Evaluation of the distribution of N, P and organic matter in sediment and the pollution status of Lakes in southeastern Hubei Province, China

Abstract Eutrophication caused by nutrients and organic matter (OM) pollution is a common problem in the world. The contents of total nitrogen (TN), total phosphorus (TP) and OM in sediments were determined in four typical lakes in southeastern Hubei, China, in the spring of 2019, and their pollution status was evaluated by organic and comprehensive pollution indexes to identify pollution sources and provide specific directions for the pollution control in key areas of the lakes. We found the following: (1) Both nutrients and OM in Cihu Lake and Bao’an Lake were relatively high. Cihu Lake had the highest nutrient and OM in both surface and core sediments. TN, TP and OM in the surface sediments of Cihu Lake reached 700.9 mg/kg, 1668.8 mg/kg and 62.8 kg/g, respectively. The contents of nutrients and OM in Daye Lake and Liangzi Lake were relatively low. OM in these two lakes was significantly lower than that in the Cihu Lake (p < 0.05). (2) The organic pollution and comprehensive pollution revealed that the pollution level of Cihu Lake and Bao’an Lake was moderate to severe. The level of pollution in Daye Lake and Liangzi Lake was mainly mild and moderate. (3) TN pollution was the main factor affecting sediment pollution in the four lakes compared to TP. The risks associated with N pollution merit increased attention. (4) Correlation analysis showed that there was not always a significant correlation between nutrients and OM in surface sediments (p > 0.05). It indicated that the sources of nutrients and OM in sediments were not identical but were affected by many factors. However, the relationship between nutrients and OM was closer in core sediments. In the future, targeted measures should be taken to reduce sediment pollution based on specific pollution sources and pollution conditions in the four lakes, especially Cihu Lake.


Introduction
Eutrophication is widespread in aquatic ecosystems including lakes, rivers, estuaries and reservoirs (Sorokovikova et al. 2022;Mosley et al. 2023;Soro et al. 2023;Zabaleta et al. 2023).Nitrogen (N) and phosphorus (P) are crucial to lake ecosystems as limiting nutrients, but their excessive abundance leads to eutrophication of lake (Álvarez et al. 2017).Eutrophic water are distributed to varying degrees from tropical to frigid zones (Abell et al. 2019;Lopes et al. 2019).Especially in the context of a warming climate, higher temperatures can also increase nutrient levels in lakes, a trend that has already been found in some Arctic lakes (Moiseenko et al. 2022).Nutrients and contaminants in lakes can be enriched in sediment through precipitation and adsorption (Ren et al. 2021;Sheng et al. 2022).Thus, large amounts of N and P are stored in the sediment.Unlike deep lakes, shallow lakes are more susceptible to wind, waves and biological activities.The material exchange between overlying water and sediments is stronger (Cheng et al. 2020;Wu et al. 2020;Gong and Zhang 2022).Consequently, sediments in shallow lakes is more prone to being resuspended, which leads to the release of nutrients in the sediment into the water, and the dynamic release of internal P during the resuspension process can increase total phosphorus (TP) in the water (Fan et al. 2020;Yuan et al. 2021).Therefore, although the levels of N and P in the sediment can be controlled after reducing the external pollution of the lake (Liao et al. 2021), internal pollution, due to the release of nutrients in sediment, becomes the main source of N and P in the water (Cheng et al. 2022).It leads to eutrophication of the lake and delays the process of lake self-repair (Bormans et al. 2016).Organic matter (OM) plays an important role in the migration and transformation of N and P in sediment (D ' Angelo and Reddy 1994).Carbon (C), N and P are also released from OM through degradation and mineralization (Ma et al. 2021).OM consumes dissolved oxygen in the water, which leads to deterioration of water quality (Guo et al. 2021).
The role of sediment in water quality and pollution in lakes has been examined in an increasing number of studies (Álvarez et al. 2017;Gerald et al. 2017;Salome and Samwel 2018).In some areas of Jianhu Lake, the sediment has become a potential source of N and P, and it has greatly increased the level of pollution (Feng et al. 2020).Similar to Jianhu Lake, total nitrogen (TN) and TP in the surface sediment of Wuliangsuhai have reached critical levels, and endogenous pollution was a major source of TN and TP in this lake (Du et al. 2022).In Lake Winnie, internal and external P loads were of similar magnitude, with about 34% of the TP in the biologically available form in the lake, so that the surface sediments will remain an important and active internal nutrient load in the future (Gerald et al. 2017).Compared to the external P load, the internal P recycle was an important nutrient source driving algal growth and eutrophication in lakes (Li et al. 2022), which has also been confirmed in four eutrophic lakes in China and Malaysia (Tay et al. 2022).OM can also be a cause of pollution in lakes.The total organic carbon (TOC) content in Lake Maninjau is in the range of high organic carbon (OC) level (>3%), and the high percentage of TOC may be evidence of frequent algal blooms in the overlying lake water (Dianto et al. 2020) .The increase of N and P content in the middle and lower reaches of the Yangtze River, one of the regions with the highest density of lakes in China, was relatively small, but the degree of OM pollution has increased significantly over the last 30 years (Zhu et al. 2019).For example, TP in the sediment of Zhazi Lake does not pollute the ecological environment, but OM and TN cause severe pollution (Yang et al. 2022).So pollution problems caused by nutrients and OM in lakes are widespread.
Southeastern Hubei Province is located in the middle reaches of the Yangtze River in China.There are many shallow lakes in this region, which are more vulnerable to eutrophication (Zhou et al. 2022).Some studies have examined the contributions of sediments to the pollution of lakes in southeastern Hubei, China (Hua et al. 2021;Rong et al. 2022), but these studies have focused on nutrient and heavy metal levels in the surface sediments of single lakes.However, few comparative studies of the surface and core sediments of multiple lakes have been conducted.In our study, we measured the content of nutrients and OM in the surface and core sediments of four typical lakes in southeastern Hubei, China, to identify the sources of sediment pollution and the areas with serious pollution.Our study can also provide specific guidance for the pollution control in key areas of lakes and facilitate the implementation of targeted measures to control relevant pollution sources in the future.

