Introduction

The eutrophication of lakes has been a hot environmental science topic in China since 1970 (Li et al. 2012). Research on the water quality in the past decades has shown a significant decline in water quality and is due to human activities in Erhai Lake’s watershed. Erhai is currently in a transition from a mesotrophic lake to a eutrophic lake (Wang et al. 2015). Among the reasons, increases in N and P in the lake have played two key roles for this transition (Wang et al. 2015; Aranguren-Riaño et al. 2018). Moreover, releases of N and P from the sediment could increase the N and P in the water column, which in turn impacts the aquatic ecosystem (Wang et al. 2008; Li et al. 2012). Accordingly, determination of the N and P species in the sediment are always useful to determine their release from the sediment and cycling between the water column and sediments and their impact on eutrophication (Gardner et al. 1995; Tang et al. 2018a, b).

Studies have demonstrated that sediment always acts not only as a sink of nutrients but also as a source (Aigars and Carman 2001). The accumulation of N and P in the sediment could increase the primary productivity of the lake (Elser et al. 2007; Li et al. 2016). And the phytoplankton could be limited with the releasing of endogenous nitrogen and phosphorus by a N to P ratio of water lower than 44.2 (Elser et al. 2009; Zhang et al. 2018). Usually, when the environmental conditions are changed, the deposition of endogenous N and P will respond and change correspondingly. Under the combined influence of microorganisms and other physical and chemical effects, such as mineralization and ion exchange, the sediment N will eventually be released and exist in a dissolved (Yu et al. 2016; Tang et al. 2018a, b). In sediment interstitial water, N will transfer from the sediment to the overlying water and re-involve in the nitrogen biogeochemical processes (Yang et al. 2017a, b).

Not all forms of N and P in the sediments can be released to the overlying water. For example, organic complexes of P cannot usually escape from the sediment, but others, such as iron, calcium, and aluminum salts, can escape from the sediments (Wang et al. 2009). Loosely sorbed P (Ex-p) can be released immediately. Different P fractions are measured using different sequential extraction schemes in sediments (Ruban et al. 1999; Li and Huang 2010). In previous studies, sediment nitrogen has always been simply divided into inorganic N and organic N. The transformation and release probability of organic N has been used to evaluate their contribution to eutrophication levels (Aigars and Carman 2001). However, now we usually use the terms transferable and non-transferable N fractions using an improved P fractional extraction method. Transferable N could be further separated into four fractions, including the ion-exchangeable form (IEF-N), the weak acid extractable form (WAEF-N), the strong alkali extractable form (SAEF-N), and the strong oxidation extractable form (SOEF-N) (Li et al. 2012). IEF-N is the nitrogen form most easily released, followed by WAEF-N, SAEF-N, and SOEF-N in terms of transferable N (Lv et al. 2005a; Wang et al. 2015). Ni et al. (2016) points out that current lake eutrophication control strategies are associated with major deficiencies due to lack of clear understanding of nutrient biogeochemical characteristics. These operational definitions help to understand the migration and transformation nitrogen. The relevant research helps to understand the nutrient biogeochemical characteristics.

Erhai Lake is mainly polluted by agricultural non-point source. The water quality entered into Erhai is grade V or worse than grade V, while the discharging one is grade II or III (Jiao et al. 2013; Xiang et al. 2016). How does the change of water quality affect the enrichment of nitrogen in the sediment of Erhai Lake? No studies have revealed it. On the other hand, Erhai Lake has been under unprecedented governance, especially in the last 5 years (Ji et al. 2017; Zhao et al. 2017). The environmental governance experience of Erhai is well worth studying. Keep abreast of distribution characteristics of the N and P species in the sediments is a basic requirement for the summary of environmental governance experience. But there is no research on it after the pollution supervision and control in Erhai Lake basin.

The objective of this study was to simultaneously investigate the latest distribution characteristics of the N and P species in the sediments of Erhai Lake with the same batch of samples along the flow direction in the central section. Previously, there was poor knowledge about N and P species and their potential release to the overlying water, and no knowledge about how the change of water quality affected the enrichment of nitrogen in the sediment. For those purposes, we used sequential extraction schemes to measure the N and P species in Erhai sediments. Nitrogen was defined in four forms: IEF-N, WAEF-N, SAEF-N, and SOEF-N. Phosphorus was defined as four forms: Ex-P, Al/Fe-P, RS-P, and Ca-P. Accordingly, our study developed a better understanding on the evolution of sediment N and P loading and ecological risk and offered guidance for controlling the release of N and P in Erhai Lake next.

