1 Introduction

Particulate matter in the water body is divided into exogenous and endogenous inputs, and suspended particles are combined with sediments after settling in the aquatic environment, with the recorded information on the environmental evolution of the basin also preserved in the lake sediments. Therefore, the occurrence of biogenic elements (carbon, nitrogen, and phosphorus) in the sediments can be applied to reconstruct the pollution history with socio-economic changes, reflecting nutrient levels in the aquatic environment and source profile of pollution in the lake ecosystem (Zan et al. 2012; Guan et al. 2014; Ye et al. 2017). As external conditions (wind and waves, benthic disturbances, etc.) change, sediments with endogenous load will markedly affect the aquatic environment and play a certain role as a nutrient source, being an important influencing factor of lake eutrophication (Qin et al. 2019; Wang et al. 2022). In particular, large shallow lakes are prone to sediments in suspension due to wind-induced and wave action, and in turn, affect the primary production of macrophytes and phytoplankton (Qin et al. 2019). Therefore, endogenous release is an urgent issue that requires to be resolved in the process of lake aquatic ecosystem management (Chen et al. 2020). Exploring the vertical distribution characteristics of carbon (C), nitrogen (N), and phosphorus (P) in sediments is of practical significance for the in-depth study of the migration and transformation patterns and environmental effects of the elements.

Previous studies have shown that sediment particle size can invert the palaeoenvironmental climate change in the basin. Different sedimentary environments are characterized by differences in particle size composition, and the compositional characteristics may contribute significantly to the distribution and transport of nutrients and organic matter (OM) (Macumber et al. 2018; Chen et al. 2022). Nitrogen stable isotopes are reliable indicators for determining the source of nutrients in the aquatic environment and can be applied to the tracing of OM and N in lakes, rivers, and oceans by combining N stable isotopes with other physicochemical indicators, enhancing the understanding of our biogeochemical C and N cycles (Zan et al. 2012; Guo et al. 2020).

In recent decades, increased human activities have led to excessive nutrients and OM entering the rivers, which converge into the lakes, resulting in frequent occurrences of water blooms in the lake (Chen et al. 2020; Zhao et al. 2022). Therefore, it is necessary to focus on the contaminated space of the basin to manage lake contamination. Taihu Lake, the third largest freshwater lake in China, is a typical shallow lake and is a drinking water source for the surrounding residents. In recent years, it has been shown that the water body of Taihu Lake is still in eutrophication and frequently affected by cyanobacterial blooms. Especially, the northwestern bay is the main "black bloom area" prone area of Taihu Lake (Qin et al. 2019; Chen et al. 2020; Wang et al. 2022), and endogenous release triggered by sediment resuspension is a significant contributor to the eutrophication of northwestern bay. Although numerous studies have been conducted to investigate the spatial distribution characteristics of nutrients in Taihu Lake (Xu et al. 2019; Li et al. 2022; Wang et al. 2022; Zhao et al. 2022), the attention on Taihu Lake has been mainly focused on preliminary analysis of hydrology and water chemistry (Xu et al. 2019; Li et al. 2022; Wang et al. 2022), and study in sediment pollution has been mostly limited to the sediments from the lake itself (Jin et al. 2006; Qin et al. 2019; Zhao et al. 2022). However, few studies have been performed on the nearby basin, and lack of analysis of the vertical variation of nutrients and pollution evaluation in the nearby basin. Nutrients and OM in the basin enter Taihu Lake through the river, whereas the northwestern Taihu Lake is the major confluence of the inflowing river with the lake. For better tracking the trophic status and anthropogenic impact in the past century as recorded in Taihu Lake, it is necessary to investigate nutrients change in the sediment cores of the northern Taihu Basin. Therefore, the purpose of this study is 1) to analyze the temporal trends of total nitrogen (TN), total phosphorus (TP), and total organic carbon (TOC) concentrations in the sediments, 2) to reveal the role of particle size on the distribution of TN, TP, and TOC, and 3) to explore the historical sources of OM and N in combination with physicochemical indicators and N stable isotopes, to provide a scientific basis for controlling the degree of eutrophication, improving water quality and ecological restoration in a typical urban lake in East China.