Study area
Southeastern Hubei is located in the middle reaches of the Yangtze River and has many shallow lakes.Considering their geographical location, properties, type and importance, we selected Cihu Lake, Daye Lake, Liangzi Lake and Bao'an Lake as typical lakes for research.Cihu Lake is located between the new and old urban areas of Huangshi City, which has been greatly affected by human activities.It is the largest urban lake in Huangshi, with an area of 10.5 km 2 , and the catchment area is 62.19 km 2 .The volume of the lake can reach 28 million km 3 .The largest inflowing river is Pengjiaqian River.The water of Cihu Lake is controlled by the Cihu sluice gate and discharged into the Yangtze River through the Shengyanggang sluice.Daye Lake is the largest natural shallow lake in Huangshi City.The western part of Daye Lake is located in the urban area of Daye City, which is greatly affected by human beings, and the intensity of human activities has been increasing in recent years.The eastern part is less affected by human beings, so it has the attributes of both a city and town lake.The lake area and watershed area are 64.6 km 2 and 1106 km 2 , respectively, and its volume is 1.2 × 10 8 m 3 .It has many tributaries, including the Sanliqi River, Luojiaqiao River and Niupi River.It also has many sub-lakes separated from the lake, and we collected most of our samples in the main lake area of Daye Lake.The water of Daye Lake is discharged into the Yangtze River through the Sigu sluice gate.Industry and mining are major causes of pollution in Daye Lake (Zhang et al. 2017).Our study area is southeastern Hubei, which includes Ezhou, Huangshi, Xianning, the south of Huanggang and the south of Wuhan.So considering the geographical location and the type of lake, we selected Liangzi Lake and Bao'an Lake.Liangzi Lake is the second largest freshwater lake in Hubei Province and its storage capacity is also the highest.It spans two municipal administrative regions, Wuhan and Ezhou City.Its area is 271 km 2 , and watershed area is 2085 km 2 .It is a typical macrophytic lake that is rich in aquatic vegetation and features a biodiverse ecosystem.In contrast to Liangzi Lake, Bao'an Lake is a transitional lake that has properties intermediate between a macrophytic lake and algal lake.The main inflowing rivers are Bao'an Donggang River, Bao'an Xigang River, Huandiqiao River and Hejinggang River.Historically, the shore of Bao'an Lake was reclaimed seriously and it has resulted in the continuous shrinkage of the lake area.Currently, the area of Bao'an Lake is only 45.1 km 2 .All four lakes have storage and irrigation functions.In addition, Cihu Lake and Daye Lake have the function of landscape entertainment, while Liangzi Lake, Bao'an Lake and Daye Lake have the function of breeding.

Sample collection and measurement
In April 2019, 10, 15, 6 and 12 sampling points were set in Cihu Lake, Daye Lake, Bao'an Lake and Liangzi Lake, respectively (Figure 1), depending on the shape, area and the outlet of the lakes, as well as the inflowing rivers.The information on the sampling points is shown in Table 1.We used a Peterson grab sampler to collect 0-10 cm surface sediment samples and used a gravity corer to collect core sediments from depths of 10-90 cm.The core sediments were sliced every 2 cm.The surface and core samples were then placed into polyethylene ziplock bags, sealed and stored at 4 °C in an incubator.Samples were air-dried until constant weight.The samples were then ground and passed through a 100-mesh sieve after animal and plant residues and other debris were removed.
TN in sediment was determined using alkaline potassium persulfate digestion and UV spectrophotometry.TP was determined using the potassium persulfate oxidation method, OM was determined using the potassium dichromate volumetric method (Wang 2014).

Organic index evaluation method
Organic indexes (OI) reflect the level of OM pollution in the sediment.Organic nitrogen (ON) is an indicator of the level of N pollution in the sediment (Sui 1996).The calculation formula is as follows: (1) Figure 1.study area.considering the geographical location, nature, type and importance of the lakes, cihu lake, Daye lake, Bao'an lake and liangzi lake were selected as typical lakes.
OrganicCarbon OC organicmatter OM The standards for evaluating the pollution status according to the OI and ON are shown in Table 2 (Wang et al. 2012).

