4.1. Analysis of Nutrient Salt and HM Distribution Characteristics
This study indicated that a portion of the N and P present in the surface sediments in the Lianhua Lake, Mati Lake and North Lake is potentially derived from external inputs, while the other portion originates from aquatic organisms [
6]. Lianhua Lake had the highest comprehensive nutrient pollution index, followed by Mati Lake, which may be related to the locations of the lakes. Lianhua Lake is located in a newly developed urban area in Yangxin County. In recent years, with the migration of the population and the development of buildings, point source pollution from production and domestic emissions has increased. Although wastewater treatment plants have been constructed near urban areas, the incomplete coverage of wastewater treatment network has resulted in an inability to intercept all domestic wastewater [
22]. Additionally, in the coastal areas of the lakes in Yangxin County, there are large areas where combined sewer systems are used for wastewater discharge. There are 15 inlets directly discharged into Lianhua Lake and 11 inlets directly discharged into Mati Lake, all of which inevitably affect water quality.
Mati Lake is located in the old urban area of Yangxin County, where the surrounding non-point source pollution is severe. In addition to the direct discharge of domestic sewage into the lake along its shore, there are significant instances of garbage dumping and feedlot farming that contribute to non-point source pollution. Moreover, the vegetation along the shore is relatively uniform, leading to a fragile ecosystem where natural restoration from environmental pollution is challenging to achieve.
Although North Lake is located on the outskirts of the city, with no large residential areas nearby and only some farmland distributed around it, there used to be privately contracted fish farming activities before 2018, involving the use of high-density organic fertilizers. Research has shown that in aquaculture, fish and other organisms can only absorb 30% of the P in their feed, whereas the rest remains in the water or sediment in free or particulate forms [
37]. The high TN content in the eastern part of North Lake may be related to coastal fish farming using fencing. This was similar to the findings of Yang et al. [
38].
Excessive nutrient levels in the water of the lakes are closely associated with aquatic plants. The periodic decay and decomposition of aquatic plants increase the accumulation of nutrients in the sediment, leading to endogenous nutrient contribution to the water body. When aquatic plants die, their remains settle on the bottom of the lake. During the decomposition process, organic matter breaks down into dissolved nutrients such as N and P, which are then released into the water [
39].
The high TN and TP contents in the northern part of Lianhua Lake were largely due to the proximity of the lakeside boardwalk to the park. To enhance the scenic beauty of the lake for tourism purposes, park managers plant aquatic plants, such as lotuses and water lilies, thereby increasing the richness of the lake’s landscape. Similar observations have been made in Taihu Lake in China, where the withering of large aquatic plants was observed to impact the increase in nutrients in the water [
40]. Additionally, researchers have found that the root systems of aquatic plants regulate the secretion of oxygen and other substances that influence the transport and transformation of P [
41]. Therefore, aquatic plants should be harvested before they wither and decay to reduce the accumulation of plant residues in sediments.
The sources of HMs in lakes, in general, include background HMs from the watershed matrix and anthropogenic pollution emissions, with the latter being the primary source [
42]. Owing to the inherent characteristics of HMs, which are persistent and difficult to degrade, they tend to accumulate easily in the aquatic sediments, leading to water, sediment, and living organism contamination [
43]. The accumulation of HMs is related to local economic development, pollution emissions, and the status of underground resource reserves [
44].
The results for the HMs in the studied lakes were generally consistent, with the Hg pollution being the most severe, followed by As. This indicates that the high risk of HMs in the watershed was mainly related to Hg and As.
Yangxin County is an important area for mineral resource development in China, and over the past 20 years, there have been a large number of mineral developments and associated transportation activities. Hg originates mainly from the combustion of petroleum products and pesticides, accumulating in sediments through atmospheric deposition and the surface runoff of industrial and transportation waste gases and wastewater [
30]. The areas surrounding Lianhua Lake and Mati Lake, characterized by the busiest traffic routes in the urban area of Yangxin County, have witnessed long-term emissions of vehicle exhaust gases settling into lake water through atmospheric deposition [
31]. The distribution of roads was closely related to the distribution of Pb, Cu, and Cr in the sediments of Lianhua Lake, confirming that Pb originates mainly from vehicle exhaust emissions and metal smelting emissions, while Cu and Cr are indicative pollutants from fossil fuel combustion [
45].
In the northern part of North Lake, pollution mainly originates from agricultural cultivation and aquaculture. Fertilizers often contain HMs, such as Hg, Cd, As, and Pb [
46]. These HMs seep into the lake through surface and underground runoff, causing non-point source pollution. Some studies have indicated that the migration capabilities of different HMs are variable. HMs with higher migration capabilities can enter rivers and lakes with surface water flow, whereas those with lower migration capabilities can transfer only small amounts of elements to water or sediment [
47]. Pollution in North Lake mainly occurs at the center of the lake, displaying a decreasing trend from the center towards the periphery. Similar trends were observed in Lianhua and Mati lakes. This may be related to the low migration capability of As, as it tends to accumulate in the central areas of lakes and gradually diminishes towards the outskirts [
48,
49]. However, the migration of HMs is closely related to the environmental pH, microbial activity, and redox reactions [
13].