2 Materials and methods

2.1 Sample collection and pretreatment

Wuxi City in the northern Taihu Basin is the core area of the Yangtze River Delta centered on Shanghai, China with a steadily increasing trend of urbanization (Yuan et al. 2014; Wuxi City Bureau of Statistics 1949–2018). In the summer of 2018, two sediment cores (S1 and S2 cores, 49.5–62-cm depth for each core) were collected using gravity coring equipment (90-mm diameter, 100-cm length) at the estuary of Meiliang Bay and Wuli Lake in the north of Taihu Lake, without disturbing the sediment ‒water interface from the estuary of the rivers (Fig. 1). After sampling, the sediment cores were transported vertically to the laboratory. To improve the resolution, the S1 sediment core was divided into 58 sub-samples by slicing at 0.5-cm intervals (-49.5 to -39.5 cm depth), 1.5-cm intervals (-4.5 cm to the surface layer), and the rest of the middle part was cut at 1 cm intervals. The S2 sediment core was divided into 72 sub-samples by slicing at 0.5-cm intervals (-62 to -52 cm depth), 1-cm intervals (-52 cm to the surface layer). All the samples were freeze-dried to prior to analysis. A total of 130 sub-samples were obtained. All samples were freeze-dried for further analysis.

Fig. 1
figure 1

Location map of sampling sites

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2.2 Measurements of N stable isotope, elemental composition, and particle size

The S1 sediment core was selected for analyzing N stable isotope ratios, and the δ15N value of the samples was measured using an elemental analyzer (FLASH IRMS, Thermo, USA) coupled with an isotope ratio mass spectrometer (DELTA V PLUS, Thermo, USA) (for details see Derrien et al. 2018). TN and TP concentration in the sediments was determined by the potassium persulfate digestion method and molybdenum-antimony spectrometry, respectively (Jin et al. 2006; Chen et al. 2022). TOC levels in the samples were analyzed by a TOC-L analyzer (TOC-L CPH, Shimadzu, Japan). The dry-weight (dw) samples were sieved through 100 mesh. Two samples of 0.2 g were placed in a clean sample boat. One was introduced into a high-temperature combustion tube at 900 °C to calcine to obtain the organic compound (TC) concentration, and one was introduced into a low-temperature combustion tube at 200 °C and phosphoric acid (4.87 mol L−1) was added to obtain the inorganic compound (IC) concentration. The TOC concentration of the sample was equal to the difference between TC and IC values (Chen et al. 2022).

A 0.50 g freeze-dried sample was used for particle size analysis. The 20 mL of H2O2 (30%) and 10 mL of HCl acid (10%) were added to the sample, to remove OM and carbonates, respectively. High-purity water was added to rinse the acidic ions. Keep the mixture for 1 d to remove the supernatant. The mixture was dispersed on an ultrasonic shaker for 15 min using a 15 mL volume of (NaPO3)6 (0.05 mol L−1). Then, the particle size distribution of the samples was performed by a laser particle size analyzer (Malvern Mastersizer 2000, UK). In reference to the Udden-Wentworth classification method (Udden 1914; Wentworth 1922), sediment particles are classified from fine to coarse as clay (< 4 µm), fine silty (4–16 µm), coarse silty (16–64 µm), and sand (> 64 µm).

2.3 Sediment core dating

The dating of the sediment core was based on 210Pbex. The sedimentation rate was obtained by using the constant initial concentration (CIC) model (Appleby and Oldfield 1983; Perez et al. 2020). An approximately 10 g freeze-dried sample was sealed for about 30 d to ensure that 226Ra and 210Pb were in a permanent decay equilibrium regime, followed by determination of the activity of 210Pb using an Ortec HPGe GWL series well-type high-purity germanium γ-spectrometer (GWL-120–15, USA). The 210Pb activity was obtained from γ-emission at 46.5 keV, while the activity of 226Ra could be calculated from the peak area of the γ-ray spectrum of 214Pb at 351.9 keV. The total 210Pb activity was subtracted from the 226Ra activity to obtain the excess 210Pb (210Pbex) activity (Chang et al. 2018). The total 210Pb activities ranged between 65.25 and 155.78 Bq kg−1, while the 210Pbex activities were 26.53–115.83 Bq kg−1, showing a gradually increasing trend from deep to surface in the sediment profile (Fig. S1). The mean sedimentary rate based on the CIC model was 0.51 cm yr−1. The dating results revealed that the 62-cm sediment core (S2 sediment core) tracked the sedimentary history of the past 146 years (1872–2018).