Comprehensive pollution index method
The OI evaluation method only considers TN and OM pollution and does not consider TP pollution.Thus, we used the comprehensive pollution index method to evaluate pollution levels.The comprehensive pollution index was calculated as follows: where S i is the single pollution index, C i is the measured value of evaluation factor i and C s is the standard value of evaluation factor i. TN and TP pose the lowest ecological risks to sediment at 550 and 600 mg/kg, respectively, according to the guidelines issued by the Ministry of Environment and Energy of Ontario, Canada (Persaud et al. 1993); that is, the C s values of TN and TP are 550 and 600 mg/kg, respectively.F and F max represent the average and largest values of S i .FF refers to the comprehensive pollution index.The standards used for interpreting the comprehensive pollution index are shown in Table 3. 2.5 ± 0.5 6 Bn(B1), Bs(B4) tn: 0.13 tP: 3.58 liangzi lake (l) 30°14'53'' n,114°32'47'' e 2.9 ± 0.5 12 le(l1), lw(l9) tn: 1.52 tP: 0.08 note: two vertical core samples were taken in each lake.cn: north of cihu lake; cs: south of cihu lake; De: east of Daye lake; Dw: west of Daye lake; Bn: north of Bao'an lake; Bs: south of Bao'an lake; le: east of liangzi lake; lw: west of liangzi lake.the depth of water refers to mean depth.

Data processing
The experimental data were statistically analyzed using MS Excel.The map of study area was created using ArcGIS10.2.The inverse distance weighted (IDW) method was used in the interpolated map.Origin 2021 (Origin Labs Inc., Northampton, MA, USA) was used to conduct correlation analysis and evaluate the significance of differences among four lakes.We used Pearson and Spearman correlation analysis to analyze the relationships between nutrients and OM in the surface sediment and core sediment.One-way analysis of variance (ANOVA) was used to analyze differences in the content of nutrients and OM in the sediments of different lakes.Before the variance analysis, we used the Shapiro-Wilk test and Levene's test to determine whether the data met the assumptions of normality and homoscedasticity (p > 0.05).Data that did not meet assumptions of normality or homoscedasticity were transformed to ensure that these assumptions were met prior to analyses.We also performed multiple comparisons of the data using the Scheffe method.
The threshold for statistical significance was p < 0.05.

Distributions of nutrients and OM in surface sediment
The content of nutrients in the surface sediment of the four lakes varied (Figure 2).TP was the highest in Cihu Lake and ranged from 408.9 to 907.3 mg/kg.TP was higher in the northern part of Cihu Lake than in the southern part (Figure 3).TP of Cihu Lake was significantly higher than Bao'an Lake and Liangzi Lake (p < 0.05).The mean value of TP in Daye Lake was 638.3 mg/kg, which was the second highest among all lakes and significantly higher than that in Liangzi Lake (p < 0.05).The highest TP content was 876.3 mg/kg near the entrance of Daye Lake (D9).TP ranged from 386.5 to 569.0 mg/kg in Bao'an Lake, and it insignificantly differs from that of Daye Lake and Liangzi Lake (p > 0.05).The maximum and minimum values were distributed in the middle (B6) and southwest (B2) of Bao'an Lake, respectively.TP in Liangzi Lake was the lowest (420.6 mg/kg), and was significantly lower than that in Cihu Lake and Daye Lake (p < 0.05).Both the maximum and minimum values were located in the northern part of Liangzi Lake, which were L12 (609.6 mg/kg) and L5 (242.4 mg/kg), respectively.The mean value of TN was the highest in Cihu Lake (1668.8mg/kg), followed by Bao'an Lake (1258.0mg/kg), Daye Lake (980.8 mg/kg) and Liangzi Lake (882.9 mg/kg).The coefficient of variation (CV) of TN was the highest in Cihu Lake, with a value of 0.33.The maximum and minimum values were observed in C1 in the northeast (2684.2mg/kg) and C6 near the center of Cihu Lake (1087.5 mg/kg), respectively.The mean value of TN in Cihu Lake was significantly higher than in Daye Lake and Liangzi Lake (p < 0.05).Differences in TN among the other lakes were insignificant (p > 0.05).The mean value of TN in Daye Lake was 980.8 mg/kg, and the maximum was 11.85 times the minimum.The TN content in the eastern part of Daye Lake was higher than that in the western part.TN in Bao'an Lake and Liangzi Lake was relatively low compared with that in Cihu Lake and Daye Lake.TN in Bao'an Lake was 910.0-1813.6 mg/kg, and the highest TN value was observed in the southwestern part of Bao'an Lake (B6).The minimum TN value in Liangzi Lake was 434.6 mg/kg (L7) in the west, and the maximum was 1257.9 mg/ kg (L11) in the east.
The distribution of OM in the surface sediments of the four lakes differed from that of nutrients.The mean value of OM in Cihu Lake (62.8 g/kg) was the highest among all lakes.The maximum value was observed in C4 (45.2 g/kg), and the minimum value was 77.0 g/kg near the center of the lake.OM was significantly higher in Cihu Lake than in Liangzi Lake and Daye Lake (p < 0.05).In Bao'an Lake, OM ranged from 45.7 to 69.4 g/ kg.Areas with higher OM were located in the western and central parts of Bao'an Lake.The mean value of OM in Liangzihu Lake was 41.0 g/kg.Overall, OM was higher in the western part of Liangzi Lake than in the eastern part.OM in Daye Lake was the lowest, which ranged from 9 to 48.2 mg/kg.Similar to the spatial distribution of TN, OM was higher in the eastern part of Daye Lake than in the western part.