Materials and methods

Study area

Erhai (N 25° 36′–25° 58′, E 100° 06′–100° 18′), the seventh largest freshwater lake in China and the second largest lake in Yunnan Province, is a typical tectonic rift lake that sits within a north-south-oriented inter-mountain basin at an altitude of 1974 m (Ni et al. 2016; Yang et al. 2017a, b). With the swift development of agriculture in the drainage basin, the water quality of Erhai Lake has been deteriorating daily and the presence of N and P from agricultural runoff has increased primary productivity. As a result, the lake has transitioned from being a mesotrophic lake to a eutrophic lake. The catchment area is 2565 km2; the lake area is 252.1 km2, and the reservoir capacity is 2.96 billion m3 (Li et al. 2016). The maximum depth of Erhai Lake is 21.3 m, and the average depth is 10.8 m. The basin has jurisdiction over 16 towns, 167 village committees, and 33 communities. At the end of 2014, the total population of the Erhai basin is 844,700.

According to the date from the Chinese Academy of Environmental Sciences, the vegetation occupied 50.36% of the catchment area, while the forest coverage rate was 37.0% (http://www.dali.gov.cn/dlzwz/5117496551576436736/20160802/309260.html). Agriculture and tourism of the basin are primarily concentrated in the western and southeastern lowland plain regions. The southern lakeside is a scenic tourist city, while the eastern is low mountainous area (Yang 2016). It is known as the “mother lake” for the people of Dali city. Erhai Lake supplies Dali city not only the primary water source for drinking but also for recreation, industrial, and agricultural production. In addition, it plays a key role in regulating the climate of Dali, promoting the agricultural development of the whole river basin, even the economic sustainable development.

Sample collection

We divided Erhai Lake into three areas, including northern, middle, and southern regions. With the help of the Erhai water depth contour map, we chose five sampling points after factoring in the hydrological characteristics and sediment pollution levels of Erhai Lake. The five sampling points along the flow direction of Erhai Lake in the central section were numbered 1 to 5, respectively. The northernmost one was an estuary of the Mizu River, which was the primary water source for Erhai Lake. In addition, the southernmost one was the only natural lake outlet. A 40-cm length of columnar sediment was sampled from each site using a sediment corer and equally divided into eight sections in June 2018. The sampling locations were shown in Fig. 1. In addition, samples of the overlying water were also collected in each session.

Fig. 1
figure 1

Sampling sites of the sediments in Erhai Lake

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All the sediment samples were stored in sterile and sealable plastic bags and placed in a refrigerator (4 °C) at the laboratory. Before the experiment, the samples were freeze dried, ground, and homogenized through a 100-mesh screen.

Sediment analysis

The TN in sediment was measured using the Kjeldahl method as previously described (Bremner and Mulvaney 1982). The total transferable nitrogen (TTN) of these samples was also investigated at the same time as described by Song and Ma (2002). This nitrogen fractionation method used the phosphorus fraction method described by Ruttenberg (1992) with some modifications. The TTN primarily consists of IEF-N, WAEF-N, SAEF-N, and SOEF-N. In addition, the extraction procedure of these four forms of nitrogen was sequential with 1 M MgCl, HAc-NaAc (pH = 5), 0.1 M NaOH, and K2S2O8 (alkaline) as described by Tang et al. (2018a). The extracts were centrifuged, and N concentration was determined with spectrophotometer (Wang et al. 2008).