2.4 Risk assessment

In this study, the ecological risk was assessed based on sediment quality assessment guidelines, which were classified into three levels (Table 1: safety level with negligible ecological toxicity, low effect level with potential ecological toxicity, serious effect level with severely damaged to benthic communities) (Mudroch and Azcue 1997; Wu et al. 2018; Qiu et al. 2022).

Table 1 The sediment quality assessment guidelines and concentration range of TN, TP and TOC in the sediment cores
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In order to achieve more scientific results reflecting the actual environmental quality, TN and TP pollution levels were further evaluated using the comprehensive pollution index (CPI) method. In addition, the organic pollution index (OI) was used to further analyze the organic pollution status of the sediments (Qiu et al. 2022). The pollution index can be described by Eqs. (1–3):

$${\text{S}}_{\text{i}}=\frac{{\text{C}}_{\text{i}}}{{\text{C}}_{\text{s}}}$$
(1)
$$\text{FF} = \sqrt{\frac{{\text{F}}^{2}+{\text{F}}_{\text{max}}^{2}}{2}}$$
(2)
$$\text{OI} = \text{TOC}\; (\%) \times \text{TN}\; (\%) \times 95\%$$
(3)

where Si is a single pollution index (SPI) or standard index (i represents TN or TP value). Ci is the evaluation factor i of the measured content. Cs is the standard content of evaluation factor i. The Cs values of TN and TP are 0.55 mg g−1 and 0.60 mg g−1, respectively, referring to the TN and TP values that can pose the lowest effect level in the sediments issued by sediment quality guidelines (Table 1) (Mudroch and Azcue 1997). FF represents the comprehensive pollution index. F is the mean value of all the pollution indices. F is the mean value of the STN and STP. Fmax indicates the maximum single pollution index (the maximum value of STN and STP). The evaluation criteria of the sediments based on the Si, FF, and OI index are provided in Table 2.

Table 2 Standards of pollution evaluation in the sediments
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2.5 Statistical analysis

Pearson correlation analysis was employed with the statistical program SPSS 21.0 (SPSS Inc. Chicago, USA), and the significance was set at p < 0.05. Mapping of sampling sites was performed using ArcGIS 10.0 (ESRI, USA). The figures were plotted using Origin 9.0 and Microsoft Excel. The socioeconomic indicator data, including GDP, population and urbanization rate in this study were obtained from Wuxi City Bureau of Statistics (1949–2018).

3 Results and discussion

3.1 Temporal trend of TN, TP, and TOC profile in the sediments

In the S1 sediment core, TN concentration ranged from 0.16 to 3.19 mg g−1 with a mean value of 0.88 mg g−1. TP concentration varied from 0.03 to 0.12 mg g−1 with a mean value of 0.06 mg g−1, and TOC concentration was between 1.08 and 37.44 mg g−1 with a mean value of 8.23 mg g−1 (Fig. 2a). In the S2 sediment core, TN concentration ranged from 0.37 to 1.68 mg g−1 (mean 0.67 mg g−1). TP concentration varied from 0.03 to 0.15 mg g−1 (mean 0.08 mg g−1), and TOC concentration was between 1.51 and 17.75 mg g−1 (mean 4.29 mg g−1) (Fig. 2b). Previous studies have reported TN, TP and TOC concentrations in the sediments from China and worldwide (Table 3). Total N concentrations in the northern Taihu Basin were comparable to those from Pinqing Lake, Baiyangdian Lake, and Bear Lake, higher than Poyang Lake, Zhelin Bay, Zhifu Bay, and Gulf of Trieste, and lower than those from Qianhu Lake, Daye Lake, Beppu Bay and Karla Lake. Total P concentrations in the northern Taihu Basin were lower than all other lakes in this study (Pinqing Lake, Qianhu Lake, Zhifu Bay, Daye Lake, and Bear Lake). The TOC level of northern Taihu Basin was higher than most of the other lakes, such as Zhelin Bay, Pinqing Lake, Zhifu Bay, Gulf of Trieste, Bear Lake, Beppu Bay, and Karla Lake, but smaller than Qianhu Lake and Daye Lake. Overall, compared with the concentrations of other lakes and coastal areas, TN, TP, and TOC concentrations were at normal–moderate levels.