Distribution of nutrients and OM in core sediment
TP in core sediment was the highest in Cihu Lake (531.8 mg/kg) (Figure 4).TP was slightly higher in Daye Lake than in Liangzi Lake, and no significant difference was observed between Daye Lake and Liangzi Lake (p > 0.05).TP in Daye Lake first decreased and then increased with depth, and TP in Liangzi Lake changed little with depth.TP was significantly lower in Bao'an Lake than in other lakes (p < 0.05), and the content of TP was only 399.4 mg/kg.In contrast to TP, TN of Bao'an Lake was higher than that of Liangzi Lake (p > 0.05) and Daye Lake (p < 0.05), while TN in Cihu Lake was still the highest among all lakes.
The vertical distribution of OM in core sediment was consistent with that of TN in core sediment.The OM content in core sediment was the highest in Cihu Lake, followed by Bao'an Lake, Liangzi Lake and Daye Lake.OM in core sediment was significantly higher in Cihu Lake and Bao'an Lake than in Daye Lake and Liangzi Lake (p < 0.05).No significant difference in OM was observed between Cihu Lake and Bao'an Lake (p > 0.05).Patterns of variation in the content of OM with depth were similar in CN, DE, BS, LE and LW.The content of OM decreased sharply with depth and remained low content.The OM content of CS decreased with depth, while the OM content of DW increased with depth.

Organic pollution evaluation in surface sediment
The pollution level according to the OI was the highest for Cihu Lake, followed by Bao'an Lake, Liangzi Lake and Daye Lake (Table 4).In Cihu Lake, the OI value ranged from 0.27 to 1.03, and ON ranged from 0.10 to 0.26.The pollution level of 50% of the area was classified as level III or IV (Figure 5).OI and ON in Cihu Lake were significantly higher than in Daye Lake and Liangzi Lake (p < 0.05).The OI value for Bao'an Lake was second only to that for Cihu Lake, but there was no significant difference in OI between Cihu Lake and Bao'an Lake (p > 0.05).The OI value ranged from 0.26 to 0.63 (levels III and IV).No significant difference in OI was observed among Liangzi Lake, Daye Lake and Bao'an Lake (p > 0.05).The OI and ON values of Liangzi Lake ranged from 0.03 to 0.46 and from 0.04 to 0.12, respectively, and 58.3% of the polluted area was classified as level III.The level of pollution was the lowest in Daye Lake, and the OI value ranged from 0.01 to 0.34 (level I to III).

Comprehensive pollution evaluation in surface sediment
The comprehensive pollution evaluation (FF) was the highest in Cihu Lake (2.61), followed by Bao'an Lake (1.95), Daye Lake (1.65) and Liangzi Lake (1.40) (Figure 6).The  most seriously polluted lakes were Cihu Lake and Bao'an Lake (mainly levels III and IV).
A total of 80% of the area of Cihu Lake was classified as level IV.FF in Cihu Lake was significantly higher than in Daye Lake and Liangzi Lake (p < 0.05).Approximately 33.3% of the area of Bao'an Lake was classified as level IV, and no significant difference was observed in FF between Cihu Lake and Bao'an Lake (p > 0.05).Daye Lake and Liangzi Lake were relatively clean, and the highest level of pollution in these lakes was classified as level I.The pollution level of 60% of the area of Daye Lake was classified as level III.
Liangzi Lake was the least polluted, and levels I, II, and III accounted for 16.7%, 41.7% and 41.7% of the area of Liangzi Lake, respectively.The level of TN pollution in all four lakes was classified as levels III and IV, and the pollution level of 66.67% of the total area was classified as levels III and IV (Figure 7).Level IV pollution according to TP was only observed for Cihu Lake, and only 10% of the area of this lake was classified as level IV.
Pollution caused by TN was more common and severe than that caused by TP.

Organic pollution evaluation in core sediment
The OI value of the core sediment of the four lakes followed the order Cihu Lake > Bao'an Lake > Daye Lake > Liangzi Lake, while ON was Cihu Lake > Bao'an Lake > Liangzi Lake > Daye Lake (Figure 8).The organic pollution in Cihu Lake was the most serious, and the pollution index for CN was higher than that for CS.The CV of OI was as high Figure 5. results of the organic pollution evaluation.the evaluation results of organic index and organic nitrogen index were relatively consistent, which was cihu lake > Bao'an lake > Daye lake > liangzi lake.the evaluation results reveal the pollution status of lakes.
Figure 6.spatial distribution of the comprehensive pollution index.the spatial distribution of the comprehensive pollution index was not consistent with the results of the organic index and the organic nitrogen index.however, the overall result was still cihu lake > Bao'an lake > Daye lake > liangzi lake.
as 106.47%, which indicates that there was high variation in OI, and that the distribution of OI was dispersed.Both OI and ON in Bao'an Lake were second only to those in Cihu Lake.The pollution level in 70.59% and 69.23% of the area of Bao'an Lake and Cihu Lake, respectively, was classified as level III (Figure 9).The level of organic pollution was the lowest in Liangzi Lake, which was mainly classified as levels II and III.ON pollution was higher in Liangzi Lake than in Daye Lake.