The TP content in the sediment was analyzed using the standard measurement and testing protocol (Ruban et al. 1999). The various phosphorus species were sequentially extracted as described by Tang et al. (2018b) with 1 M NH4Cl, 0.1 M NaHCO3-NaS2O3, 1 M NaOH, and 0.5 M HCl, respectively. In this method, the inorganic phosphorus (IP) fractions would be divided into exchangeable or loosely sorbed phosphorus (Ex-P), aluminum- or iron-bound phosphorus (Al/Fe-P), reductant-soluble phosphorus (RS-P), and calcium-bound phosphorus (Ca-P). The analysis of phosphorus in each extracting solution was conducted using the molybdenum blue method (GB3838-2002) (MEP 2002). All the instruments used sediment sample analysis had passed national measurement examination. And the reference materials (GSD-9) and standard reagents were used for quality control. Set one parallel for every five samples.

Eco-risk assessment methods

The guide issued by the Environment and Energy Ministry of Ontario, Canada was used to assess the nitrogen and phosphorus pollution risk of the Erhai sediments (Leivuori and Niemistö 1995). The guide used a standard index for each contaminant i using the following equation:

$$ {S}_{\mathrm{i}}={C}_{\mathrm{i}}/{C}_{\mathrm{s}} $$
(1)
$$ \mathrm{FF}=\sqrt{\kern0.5em \frac{F^2+{F}_{\mathrm{max}}^2}{2}} $$
(2)

where Si was a single evaluation index or standard index, and if Si > 1 this indicated that the contaminant concentrations exceeds the standard value. Ci was the measured concentration of contaminant i (g/kg). Cs was the standard value of contaminant i (g/kg). FF was the comprehensive pollution index. F was the average of all the pollution indices (the average of STN and STP). Fmax was the maximum single pollution index (the maximum of STN and STP). The minimum levels of TN and TP that could cause the ecological risk were 550 and 600 mg/kg, respectively.

Data processing

The raw data was processed using Microsoft Excel 2010 (Microsoft, Redmond, Washington, USA). The analysis of variance (ANOVA) was conducted to determine the significance of difference using the SPSS software package (SPSS 19.0 for Windows, USA). The scattergram was prepared using Origin 8.0 (Origin Lab Corporation, Northampton, MA, USA).

Results and discussion

Recovery evaluation

Recovery experiments were performed to ensure the dependability of the nitrogen and phosphorus extraction method in certified reference material (GSD-9). The obtained recovery of TN was 98.13% with a relative standard deviation of less than 3.57%. The obtained recovery of TP was 95.29% with a relative standard deviation of less than 2.76% (Table S1). The results indicated that the recovery processes were satisfactory for the experimental requirements for the determination of the N and P.

Total nitrogen in the sediments

The TN in the sediments and overlying water parameters were presented in Fig. 2 and Table S2, respectively. It ranged from 1583.3 to 8018.5 mg/kg, and No. 5 and No. 3 samples are dramatically higher near the surface of the sediment. All results exceeded 1000 mg/kg which was the Chinese environmental protection and EPA standard (EPA 2002; Wang et al. 2010). The standard was a level where the sediments should be dredged to protect the environment. With increasing depth of sediment, the TN content basically decreased. These relative TN concentrations were similar to other freshwater lake sediment studies (Yang et al. 2017a, b). Within the range of 0–25 cm, the decrease in the TN content was rapid as the depth increased, and it stabilized below 25 cm. The southern area of Erhai showed the highest TN average concentration of the sediments, followed by the northern area and the central areas. In contrast, Zhao et al. (2013a) reported that the spatial distribution of TN in the Erhai surface sediments was similar to that in the northern area and was higher than the southern area followed by the central area. It might be the reason that the economic development of the southern urban areas had increased nitrogen emissions over the previous years. Along the flow direction of Erhai Lake, north to south, TN and TDN contents of the overwater at the sampling points showed an opposite distribution when compared with their sediments TN contents (shown in Table 1). It indicated that N might be still accumulating in Erhai sediment. Similar to the results, Wang et al. (2015) emphasized that Erhai was currently in a transition from Mesotrophic Lake to a Eutrophic Lake.