Fig. 2
figure 2

Vertical distributions of TN, TP, and TOC value with economic development in the S1 (a) and S2 (b) sediment cores. UR, urbanization rate

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Table 3 Comparison of TN, TP and TOC concentrations in the sediments, across China, and worldwide (mg g−1)
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Overall, two temporal trends for the concentration of nutrients and OM in the S1 sediment core were characterized (Fig. 2a). In the first, slight changes of TN, TP, and TOC values in the deeper segments were observed, reflecting the natural deposition of lake sediments before the 1960s. The reason is that industry and agriculture in the study area were not well developed before the 1960s, and the effect of anthropogenic activities had no or very weak effects on the nutrient and OM in the sediments. The second stage showed a trend of gradual increase in concentrations of TN, TP, and TOC from the 1960s onwards in the S1 sediment core (Fig. 2a). Among them, before 1980, TN concentration showed slow and fluctuating growth. after 1980, it had increased dramatically due to the development of the local economy and population explosion after the "reform and opening up" in 1978. Despite the controlled N input from industrial effluent and aquaculture discharge during this period, N input from agricultural wastewater, domestic sewage, and atmospheric deposition was enhanced, resulting in the accumulation of TN in sediments (Guan et al. 2014; Qin et al. 2019; Zhao et al. 2022). Total P levels remained stable until 1960 and rose sharply after 1960. The dramatic economic construction in the early socialist period (the late 1950s to early 1960s) intensified the nutrient input into the lake and consequently increased TP concentration in the sediment. Total OC concentration was essentially steady at about 1.43 mg g−1 before 1948 and started to progressively grow after the 1940s.

As illustrated in Fig. 2b, from the deep to the surface layer in the S2 sediment core, TN and TOC levels kept a stable state basically before 1960 and followed a progressive upward trend after 1960. Total P levels decreased continuously with the increasing depth until the 1930s and started to grow strongly between the 1930s and 1960s and peaked in 1968 (0.15 mg g−1). It markedly dropped to 0.04 mg g−1 in 1972, coinciding with the collapse and slowdown of industrial and agricultural development in China during the ten years (1966–1976), after that, the level was increasing continuously again. Rapid commercial and industrial development in Wuxi City since the 1960s caused land use and land cover changes, and nutrient loss, and subsequently these significant disturbances resulting from human activities contributed to increased levels of nutrients in the lake. The socioeconomic data of Wuxi City (Fig. 2b) showed that a sharp increase in the GDP (0.09–112.03 × 1010 RMB), population (306.30–497.21 × 104), and urbanization level (22.48–80.40%) (Wuxi City Bureau of Statistics 1949–2018). The quality of sediment was increasingly affected by exogenous inputs since the 1960s, and the contamination of the lake environment by production and domestic wastewater has been worsening.

3.2 Vertical distribution characteristics of particle size

The composition of particle size in sediments plays an essential role in an inversion of the environmental behavior of climatic and geochemical elements, which is strongly affected by the combination of hydrodynamics, material sources, transport media, and transport processes (Lu et al. 2018; Macumber et al. 2018). It was demonstrated that there was a remarkable effect of particle size on the accumulation of OM and nutrients in the environment.