Comprehensive pollution in core sediment
The value of FF in core sediment followed the order Cihu Lake > Bao'an Lake > Liangzi Lake > Daye Lake (Figure 10).The FF of core sediment was the highest in Cihu Lake (1.83).The pollution level of 36.67% of the area of CS was classified as level IV (Figure 11).The FF of the core sediment in Daye Lake was the lowest (1.18), and approximately 88.09% of the area of Daye Lake was classified as level I to II.The pollution level of Bao'an Lake was mainly classified as level II to III, and the pollution level of Bao'an Lake was the second highest after Cihu Lake.The FF in Bao'an Lake decreased with depth.The FF value of Liangzi Lake ranged from level I to level IV.Variation in FF was observed in LE at different depths.Severe pollution was observed for 34.2% of the total area, and FF generally decreased with depth in LW.

Spatial distribution of nutrients, organics and pollution in the surface sediment
Both the content of nutrients and OM in surface sediment were the highest in Cihu Lake, and severe pollution was observed in as much as 80% of the sampling area.Factors that affect the content of nutrients and OM in sediment include aquatic vegetation, sediment particle size, bioturbation and hydrodynamics (Wang et al. 2016;Chen et al. 2021;Gong and Zhang 2022) .The effects of human activities are likely greater at Cihu Lake than at the other lakes because it is the largest urban lake in Huangshi.Southwestern and northwestern areas of Cihu Lake are the most polluted according to OI values.This likely stems from the fact that these two areas are a part of the new urban area of Huangshi, and residential areas are widespread.In addition, the largest inflowing river, Pengjiaqiangang River, is also located in the southwest.Although domestic sewage flows into the Pengjiaqiangang River after passing through the sewage treatment plant, some pollutants still enter Cihu Lake.Pollutants are continuously enriched in the sediment over time, and this has caused severe pollution in the southwestern and northwestern parts of Cihu Lake.
The content of nutrients in Cihu Lake was higher than in Chaohu Lake (Wang et al. 2021b) and Dongting Lake (Yin et al. 2021).Sediments rich in high content of nutrients and OM can be properly recycled for crop cultivation to make effective use of the sediments as long as the heavy metal content in the sediments is not high (Kiani et al. 2023).
The content of nutrients in surface sediment was the lowest in Liangzi Lake, and this was related to the fact that Liangzi Lake is a shallow, macrophytic lake.The aquatic vegetation in Liangzi Lake is well developed, and submerged plants present in the water include Potamogeton crispus L. (P.crispus), Vallisneria natans (Lour.)Hara, and Potamogeton wrightii Morong (P.wrightii).Although the decomposition of an appropriate amount of plants can reduce the content of inorganic nitrogen in the water, however, submerged plants can store different amounts of N physiologically (Catherine et al. 2012).And OM in the sediment often contains residues such as aquatic plants or algae (Bai et al. 2017).The decay of these residues and litter will produce a large amount of ON, which is stored in the sediment (Thorén et al. 2004;Cao et al. 2015), and this leads to increases in OM and TN in the surface sediment (Deng et al. 2022).The East Lake of Taihu Lake, which was dominated by aquatic plants, was also found to have the highest OM load.However, macrophyte species can have strong effects on changes in C, N and P stocks in shallow lakes.Wang et al. (2016) found that because P. wrightii has a higher biomass than P. crispus, it can redistribute more elements from the water column into the sediment after dying, and plant debris can remain in the sediment for up to 6 years; by contrast, the debris of P. crispus can only remain in the sediment for 1 year.In addition, aquatic plants can also affect the transport and transformation of P through the rhizosphere secretion of oxygen and other substances (e.g.alkaline phosphatase) (James et al. 2004).So moderate harvesting of aquatic plants before wilting is recommended to reduce the deposition of plant in the surface sediments.The content of nutrients and OM were relatively high in L4, L9 and L12 because these sampling points were located close to Longwan Peninsula Resort and Liangzi Island Tourist Scenic Area.There were many residents on the peninsula, and commercial activities were widespread.The study by Knoll et al. (2015) has shown that the percentage of area covered by crops in a watershed has a direct impact on the eutrophication of the water body.The shore of Liangzi Lake was dominated by farmland and forests.Chemical fertilizers and pesticides were used in large quantities or in excess in agricultural production activities.Only 10-20% of the nutrients applied are used in the first crop cycle, and excess and unused nutrients are lost and discharged into water bodies (Sattari et al. 2012), so substances containing large amounts of N and P in pesticides and fertilizers are washed into the lake.TN and TP in the surface sediment of Liangzi Lake ranged from 598 to 