Aquaculture activities in North Lake, Lianhua Lake, and Mati Lake are essential economic sources for the residents [
22]. Feed used in aquaculture contains HMs such as Cd, Cu, and Zn. A large amount of untreated feed is directly released into the water and sediments, and through absorption and excretion by aquatic organisms, these HMs are deposited directly or indirectly into the water, leading to their accumulation [
47]. Due to the relatively few inflowing rivers in North Lake and the stable sedimentary environment within it, HMs tend to accumulate in the central part of the lakes.
4.2. Analysis of Pollution Sources
The correlations obtained via PCA (
Figure 7) for the three lakes confirmed that Cu, Pb, Cd, and Ni undergo similar geochemical processes and multiple elements contribute to the pollution of the same environment. Except for Cd, the concentrations of the other HMs did not exceed the background values of the soil HMs in Hubei Province [
15].
Yangxin County has a long history of Cu mining, with remnants of human excavations of Cu mines dating back to the Warring States period still preserved today. In 2020, a Cu smelting plant with a capacity of 400,000 t was established in Yangxin County, making Cu the primary element for mineral development in this area [
35]. Studies have shown that fishmeal fertilizers used in fish feed contain large amounts of Cd elements [
50]. Some researchers believe that Cd is associated with the use of fertilizers in agricultural production [
49]. Agricultural activities are important sources of Pb, Ni, and Zn [
40]. Agricultural land is still distributed in the northern part of North Lake; therefore, it is believed that the primary sources of the first principal component in these three lakes are anthropogenic activities, such as mining and metallurgical industries; aquaculture feed; and agricultural fertilizers.
In Lianhua and Mati lakes, Hg and TN displayed high positive loadings on the second principal component, signifying a significant correlation, suggesting a common pollution source. The spatial distribution of Hg in the study area exhibited a clear decrease: a north-to-south concentration decrease in North Lake, higher concentrations in the east compared to the west in Mati Lake, and higher concentrations in the west than the east in Lianhua Lake. This pattern aligns precisely with the distribution of inlets from harbors and channels around the county seat. Research indicates that Hg, commonly found in fossil fuels, is significantly influenced by non-point source pollution [
51], correlating with TN, another parameter affected by non-point source pollution. Considering the layout of surrounding industries, transportation routes, and older communities, it can be inferred that Hg and nutrients in the sediment originate from the atmospheric deposition of wastewater and traffic exhaust from the surrounding areas as they had relatively high positive loading, with no significant correlation with other elements (see PCA,
Figure 7). Research indicates that As is primarily found in agricultural run-off and industrial effluents. Cr exhibited a relatively high positive loading in North Lake, with no significant correlation with other HMs, differing from Lianhua Lake and Mati Lake. Research also suggests that in the Pearl River Delta region of China, Cr in the soil primarily originates from the weathering of parent rock materials [
52]; similar results have been obtained in other countries [
53]. Therefore, it can be inferred that Cr in North Lake mainly originated from the soil parent material.
4.3. Comparison of Pollution in Surface Sediments of Different Areas
The sediments of typical lakes in Yangxin County exhibit a relatively high nutrient salt content. The average TP and TN contents in the sediments of the three typical lakes in Yangxin County were higher than those in the middle and lower reaches of the Yangtze River (
Table 7), including Dongting Lake, Taihu Lake, Chizhou Lake, Hongze Lake, and Changtan Reservoir in Zhejiang (
Table 8). They also exceeded levels in northern lakes such as Qinghai Lake and Shandong Swan Lake and the Tianjin Haihe River but were lower than those in Mulan Lake in Inner Mongolia, Tianmu Lake in Jiangsu, Dianchi Lake in Yunnan, and Wanghu Lake in the same region.
Compared with other lakes in China, the three lakes in Yangxin County exhibited higher concentrations of Hg and As; higher average concentrations of Cd, Cr, Cu, Zn, and Ni; and lower concentrations of Pb. The As content in Lianhua Lake was eight times that of Taihu Lake, and the Hg content was three times that of Taihu Lake.
HM pollution in water bodies is a significant environmental issue [
66]. Due to their inherent biotoxicity, they can directly harm the cells and tissues of living organisms, impacting growth and reproduction, ultimately leading to a decrease in population and the decline of ecosystems [
67]. As humans occupy the top of the food chain, toxins accumulate layer by layer along the food chain. The occurrence of Minamata disease in Japan illustrates this phenomenon, where local residents consume seafood contaminated with Hg and Cd, marking one of the earliest instances of disease-causing environmental pollution. Therefore, HM pollution of aquatic environments cannot be overlooked.