Fig. 2
figure 2

Concentrations of TN at various depths in Erhai sediments

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Table 1 The overwater quality of Erhai Lake (mg/L)
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Currently, Erhai is a mesotrophic lake. However, the TN concentration in Erhai sediments are close to or even higher than the eutrophic lakes in the middle and lower reaches of the Yangtze River area, such as Taihu and Chaohu Lakes (Wang et al. 2008, 2010). Similar results were reported in Dianchi Lake, another famous plateau lake in Yunnan province, whose TN concentration of surface sediments ranged from 1775.4 to 22,384.1 mg/kg (Li et al. 2012). The reason might be that they were located at high altitudes, and both were close to a large city where most of the domestic and industrial sewage had not been adequately treated and was directly discharged into the lake (Li et al. 2012). Similar to the results, Catalan and Donato Rondón (2016) emphasized that human settlement and farming could lead to potential eutrophication in mountain lakes at elevations that were less impacted by watershed development at temperate latitudes. As shown in Fig. 2, sampling point No. 5, which was adjacent to the downtown area of Dali City and might be also polluted by domestic and industrial sewage that directly discharged into the lake, contained the highest TN content. For the No. 3 sampling point, the sediment had a lower temperature and less flow disturbance caused because the water depth was 18.2 m. These were the reasons why we found high concentrations of TN at No. 3 point where less nitrogen was released (shown in Table S2) (Yu et al. 2016; Yang et al. 2017a, b).

Total phosphorus in the sediments

The TP concentrations at various depths of the sediments are presented in Fig. 3, which ranged from 814.9 to 1442.3 mg/kg. The TP concentration decreased as a whole with increasing depth. Similar vertical distribution features were reported in other various freshwater lake sediments, such as Taihu and Dianchi Lakes (Zhang et al. 2007; Yang et al. 2017a, b; An and Li 2009), but the P sediment concentrations in most Chinese lake sediments were seldom above 750 mg/kg. However, the TP sediment concentrations in this study were obviously higher than that. In addition, it was even higher than in the famous eutrophic lakes, such as Taihu Lake in China, where the TP concentration was under 1000 mg/kg in most regions (Zhang et al. 2007). However, in similar study on the Dianchi Lake, the P concentration reached 3200 mg/kg (Yang et al. 2017a, b). Yang et al. 2017a, b showed that high concentration of sediment P concentration was due to the application of large amounts of phosphorus fertilizers. The high content of TP at the No. 3 sampling point might be due to the same reason. The farmland around the No. 3 sampling point was primarily used for garlic cultivation and higher phosphorus fertilizers applications were used. Lago de Tota in the Andean was also a high mountain lake. Aranguren-Riaño et al. 2018 studied the carbon and nitrogen stable isotopic signatures of invertebrates in this lake and pointed out onion agriculture in the watershed contributed the bulk of nutrients supporting elevated productivity in Lago de Tota.

Fig. 3
figure 3

Concentrations of TP at various depths in Erhai sediments

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Nitrogen fractions in the sediments

To estimate potential nitrogen release, the vertical distributions of the IEF-N, WAEF-N, SAEF-N, and SOEF-N contents were measured, and the results were shown in Fig. 4a–d. The concentrations of IEF-N were between 16.1 and 166.0 mg/kg. The average concentration of IEF-N was 67.5 mg/kg. The relative content and variation trend of IEF-N at each sample point were consistent with their TN content (Fig. 4a). In addition, the estimated value of the IEF-N was significantly greater than that reported in the estuaries of Dianchi Lake (Yang et al. 2017a, b). Some TN values even reached the level of the Yangtze River area (Wang et al. 2008). As the IEF-N was very easily released into the overlying water (Lv et al. 2005a), Barik et al. (2016) stated that IEF-N was extremely likely to participate in the N cycle of the lake through the interaction between the sediments and interstitial water. Their retention and release participated in the seasonal variation of the N pool in sediments. Therefore, the study of IEF-N in Erhai sediments indicated the risk of endogenous nitrogen pollution.

Fig. 4
figure 4

Concentrations of a ion-exchangeable form (IEF-N), b weak acid extractable form (WAEF-N), c strong alkali extractable form (SAEF-N), and d strong oxidation extractable form (SOEF-N) for various sediment depths

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The WAEF-N form is relatively sensitive to the pH value as the name implies. When the environment became acidic, the WAEF-N would release N and participate in the N cycle of the sediments (Lv et al. 2005b; Zhang et al. 2016). As shown in Fig. 4b, the WAEF-N content ranged from 311.9 to 446.3 mg/kg. It gradually decreased with the increasing depth of the sediment. The trend was also consistent with the TN and IEF-N contents. The maximum value appeared at the southern exit of the lake (No. 5 sample point), and the minimum value appeared in the northern entrance of the lake (No. 1 sample point). The average concentration of WAEF-N in Erhai was 370.2 mg/kg. However, the risk of the WAEF-N fraction release from the sediment would not be very high considering that the pH of the Erhai Lake water was slightly alkaline as shown in Table 2. In addition, Wang et al. (2014) reported that WAEF-N could convert to SOEF-N through microbial mineralization.