1372 mg/kg and from 323 to 804 mg/ kg in 2014 (Gao et al. 2016), respectively.It indicated that TN and TP loads in the surface sediment of Liangzi Lake decreased from 2014 to 2019.This likely stemed from the fact that before 2014, the breeding area of hairy crabs in Liangzi Lake was as high as 360,000 acres.Since 2018, Liangzi Lake has adopted a non-fishing policy, and crab breeding areas are now restricted to fenced areas, which has reduced the total breeding area to approximately 10,000 acres.However, mild pollution and moderate pollution still accounted for 41.7% of the total area of Liangzi Lake because the pollution of surface sediments caused by long-term breeding activities cannot be eliminated in a short time.A similar situation exists in Maninjau Lake, West Sumatra, where floating fish cage activity enriched the total organic matter (TOM) content in the surface sediment (Dianto et al. 2020).
The pollution level of Bao'an Lake was second only to Cihu Lake and was mainly classified as levels III and IV.The main source of income of the villages and towns in the Bao'an Lake basin is agriculture, and there are a large number of intensive fishponds and farmland around the lake.The pollution level of the middle and southwestern parts of Bao'an Lake was classified as severe because the inflowing rivers, such as the Bao'an Donggang River, Bao'an Xigang River and Huandiqiao River, flow through densely populated areas, and production activities and domestic sewage were major causes of pollution in these rivers.In 2019, compared with Environmental Quality Standards for Surface Water (GB 3838-2002), TN was 35% lower than the standard of I water (TN ≤ 0.2 mg/L), while TP exceeded 8.95 times the standard value of inferior V water in these rivers.These rivers finally flowed into the southern part of Bao'an Lake and increase P and N loading in the water.In Winnipeg Lake, high levels of P input from the Assiniboine and Red Rivers in the south also resulted in higher TP and bioavailable phosphorus concentrations in surface sediments in the southern basin than in the northern basin (Gerald et al. 2017).A similar pattern was found at Baikal Lake (Sorokovikova et al. 2022).So the high concentration of nutrients carried by the inflowing rivers is one of the important reasons for the accumulation of nutrients in the lake.In our sediment samples, TN and TP were 75.8% and 36.2%lower, respectively, than in samples collected in 1996 (Zhang et al. 2000).Since 2009, the Daye Municipal Government began to implement wetland protection policies for Bao'an Lake, demolished the high-density breeding fences in Bao'an Lake and limited water-polluting activities to specific areas within the wetland park.In addition, nearby scattered mines were consolidated.Domestic sewage treatment plants and rainwater diversion pipes have been built in Huandiqiao Town, Bao'an Town and Dongfeng Farm in the Bao'an Lake basin in recent years, and it has reduced the content of nutrients in surface sediment.TP was lower in Bao'an Lake than in Chaohu Lake (Wang et al. 2021c), Dongting Lake (Yin et al. 2021) and Taihu Lake (Deng et al. 2020).However, TN was 15.77%, 22.20% and 24.50% higher in Bao'an Lake than in Chaohu Lake, Dongting Lake and Taihu Lake, respectively.in the surface sediments of Daye Lake was also high and only slightly lower than that of Cihu Lake.It was related to the high intensity of human activities in the Daye Lake basin.The Daye Lake basin is rich in mineral resources and is the main mining area in Daye City, and many mining and non-ferrous metal smelting enterprises are present in this region.The mining of minerals will lead to unidirectional phosphorus flow from land to water, where lakes are the main affected inland water bodies (Elser and Bennett 2011).The Daye Lake basin is a vast and densely populated area, and this results in the production of a large amount of sewage.Hydrodynamics also affects the spatial distribution of nutrients in surface sediment (Liu et al. 2013).The western part of Taihu Lake is the upstream basin, the eastern part is the downstream basin and part of the external N is carried by inflowing rivers on the west bank.Then, an inflow-outflow induced current delivers part of N to the eastern part of Taihu Lake (Chen et al. 2021).In the west of Daye Lake, human activities were intensive; a large amount of nutrient and OM entered Daye Lake, and the water of Daye Lake flowed from west to east.Under hydrodynamic forces, nutrients and OM might also be carried to the east.Additionally, the eastern part of the lake, which is close to the outlet, has weak hydrodynamic forces.Weak hydrodynamic conditions at the outlet are conducive to the deposition of particles, pollutants and other substances.In addition, particles have adsorption effects on nutrients.Thus, the level of sediment pollution was higher in eastern Daye Lake than in western Daye Lake.However, the spatial distribution of TP in the west was higher than that in the east.It was consistent with previous observations of Taihu Lake, which might be related to the different deposition and transportation modes of N and P (Wu et al. 2019).