The distribution characteristics of particle size in the sediment cores were shown in Fig. 3. The results indicated that the particle size fraction in the S1 sediment core was dominated by fine silty and coarse silty, followed by clay and sand, with concentrations of 31.60–44.97% (mean 37%), 26.90–41.47% (mean 37.11%), 18.53–27.33% (mean 21.82%) and 0.27–9.03% (mean 4.07%), respectively Fig. 3a. The median particle size ranged from 9.65 to 15.79 μm (mean 13.86 μm). Particle size composition in the S2 sediment core was similarly dominated by fine and coarse silty, followed by clay and sand, with the proportions of 31.72–40.35% (mean 40.35%), 23.97–42.78% (mean 36.40%), 15.39–26.50% (mean 21.69%) and 0.30–24.90% (mean 1.56%), respectively (Fig. 3b). Median particle size varied from 9.28 to 15.41 μm (mean 11.57 μm). Compared with Qilu Lake, Qinghai Lake, and Wusuliliang Lake (Li et al. 2010), the concentration of sand in the S1 and S2 sediment core was at a low level. It can be explained by the relatively poor hydrodynamic conditions of the lower Yangtze River plain, and the study area is located at the north bay of Taihu Lake, which is favorable to the deposition of fine particles. With decreasing depth, the sediment grain size in sediment cores showed a trend of gradual refinement starting from the 1980s, with a slight increase in clay and fine silty and a marginal reduction in coarse silty and sand, demonstrating a trend of decreasing environmental hydrodynamics of sediments in the study area. In general, the particle size of the sediment is finer, the more likely to form an anaerobic environment, resulting in difficulties in redox reactions of OM, with low decomposition rates and reduced nutrients entering the water body (Ma et al. 2021), ultimately causing nutrients to be enriched in the sediments. It is speculated that the increase of clay and silty is one of the reasons for the growth of nutrients in the sediments. Overall, the variation of sediment particle size composition with depositional age was minor, reflecting that the hydrodynamic conditions in the basin were relatively stable during the sedimentary history period.

Fig. 3
figure 3

Historical distribution of particle size in the S1 (a) and S2 (b) sediment cores. MPS, median particle size

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3.3 Factors affecting nutrient distribution in the sediments

It was reported that the distribution of OM and particle size played a key role in the transport, aggregation, and fate of compounds in the sediments (Wang et al. 2021; Chen et al. 2022). In this study, the correlation analysis of TN, TP, and TOC concentrations with different particle sizes was carried out (Fig. 4). The results demonstrated that there was an apparent positive coefficient between the TN, TP, TOC, and particle size in the S1 and S2 sediment cores (p ≤ 0.01). Total N in the sediments was remarkably related to TP, suggesting that N and P in the process of lake eutrophication have similar sources and depositional behavior. There were high correlations of TOC with TN, and TP in the sediments, which illustrated that there was a relationship between N and P in the sediment and the mineralization process of OM, and most of the N existed in the OM in the form of an organic state. Furthermore, sediment OM will consume dissolved oxygen at the water–sediment interface during biodegradation and mineralization, accelerating the effective release of N and P (Ding et al. 2018; Qin et al. 2019). It was observed that controlling the decomposition process of OM in the study area was the key to controlling the release of N and P from the sediment. Meanwhile, excessive N and P inputs to lakes result in outbreaks of cyanobacteria and red tide. The surface of algae is adsorptive to OM, with its ability to store dissolved and particulate OM (Dachs et al. 2000; He et al. 2021). At the end of the life cycle of phytoplankton, the decaying residue gets deposited (Turner 2015), resulting in increased concentrations of particulate organic C in the water body and sediments. Further analysis of R revealed that the correlation between TOC and TN was slightly stronger than that between TOC and TP in the two sediment cores. Due to the complex occurrence form of P in the sediments, especially in shallow lakes, whereas the N in the sediments generally exists predominantly in the form of organic N (Yang et al. 2014). Therefore, the correlation coefficient between TOC and TN was relatively higher.

