Continuous Field Determination and Ecological Risk Assessment of Pb in the Yellow Sea of China

Abstract

Field determination and ecological risk assessment of dissolved lead (Pb) were performed at two Yellow Sea sites in China using a continuous automated electrochemical system (CAEDS). This CAEDS instrument includes an automatic triple filter sampler and an electrochemical detection water quality analyzer, which might be operated automatically four times daily. The dissolved Pb concentrations varied from 0.29 to 1.57 μg/L in the South Yellow Sea over 16 days and from 0.32 to 2.28 μg/L in the North Yellow Sea over 13 days. During the typhoon and algal bloom periods, the Pb concentration was as high as ten times greater than usual. According to the calculation of contamination factors (C_f) and subsequent analysis, seawater quality was classified as Grade II. Through species sensitivity distribution (SSD) method experiments and ecological risk analysis, an average risk quotient (RQ) below 1 for both areas was obtained, indicating a low-to-moderate ecological risk. This system will be helpful for Pb monitoring and assessment in seawater and contribute to the biogeochemical cycling study of Pb.

1. Introduction

As the Earth’s most expansive ecosystem, oceans play a vital role in supporting a myriad of life forms and significantly impact the global climate. Nonetheless, the marine environment remains vulnerable to contamination. Trace metals are particularly hazardous due to their toxicity, bioaccumulation tendency, and environmental persistence, representing some of the most severe pollutants threatening marine ecosystems. Prolonged exposure to high concentrations of trace metals can disrupt the structure and function of marine ecosystems, impacting biodiversity [1]. These changes may include shifts in the composition of biological communities and interactions among organisms, thereby impacting the health and stability of ecosystems. As a result, the environmental quality of the ocean has become a pivotal concern. Field water quality monitoring is often required to protect and enhance seawater quality, followed by developing appropriate measures [2]. Therefore, identifying a suitable method for seawater quality testing is essential.

Toxic heavy metals, such as lead (Pb), which are prevalent in the environment, pose significant health risks [3]. Despite low oceanic concentrations, trace amounts of Pb pose significant risks to marine life and human health due to toxicity and bioaccumulation. Industrial discharge, ship paint, and mining activities might release Pb into the marine environment, where it can bioaccumulate through the food web, posing a threat to marine ecosystems [4]. High Pb intake in humans may cause memory issues, kidney disease, impaired bone growth, and gastrointestinal and neurological disorders [5,6], posing health risks. Long-term in situ monitoring [4] of Pb concentrations is crucial for timely, accurate detection near coastal areas, enhancing our understanding of Pb biogeochemical cycling and pollution management. Despite the importance of detecting Pb for marine health, long-term water quality data for trace metal contamination still need to be revised and indicated in a timely manner. This lack of precise and timely monitoring underscores the need for more efficient, effective water quality monitoring systems. Traditional analytical methods, including inductively coupled plasma-atomic emission spectrometry (ICP–AES), are commonly employed to detect dissolved trace metals [7]. Despite their accuracy, these methods necessitate costly equipment, intricate sample preparation, and extensive analysis, precluding real-time, long-term in situ monitoring. Moreover, the chemical properties of samples may be altered during transportation and storage, potentially impacting the analytical results. Furthermore, the quantity of samples suitable for laboratory analysis is limited. Consequently, these methods may miss crucial data on oceanic trace metal concentration fluctuations. Electrochemical methods are popular due to their operational simplicity, cost-effectiveness, high sensitivity, and field suitability. Numerous practical applications have demonstrated the effectiveness of electrochemistry for Pb detection. In our laboratory, Liang et al. [3] utilized an automated electrochemical system to measure dissolved trace metals (Cu, Pb, and Cd) in three key seawater aquaculture zones. Moreover, online or in situ measurement methods significantly reduce the risk of electrode contamination [8]. Applying an agarose gel layer to the working electrode can enhance the system’s stability and reliability [9,10]. Despite these advancements, the development of continuous, automated in situ trace metal detection systems still requires further technological innovation and practical validation, especially in coastal environments. Moreover, this will enhance our understanding of the interactions between Pb and the natural environment.

To overcome the limitations of current electrochemical methods for automated, continuous in situ monitoring, this study builds upon Liang et al.’s [3] automated electrochemical detection system, proposing an enhanced system, CAEDS, designed for CON. Our objective was to refine the electrochemical instrument to withstand the interfering substances in seawater, facilitating long-term in situ Pb monitoring. These optimizations encompass instrument enhancements, detection parameter refinement, and the implementation of novel cleaning and maintenance mechanisms. To automate water sample filtration and circumvent laborious manual 0.45 μm membrane filtration, a pretreatment unit was developed to streamline the sampling and filtration process, enhancing its suitability for extended in situ monitoring. The pretreatment device automatically filters out particulate impurities from water samples, reducing mercury film contamination on the electrode surface and prolonging its lifespan. Further laboratory validation of the CAEDS system’s stability will facilitate its deployment in the field, allowing for automated and real-time monitoring throughout the day.

This study achieved continuous field determination of Pb in seawater using a continuous automated electrochemical detection system (CAEDS) in the North and South Yellow Seas of China. Water quality assessments were also conducted at both locations. Significant enhancements in the accuracy and reliability of Pb detection in complex seawater environments are anticipated. This advancement was considered crucial for developing and implementing environmental protection policies. Additionally, this study facilitated the sophisticated development of trace metal detection equipment for robust field applications, significantly augmenting our capacity to respond to marine pollution. This study investigated the variations in the migration and transformation behaviors of Pb under natural factors such as typhoons, enriching the theoretical framework of Pb in environmental science.