Table 2 Comprehensive pollution risk assessment for the surface sediments in Erhai Lake
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SAEF-N was a nitrogen form, which was sensitive to the redox environment and bound to ferromanganese (Zhao et al. 2013a). As shown in Fig. 4c, the SAEF-N content ranged from 81.7 to 431.6 mg/kg. The average concentration of the SAEF-N in Erhai was 183.1 mg/kg. The maximum value appeared at the No. 5 sample point. SOEF-N primarily referred to the nitrogen form, which was combined with organic matter and sulfide (Zhao et al. 2013a). As shown in Fig. 4d, the SOEF-N content ranged from 1191.2 to 1548.2 mg/kg. The average concentration of SOEF-N in Erhai was 1337.6 mg/kg. Compared with IEF-N and WAEF-N, the SOEF-N was more difficult to release (Yang et al. 2017a, b). Lv et al. (2005a), suggesting that the total benthos biomass had a positive relationship with SOEF-N in the sediment and the organic-binding nitrogen was easily utilized by the benthos biomass. This research indicated that microorganisms were active under environments with high concentrations of SOEF-N.

Overall, the proportions of the various N fractions in the sediments of the study areas ranked as follows: SOEF-N > WAEF-N > SAEF-N > IEF-N. Wang et al. (2008) indicated that the rank in the middle and lower Yangtze river was as follows: SOEF-N > IEF-N > WAEF-N > SAEF-N. Therefore, the IEF-N was relatively small in the sediments of Erhai Lake. IEF-N is the primary nitrogen form exchanged at the sediment-water interface. This study indicated that the nitrogen release amount of the sediments of Erhai Lake was small, but the mass fraction and proportion of bioavailable nitrogen, such as WAEF-N was high. Thus, the nitrogen release risk of the Erhai Lake sediments was relatively high.

Phosphorus fractions in the sediments

To determine the potential release of phosphorus in the Erhai sediments, the vertical distributions of the Ex-P, Al/Fe-P, RS-P, and Ca-P concentrations for various depths were measured and shown in Fig. 5a–d. The concentrations of Ex-P were between 4.7 and 43.8 mg/kg. The average concentration of Ex-P was 11.1 mg kg−1. Ex-P is a phosphorus form that was very easily released to the overlying water in the sediment (Ruban et al. 1999). As shown in Fig. 5, the Ex-P concentration in the Erhai sediments accounted for only 0.4–4.6% of the TP content, which was the lowest among the four phosphorus forms. This average proportion was lower than that found in Chaohu Lake (4.3%) and similar to that in Dianchi Lake (0.1–3.5%) (Jiao et al. 2018; Yang et al. 2018). It was relatively high of depth of 0 to 10 cm and was stabilized for deeper depth. The obvious difference at the No. 5 sampling point might be caused by phosphate adsorption by the aquatic macrophytes (Yang et al. 2018).

Fig. 5
figure 5

Concentrations of a exchangeable or loosely-sorbedphosphorus (Ex-P), b aluminum- or iron-bound phosphorus (Al/Fe-P), c reductant-soluble phosphorus (RS-P), and d calcium-bound phosphorus (Ca-P) for various sediment depths

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Al/Fe-P is another form of phosphorus that can be more easily released and has a significant correlation with the total phosphorus content in pore water (Wang et al. 2012). It had the highest fraction among the four phosphorus forms. The concentrations were between 360.7 mg/kg and 537.5 mg/kg, and the average concentration was 451.9 mg/kg. As shown in Fig. 5, Al/Fe-P both increased gradually with depth and then remained stable. This trend was consistent with a previous study on Erhai Lake (Zhao et al. 2013b). Al/Fe-P was particularly abundant at the lake entrance (No. 1) and low in the center of the lake (No. 3). However, the TP concentration at the No. 3 sampling point was the highest. This shows that there were different sources of sediment phosphorus from the catchment basin around Erhai Lake (Jiao et al. 2018).