Spatial distribution of nutrients and OM in core sediment and pollution analysis
The content of nutrients and OM in sediment often decreases with depth.This is related to the form of N and P. Taking P as an example, insoluble phosphates mainly accumulate on the surface of the sediment.Ca-P is a slightly soluble form of P that is rarely involved in the P migration and transformation process, and it is often buried at the bottom (Kaiserli et al. 2002).In addition, as the depth of deposition increases, the anaerobic degree is strengthened, which facilitates the release of unstable forms of P from the sediment, such as Fe/Al-P, and results in the release of P from the sediment into the overlying water (Zhou et al. 2014).The biogeochemical effects of N and P also require consideration, because the N released through organic mineralization gradually weakens with depth (Chen et al. 2008).But changes in lake sediment are also closely related to human activities (Wang et al. 2021a).The content of nutrients and OM was the highest in the core sediment of Cihu Lake among the four lakes in our study, and the level of pollution of a large proportion of the samples was classified as moderate and severe.Cihu Lake is located in the center of Huangshi, and there are a large number of residents in the densely populated basin.The construction of rain and sewage diversion pipes was not very ideal, and the treatment capacity of domestic sewage treatment plants was insufficient before the beginning of the twenty-first century; this resulted in the flow of a large amount of domestic sewage into Cihu Lake.In the 1970s, industrial activity on the south bank of Cihu Lake and fish aquaculture activities increased.A large amount of unused residual feed in farming activities and nutrients in fish excrement remain in the water or were deposited in the sediment (Hyun et al. 2013;Zhang et al. 2018).And the contribution of fish manure in aquaculture activities was significantly higher than other sources of OM (Go et al. 2023).It can lead to an increase in the content of nutrients and OM in the sediment (Zhang et al. 2015), which results in the enrichment of OC and hypoxia (Chen al. 2015).It was worth noting that hypoxia was very likely to reduce the mineralization rate of OM and increase the burial of OM in sediments (Lukawska-Matuszewska and Kielczewska 2016).Zhao et al. (2022) found that high-density pen culture and overfeeding were the main causes of N accumulation in the sediment in the traditional culture area east of Taihu Lake.Similar observations have been made in Luoma Lake (Liu et al. 2021) and Poyang Lake (Wang et al. 2021c).However, if aquatic plants are planted at the same time that farming activities are conducted, the content of nutrients in the sediment can be effectively reduced (Chen et al. 2008;Liu et al. 2019).
The core sediment pollution level of Bao'an Lake was second only to that of Cihu Lake, which was related to human activities that have historically occurred around Bao'an Lake.There have always been a large number of fish ponds around the northern part of Bao'an Lake.The aquaculture area was large, and the discharge of aquaculture wastewater and regular water exchange cause pollution in Bao'an Lake.Enclosure aquaculture facilities were also present in Bao'an Lake.Bao'an Town is located on the southern part of Bao'an Lake, which has historically been an active iron mining area, and there are still many cement industries, coal mining industries and iron mining industries around Bao'an Lake.Additionally, the inflowing rivers, located in the eastern and southern parts of Bao'an Lake, can carry a large number of pollutants into Bao'an Lake.There were differences in the distribution of nutrients and OM content in the core sediment of Liangzi Lake.The nutrient content was higher in LE than in LW.However, the CV of TP in LE among samples collected at various depths was only 0.1, indicating that TP was relatively stable.OM was higher in LW than in LE because LW was historically an aquaculture area, especially hairy crab aquaculture, and this has caused the content of OM to be higher in LW than in LE at all depths.Although the aquaculture area has gradually decreased in recent years, the rise in tourism activities has led to a sharp increase in the OM content.
The content of nutrients in core sediment and organic pollution were more serious in DW than DE.In DW, the content of nutrients and OM did not change with depth.However, the content of nutrients and OM changed greatly with depth in DE, especially between 0.1 and 0.3 m, because most of the main inflowing rivers were distributed in the west of Daye Lake, and the intensity of industrial and agricultural activities was high in upstream areas.The content of OM in Liangzi Lake decreased with depth, while the content in DW increased with depth.According to the condition of oxygen, OM degradation can occur through different pathways and intensities (Kristensen and Holmer 2001).So the phenomenon of DW may be related to the high oxygen content and thus aerobic microbial activities in shallow sediment, which mediates the decomposition of OM in the sediment near the surface (Wen et al. 2019).

Correlation analysis of nutrients and OM in surface sediment
Table 5 shows the correlations between nutrients and OM in surface sediment.The content of nutrients and OM in the surface sediments of Liangzi Lake and Bao'an Lake was insignificantly correlated (p > 0.05), suggesting that the sources of nutrients and OM in Liangzi Lake and Bao'an Lake were affected by many factors.Additionally, this might be explained by the small number of sampling points in Bao'an Lake, which resulted in an insignificant correlation (p > 0.05).TN and OM of Daye Lake were significantly positively correlated (r = 0.652, p < 0.05).The similarity in the spatial distribution of TN and OM indicates that TN and OM in Daye Lake sediment might be derived from the same source.N mainly exists in the form of OM, and the mineralization of OM can result in release of ON compounds (D ' Angelo and Reddy 1994).However, no significant correlation between TP and OM was observed in Daye Lake (p > 0.05).Compared with the mineralization of OM, TP may not be caused by the enrichment of OM in sediments but mainly consists of inorganic phosphorus.Human activities might also play a role in the increases of TP, as has been observed in Huangda Lake (Ma et al. 2021) and Wuhu Lake (He et al. 2022).By contrast, the correlation coefficient of TP and OM in Cihu Lake reached 0.805 (p < 0.01), suggesting that P in Cihu Lake mainly exists in the form of organic phosphorus (OP).And OM is an important carrier of P, and the content of OP in sediment accounts for approximately 20-80% of TP.