There are differences in the biodegradability and chemical composition of the various particle sizes in the sediment. Therefore, they possess dissimilar sorption capacities. A positive correlation was observed between clay, fine silty, TN, TP, and TOC (p ≤ 0.01) in the S1 sediment core. Whereas, coarse silty and sand had a negative correlation with TN, TP, and TOC (p ≤ 0.01), respectively. In the S2 sediment core, coarse silty was positively correlated with TN, TP, and TOC (p ≤ 0.01) respectively. In contrast, clay and fine silty were negatively correlated with these elements (p ≤ 0.01), respectively. The result supports the hypothesis that the distribution of nutrients and OM is influenced by sediment particle size. In general, the specific surface area of sediment particles increased with decreasing particle size, and the adsorption capacity of nutrients was enhanced (Chen et al. 2022). Therefore, the clay concentration in the S1 sediment core has remarkable effects on the distribution characteristics of nutrients and OM. Further analysis of R showed that TN, TP, and TOC had a higher positively correlated with larger particles (4–64 µm, i.e., fine silt and coarse silt) in the sediment cores, and the results were supported by the findings (Wang et al. 2016a, b; Yang et al. 2021). Previous study showed that microorganisms were more active in fine particles and with higher decomposition rates (Tao et al. 2010). Therefore, the concentration of TN, TP, and TOC may not increase with the increasing concentration of clay (< 4 µm), while the silt (4–64 µm) has more pronounced effects on the vertical distribution of these elements. It is possible to speculate that the particle size for OM adsorption is affected by the specific surface area and pore diameter of the particles (Yang et al. 2021). Large particles provide larger pore diameters of the particles with more OM adsorption sites internally and fewer microorganisms attached to the surface, consequently the ability of OM decomposition is weak (Châtelet et al. 2009; Yang et al. 2021). Thus, long-term burial may contribute to a lower OM concentration attached to fine particles compared to coarse grain material (Fig. 4).

Fig. 4
figure 4

The correlation between TN, TP, TOC, and particle size in the S1 (a) and S2 (b) sediment cores

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3.4 Source analysis of OM and N in sediment profiles

The C/N ratio in the sediments provides an indication of the source of OM in the lake. The C/N ratio of native algae and aquatic vascular plants in lakes commonly ranges from 4 to 10; while the C/N ratio of terrestrial plants like grasses and sedge families is tending to be > 12 and even up to 20 (Zan et al. 2012; Freimuth et al. 2020; Chen et al. 2022). The C/N ratio in the S1 sediment core ranged from 1.55 to 25.39, with a mean value of 8.34 (Fig. 5). The value generally showed a fluctuating increasing trend, with all C/N ratios < 10 in all sedimentary layers before 1948. The smaller the C/N ratio, the greater proportion of autochthonous i.e., bacteria, algae, and phytoplankton sources (Wang et al. 2021). It indicated that OM in the sediments was mainly influenced by phytoplanktonic sources during this period. From 1950 to 2018, the percentage of samples with C/N ratios > 10 was 77%, suggesting that OM was primarily input from terrestrial sources. During this period, the C/N ratio reached a peak value in 1955, which was attributed to the flooding in the Taihu Basin in 1954 with increased input of OM from terrestrial sources (Ni et al. 2011). In the S2 sediment core, the C/N ratio was between 3.47 and 10.74, with a mean value of 5.89. The ratio in 96% of the sediment layers was below 10, showing that the OM in the sediments was dominated by endogenous inputs during the historical period. A larger C/N ratio indicates a greater proportion of OM input from terrestrial plants. Overall, the C/N ratios in S1 and S2 sediment cores showed an increasing trend over time, reflecting the increasing input of exogenous sources. It is attributed to the fact that Taihu Lake, as the third largest freshwater lake in China, is located in the Yangtze River Delta region, which is economically developed in the eastern part of the country, and the rivers entering the lake pass through the surrounding developed cities bringing a large amount of pollutants.

Fig. 5
figure 5

Vertical distributions of C/N value in the S1 (a) and S2 (b) sediment cores

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The C/N ratio is a preliminary indication of the source of OM, and the use of N isotopes provides a clearer picture of the OM source in the sediments (Herczeg et al. 2001; Zan et al. 2012; Guo et al. 2020). Therefore, the historical source of sediment OM was further determined by the analysis of δ15N measurements in the S1 sediment core. As illustrated in Fig. 6, the variation of δ15N values in the sediment core of the northern Taihu Basin ranged from 2.96 to 9.96‰, with a mean value of 5.83‰. Historical variation of δ15N values was roughly classified into three periods. Period I: 1900s–early 1960s, the δ15N values in the sediments had an overall decreasing trend, and stabilized at approximately 7.80%o over the period from 1902 to 1948, then continued to decline to 3.15‰ up to 1962. Period II: early 1960s–late 2000s, δ15N values in the sediments showed a slight increase (3.15–5.44%o) during 1962–2008. Afterward, the value progressively decreased to 2.96‰ in the surface layer.

