²¹⁰Pb dating method, which is similar to the dating results of other shallow lakes in the middle and lower streams of the Yangtze River.

2. The fine silt deposition occupied a predominance of lake sediment in Honghu Lake. Meanwhile, the sediment content of each grain size level in two-column cores does not vary much, which indicates that the hydrodynamics of the whole lake is relatively smooth. The grain size of sediment core HH-A gradually becomes coarser from the bottom upwards, indicating that the influence of lake sedimentation gradually shifted from a process dominated by natural factors to a process dominated by human activities. Compared to HH-A, grain sizes of sediment core HH-B change less in the vertical profile, because an interception dam has been constructed at the upper branch of the nearby inflow direction.

3. The dominant factor of sedimentation transfers from the physical geographic environment to human activities in Honghu Lake. The process of sedimentation goes through four stages over time: before 1900, the sedimentation rate was low and dominated by natural sedimentation; from 1900 to 1949, dominated by the physical geographic environment, the sedimentation rate increased slowly with a small range of fluctuation; from 1949 to 1980, there was a rapid rise as well as a large fluctuation which gradually influenced by human activities in sedimentation rate; and from 1980 to 2011, a fluctuation decrease occurred in the sedimentation rate wholly, and the process was still dominated by human activities.

4. The nutrients in the sediments of Honghu Lake have a trend of synchronous change. With the decrease of sediment depth in Honghu Lake, the concentrations of TOC, TN and TP gradually increased, and all showed relatively stable before 1953. Their concentrations tend to increase rapidly at the depth of 16–35 cm (1953–1992) and grow slowly at the depth of 5–16 cm (1992–2008). The comparison of nutrient content between the two cores shows that the TN content in core HH-A is much larger than that in core HH-B due to the influence of human activities. According to the organic index and the evaluation method of organic nitrogen, the HH-A of the Honghu column core is more polluted than HH-B because it receives more nutrients containing nitrogen for a long time. Therefore, further attention should be paid to the local nitrogen pollution problem.

Author Contributions

Conceptualisation, M. L. Zhang; Methodology, H. Wang, Y. H. Wang, and J. Wu; Validation, H. Wang, Y. H. Wang, and J. Wu; writing-original draft preparation, H. Wang, Y. H. Wang, and J. Wu; writing-review and editing, H. Wang, and M. L. Zhang; supervision, M. L. Zhang; funding acquisition, M. L. Zhang. All authors have read and agreed to the published version of the manuscript.

Acknowledgments

This work was supported by the National Key Research and Development Program of China [grant numbers: No. 2021YFC3201500] and the Research Project of Nanjing Normal University [grant numbers: No. 1812200046KCSZ2223 and No. 1812200050KCSZ2376].

Disclosure statement

No potential conflict of interest was reported by the author(s).

Data availability statement

Data will be made available on reasonable request.

Additional information

Funding

This work was supported by the National Key Research and Development Program of China [2021YFC3201500]; Research Project of Nanjing Normal University [1812200050KCSZ2376]; Research Project of Nanjing Normal University [1812200046KCSZ2223].