RS-P was primarily affected by the redox of the water environment. The content in Erhai Lake was between 52.2 and 365.5 mg kg−1 (Fig. 5c). Overall, RS-P tended to decrease with the increasing depth. The reason may be that decreases in the DO promoted the release of RS-P from the sediments. Erhai Lake was a permanent aerobic lake with little change in redox potential (Xiang and Zhou 2010). Thus, very little RS-P would be released little at this point. The Ca-P content ranged from 175.6 to 375.6 mg/kg. It tended to increase with the depth increase. Ni and Wang (2015) pointed out that Ca-P was unavailable to algae, and the content was largely dependent on the lake position and naturally occurring background calcium levels in the soils within the watershed. It also explained why the content of Ca-P did not change as much with depth as the other forms of phosphorus content at the majority of the sampling sites.

Pollution risk of nitrogen and phosphorus from the sediments

A comprehensive pollution assessment for the surface sediments in Erhai Lake was conducted. The detailed assessment results are shown in Table 2. The standard index of the TN (STN) in the surface sediments ranged from 2.88 to 14.58. The standard index of TP (STP) in the surface sediments ranged from 0.83 to 2.40. The comprehensive pollution index (FF) in the surface sediments ranged from 2.52 to 11.88. After making a comparison with the comprehensive pollution assessment standard for the Environment and Energy Ministry of Ontario, Canada methods (shown in Table 3), all the surface sediments were at level 4 of pollution which illustrated the heavy contamination by nitrogen and phosphorus. The Environment and Energy Ministry of Ontario Canada had also emphasized that the total nitrogen concentration of sediment with serious ecological risk effect was 4800 mg/kg (Leivuori and Niemistö 1995). However, the highest content of TN in the Erhai sediment at NO.3 and NO.5 sampling point both were exceed this standard value. This result ensured the serious ecological pollution risk of Erhai Lake again. Lake Baikal is the deepest lake in the world and has no releasing risk of nitrogen and phosphorus from its sediments. During the same period, TN of Baikal sediments was 1630 mg/kg at most which was far less than that in Erhai (Semenov et al. 2018). Similar to the results of this study, Zhao et al. (2013a) pointed out TN of Erhai sediments ranged from 2354 to 6174 mg/kg and jugged the great nitrogen releasing risk clear for the same Lake. But they had not given a comprehensive pollution assessment in combination with nitrogen and phosphorus. Five years later, the overall pollution risk in Erhai is still rising.

Table 3 Standard and level of comprehensive pollution assessment for the methods issued by the Environment and Energy Ministry of Ontario, Canada
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Conclusions

This study investigated the fractions, vertical distribution, and ecological risks of nitrogen and phosphorus in the Erhai sediments. After a comprehensive analysis and discussion of the results, some conclusions could be drawn as follows:

  1. (1)

    The contents of TN in the Erhai sediments were close to or even higher than those in eutrophic lakes in the middle and lower reaches of the Yangtze River area. The southern area showed the highest average TN concentration, followed by the northern and central areas. It indicated that the economic development of the southern urban areas had increased nitrogen emissions over the previous years. And N might still be accumulating in Erhai sediment.

  2. (2)

    The proportions of various N fractions in the sediments of the study areas ranked as follows: SOEF-N > WAEF-N > SAEF-N > IEF-N. IEF-N was the primary nitrogen form exchanged at the sediment-water interface. Although the content of IEF-N was relatively low in the sediments, the mass fraction and proportion of bioavailable nitrogen (WAEF-N) were high. Thus, the nitrogen release risk of the Erhai Lake sediments was relatively high.

  3. (3)

    The levels of TP in Erhai sediment were obviously higher than those in most Chinese lake sediments, even the famous eutrophic lakes such as Taihu Lake in China. The vertical distribution of each fraction of phosphorus (Ex-P, Al/Fe-P, RS-P, and Ca-P) indicated that there were different sources of sediment phosphorus for Erhai Lake.