Fig. 6
figure 6

Historical variation in δ15N values (a) and analysis results of the qualitative source identification of organic matter in the sediments (b)

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The δ15N value in the sediment was between -5 to 18‰ and 3 to 6‰, suggesting that the OM was derived from terrestrial plants (Guo et al. 2020). While the value varied between 4 and 10‰, suggesting the OM originated from typical planktonic and lacustrine algae (Gearing 1988; Freimuth et al. 2020; Chen et al. 2022). C3 and C4 plants were classified as terrestrial plants, and lake autochthonous algae and plankton were classified as plankton. Terrestrial plants and plankton were identified as end-member substances in this study. Historical variation of δ15N values and the C/N ratio of each end-member were illustrated in Fig. 6. The results revealed that the distribution of the C/N ratio (1.55–8.69) and δ15N value (6.28 to 9.96‰) in the period from the 1900s to 1940s were totally within the range of aquatic algae source, indicating that the sediment OM was principally from plankton in this stage. Thereafter, there was > 74% of the C/N ratio (5.85 to 25.39) and all of the δ15N values (2.96 and 5.48‰) fall within the range of terrestrial OM, indicating that the terrestrial plants input was the predominant contributor of OM in the sediments during this period. The input of exogenous OM is markedly disturbed by anthropogenic disturbances, especially by exploitation activities in the basin (Guo et al. 2020). In the period from the 1960s to 2000s, the δ15N values were on the rise, demonstrating a gradual increase in terrestrial input, which was consistent with the industrial and agricultural pollution caused by economic development in the study area during this period (Li et al. 2019). After peaking in 2008, the δ15N values began to decrease sharply. It is possible that after the "Water Crisis" in Wuxi City in 2007, the ecological restoration project and the implementation of environmental protection policies by the government had an impact on the accumulation of sediment OM (Lei et al. 2016; Li et al. 2019), and the exogenous input was kept under control to a certain extent. In general, the source of sediment OM in the northern Taihu Basin changed progressively from autochthonous sources to allochthonous sources.

Meanwhile, the sedimentary record of δ15N can be used to invert the source information of N (Heaton 1986; Kendall 1998; Sigman et al. 1999). Typically, δ15N value in the sediments varying between 0.2 and 0.8‰ is attributed to atmospheric deposition source. δ15N values in the S1 sediment core fall within the range of 4 to 9‰ identified as a source of soil loss. δ15N values for synthetic fertilizers have been determined to vary from -4 to 4‰ and sewage from 10 to 20‰. δ15N values in the sediment core ranged from 2.96 to 9.96‰, with 16% of the samples presenting δ15N values between -4 and 4‰, indicating N input from synthetic fertilizers. δ15N values fluctuated within 4–9‰ in 83% of the sediment layers, reflecting that sediment N was predominantly influenced by a combination of soil N loss and agricultural non-point source pollution. It can be attributed to the soil and N loss caused by human activities such as paddock farming and the rapid development of agriculture and industry in the Taihu Basin in the past decades.

3.5 Pollution risk of N and P from the sediments

For the S1 sediment core, the results showed that TP concentration in the sediment was at a safety level. TN and TOC have different degrees of toxicity to benthic organisms. The values of TN and TOC in 63.79% and 29.31% of the sediment layers were at a low effect level, respectively. For the S2 sediment core, the results of ecotoxicity analysis of TN, TP, and TOC reveal that TP concentration falls within the safety level. TN and TOC concentrations in 59.72% and 2.78% of the sediment layers were in the low effect level, respectively. The result concludes that TN and TOC in the sediments present potential ecological risks.