Correlation analysis of nutrients and OM in core sediment
TN and TP were positively correlated in CN, BS and DE (Table 6).The correlation coefficient between TP and TN in BS was 0.833 (p < 0.01), demonstrating that TP and TN in BS were derived from the same source, which might be the domestic pollution carried by inflowing rivers.However, a significant negative correlation was observed between TN and TP in CS (r = −0.708,p < 0.01), and no significant correlations were observed between TP and TN in BN and DW, suggesting that the source of TN and TP may not be the same and that human activities might contribute to these patterns.
OM decomposes during the process of mineralization, which results in the release of N and P. Significant positive correlations were found between TP and OM (p < 0.01) in CN, CS, DE, BS and LW.The strongest correlation was observed for BS, and its correlation coefficient was 0.824.It indicated that P in BS was mainly composed of OP, which might be derived from organic mineralization.The correlation coefficient between TN and OM in BN was 0.915 (p < 0.01), which might be related to the long-term aquaculture industry in northern part of Bao'an Lake.The N content of the feed used in farming activities is high, and unused feed was discharged into Bao'an Lake along with farming wastewater.When aquatic organisms such as fish die, a large amount of OM can be decomposed, resulting in the deposition of nutrients and OM in lakes.

Conclusions
• Both nutrients and OM in Cihu Lake and Bao'an Lake were higher in the four lakes.Especially, Cihu Lake had the highest nutrients and OM in both surface and core sediments.TN, TP and OM in the surface sediments of Cihu Lake reached 700.9 mg/kg, 1668.8 mg/kg and 62.8 kg/g, respectively.The contents of nutrients and OM in Daye Lake and Liangzi Lake were relatively low. 1.The organic pollution and comprehensive pollution revealed that the pollution levels in Cihu Lake and Bao'an Lake were moderate to severe (levels III-IV).A total of 80% of the area of Cihu Lake was severely polluted, and the levels of pollution in Daye Lake and Liangzi Lake were mainly mild and moderate (levels II-III).The highest level of pollution in them was level I. 2. TN pollution was the main factor affecting sediment pollution in the four lakes compared with TP.The risks associated with N pollution merit increased attention.3. Nutrients and OM in the sediments of the four lakes were not always significantly correlated (p > 0.05).It indicated that the sources of nutrients and OM in sediments were not identical but were affected by many factors.In the surface sediments, only TP in the Cihu Lake and TN in the Daye Lake were significantly positively correlated to OM (p < 0.05).However, the coupling relationship between nutrients and OM was better in core sediments.

Figure 2 .
Figure 2. nutrients and oM content in the surface sediment.Different lowercase letters are significantly different in tP, tn and oM (p < 0.05).the black data points represent the nutrient and organic matter content of each sampling point in each lake.

Figure 3 .
Figure 3. spatial distribution of nutrients and oM in the surface sediment.the interpolation map obtained by IDw clearly shows the changes of nutrients and oM content in the lake.

Figure 4 .
Figure 4. Vertical distribution of nutrients and oM in core sediment.the line chart reflects the variation of nutrient and oM content with depth at each depth, and reveals the pollution of nutrients and oM in historical period.

Figure 7 .
Figure 7. evaluation of variation in S tP (a) and S tn (b) among lakes.the change of the proportion of pollution level can intuitively reflect the overall pollution status of each lake.compared with tP pollution, tn pollution was more worthy of attention.

Figure 8 .
Figure 8. oI and on in core sediment in four lakes.the evaluation results of oI and on directly reflect the pollution of nutrients and oM in core sediments with the change of depth.

Figure 10 .
Figure 10.comprehensive pollution evaluation of core sediment.the change of tn and tP contents caused the change of ff at different depths.

Figure 11 .
Figure11.the percentages of different pollution levels of core sediment.the comprehensive pollution evaluation showed that the proportion of level III to IV pollution in cihu lake and Baoan lake was relatively large, and the pollution of tn and tP in the two lakes was relatively high.

Figure 9 .
Figure 9. the percentages of oI (a) and on (b) levels in the core sediment of four lakes.the evaluation results of oI and on were relatively consistent, but the oM pollution was more serious.

Table 1 .
Basic information on the lakes in southeastern hubei, china.

Table 2 .
classified standard used to evaluate the magnitude of pollution according to the oI and on.

Table 3 .
classified standard used to evaluate the comprehensive pollution index in the sediment.

Table 4 .
the value of oI and on in sediment in four typical lakes.

Table 5 .
correlation analysis of nutrients and oM in surface sediment.refers to significant correlation at the 0.05 level (double tail).**refers to significant correlation at the 0.01 level (double tail). *

Table 6 .
correlations between nutrients and oM in core sediment in different samples.
*refers to significant correlation at the 0.05 level (double tail).**refers to significant correlation at the 0.01 level (double tail).