The environmental consequence of nutrients in the sediments was evaluated by the pollution index, and the calculated TN, TP, and OM pollution index was shown in Figs. 7 and S2. The STN of the S1 and S2 sediment cores was 0.30–5.79 (mean 1.6) and 0.67–3.06 (mean 1.22), respectively. The results indicated that more than half of the sediment layers (S1 core: 63.79%, S2 core: 59.72%) were at mild and above mild pollution levels. Among them, even 34.48% and 1.39% of layers were heavily polluted in the S1 and S2 sediment cores, respectively (Fig. 7). The STP of the S1 and S2 sediment cores was 0.05–0.19 (mean 0.09) and 0.05–0.25 (mean 0.13), which suggested that all of the sediment layers were at the clean level. The FF of N and P was 0.25–4.61 (1.28) and 0.54–2.46 (0.99) for the S1 and S2 sediment cores, respectively. There were 13.79% and 37.50% of the sediment layers at the mild pollution level, 37.93% and 5.56% at the level of moderate pollution or heavy pollution in the S1 and S2 sediment cores, respectively, which indicated that the polluted sediments were likely to pose a threat to water quality. The range of the OI index was 0.22 × 10–2–1.13 (mean 0.10) and 0.01–0.28 (mean 0.03) for the S1 and S2 sediment cores, respectively. Of these, 29.31% and 1.39% of the samples were at the mild pollution level, and 18.97% and 1.39% were moderately polluted in the S1 and S2 sediment cores, respectively. Only one sediment layer in the S1 sediment cores, respectively, was heavily contaminated. It is evident that the environmental quality of sedimentation in the northern Taihu Basin was predominantly influenced by TN and TOC.

Fig. 7
figure 7

Vertical distributions of pollution risk in the S1 (a) and S2 (b) sediment cores of the northern Taihu Basin. STN is the single factor pollution index of total nitrogen (TN); STP is the single factor pollution index of total phosphorus (TP); FF is the comprehensive pollution index; OI is the organic pollution index

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As illustrated in Fig. 7, from deep to surface layers, the time variation of the FF and STN in the profile of the sediment core showed consistency, demonstrating that FF was influenced by STN and TN was the dominant factor affecting the quality of the sedimentary environment. Since the 1960s, the STN and FF in all sediment layers were more than 1 in both sediment cores, and the OI index in all sediment layers was greater than 0.05 since the 1960s and 2000s in the S1 and S2 sediment cores, respectively. It means that the sediments were contaminated with N and P at this stage. It can be attributed to the rapid development of the economy in China since the 1960s, especially, Wuxi City's GDP showed a dramatic growth with a rise of 852% during the period 2000–2018 (Fig. 2) (Wuxi City Bureau of Statistics 1949–2018; Qin et al. 2019). Although many intensive actions have been taken since the 2000s to improve the water quality of Taihu Lake, it has not led to detectable improvements in water quality until now (Qin et al. 2019). It is probably for the reason that most of the environmental management of Taihu Lake is limited to the lake itself. The targeted governance measures in the nearby basin and the control of pollution sources for the lake need to be further strengthened. The N and P loads in the northern Taihu Basin are associated with the exogenous inputs. The northern area of Taihu Lake is the confluence of rivers entering the lake (Xu et al. 2019). Compared with other areas in Taihu Basin, cities along these rivers had developed industry and agriculture and high population density with numerous ports (Qin et al. 2019; Wang et al. 2022). Therefore, this area received a large amount of nutrient-rich wastewater from nearby cities (e.g., Wuxi City) that far exceeded their assimilative capacity and contribute nearly 80% of N and P to Taihu Lake (Qin et al. 2019). With weak flow movement and almost blocked water exchange, the study area showed poor self-purification ability (Chen et al. 2020). Thus, pollutants in the northern Taihu Basin were not easy to transfer to the open lake area and then accumulated in situ.

4 Conclusions

The 210Pb-dated sediment records from the northern Taihu Basin recorded the nutrients for 146 years before sampling (1872–2018). These showed increasing trends of TN, TP, and TOC concentration, with a turning point in the 1960s, consistent with the urbanization and industrial development of Wuxi City. The concentrations of TN, TP, and TOC were at normal–moderate levels compared to other lakes and coastal areas in the world. The increase in the silty sand (coarse silty and fine silty) and TOC was accompanied by an increase in the levels of TN and TP (p ≤ 0.01), and the critical factor in controlling the release of N and P from sediments was to control the decomposition process of OM in the study area. Prior to the 1940s, the OM in the sediments mainly originated from algae. Since the 1940s, increasing TN, TP, and TOC concentrations together with the change of δ15N suggested anthropogenic perturbations in sediment OM, and the accumulation of N in the sediment was affected predominantly by soil N loss and N-containing agricultural effluent. The single factor index and comprehensive pollution index showed that the ecological risk of N and organic contamination in the sediments was serious. There was a potential danger of endogenous N release which should receive greater attention.