2. Experimental Design

2.1. Materials and Reagents

Polyethylene bottles for storing reagents and standard solutions were immersed overnight in nitric acid (HNO₃) and thoroughly rinsed with deionized water. The Pb²⁺ standard stock solution, with a concentration of 1000 mg/L, was procured from the National Institute of Standards and Technology (Beijing, China), while the 5 μg/L Pb²⁺ standard solution were derived through successive dilution of the stock solution. The electrolyte solution was composed of 5% sodium acetate anhydrous (C₂H₃NaO₂) and 3.75% acetic acid (CH₃COOH) in deionized water and was obtained from Sinopharm Chemical Reagent Co., Ltd., Shanghai, China. The digestion solution contained 3% nitric acid (HNO₃) in deionized water supplied by Sinopharm Chemical Reagent Co., Ltd., China. The plating solution was prepared at a 2% concentration using the standard plating solution from Lihe Technology (Hunan) Co., Ltd., Changsha, China. Artificial seawater was formulated with varying concentrations of sodium chloride (NaCl) up to a specific percentage in deionized water, also obtained from Sinopharm Chemical Reagent Co., Ltd., China. The ultrapure deionized water (18.2 MΩ·cm) retained in this study was generated utilizing a Pall Cas-cada™ lab water purification system (Pall Corporation, New York, NY, USA). All chemicals utilized in this investigation were of analytical grade.

2.2. Instrumentations

This investigation employed a water quality analyzer collaboratively developed and optimized with Lihe Technology (Hunan) Co., Ltd., China. The analyzer consisted of an injection system, an electrochemical detection system, and a data control output system. The essential elements included an electrochemical cell with a three-electrode configuration (with a platinum auxiliary electrode, an Ag/AgCl reference electrode, and a glassy carbon working electrode). All connecting conduits were constructed from polytetrafluoroethylene, featuring an internal diameter of 0.6 mm. Within our laboratory, Liang et al. [3] demonstrated the instrument’s efficacy in seawater analysis by juxtaposing its outputs against those derived from an ICP-MS (ELAN DRC II, PerkinElmer^®, Waltham, MA, USA) alongside certified seawater reference solutions. The instrument’s accuracy in gauging dissolved concentrations of Cu, Pb, and Cd was confirmed by comparing data obtained from ICP-MS analysis and certified seawater reference solutions. Although the comparison had been carried out in our previous work [3], new comparison experiments were added. We selected three different areas and retrieved seawater in sampling bottles. Perchloric acid was subsequently introduced into all samples to adjust their pH to 2.0, followed by their analysis using ICP-MS. Untreated samples were tested directly with CAED. Table S1 presents the results, demonstrating that the developed CAEDS exhibited good reliability and was suitable for field determination and analysis. Additionally, the instrument facilitated the establishment of a calibration curve for Pb concentration by employing differential pulse anodic stripping voltammetry. Within the national standard samples, the derived regression equation for Pb was Ip = 2.6818 + 0.6566 C, covering a range of 0.01–10 μg/L, with a limit of detection (LOD) of 0.003 μg/L and a coefficient of determination (R²) of 0.991 [3]. This demonstrates a linear correlation between the peak current and the Pb concentration, confirming the instrument’s ability to detect low levels of Pb.

However, considering the challenges, such as sea level variations, seawater composition complexity, and the potential damage from mercury film on internal electrodes, a continuous, automated sampler featuring a multistage filtration system was developed. To ensure the detection system’s sustained, stable, and automated operation, enhancements were made to the sampling mechanism. This apparatus, integrating a peristaltic pump, timer, and filter, enabled the synchronization of detection cycles with pump actions. The manual interface facilitated adjustment of the timer intervals to the desired settings, ensuring precise detection of Pb despite the high levels of impurities in seawater. This configuration allowed for the scheduled, quantitative automatic extraction and filtration of water samples, enabling accurate determination of the Pb content. Each filtration stage had a designated lifespan, and the filter head could be replaced based on the cleanliness of the seawater and duration of use, with the replacement process being both convenient and swift. Together, the water quality analyzer and sampling device constituted a continuous automated electrochemical detection system (CAEDS), as depicted in Figure 1a. The operational procedures of the CAEDS system are depicted in Figure 1b. These steps included drawing 1 mL of washing solution into the plunger pump, injecting it into the washing cell, and diluting it to 5 mL, followed by filtration and introduction of the actual sample into the cell for aeration, stirring, and heating for digestion (step 1). Then, 5 mL of diluted electrolyte was added to the detection cell, along with 1 mL of standard Pb²⁺ solution for dissolution and enrichment, and the peak height was measured (step 2). Subsequent steps involved cleaning the detection cell with deionized water after standard sample analysis (step 3), filling the detection cell with electrolyte and the digested sample for repeated enrichment and dissolution (step 4), cleaning the cell with deionized water after analyzing the actual sample (step 5), and determining the seawater concentration by comparing the peak values of the standard solution with those of the actual sample (step 6). Our system takes approximately 45 min to complete one Pb detection and can achieve a detection frequency of once per hour throughout the day.

2.3. Study Areas

This study focused on prolonged observation at two strategically significant sites along the Shandong Peninsula, adjacent to the Yellow Sea: Haiyang Center Fishing Harbor and Yantai Harbor, located on the southern and northern shores, respectively (Figure 2). These regions, characterized by a moderate monsoon climate and comparable seasonal variations, play integral roles in China’s maritime strategy and regional economic growth. Serving as vital hubs in the global shipping network and being historically significant for local aquaculture, these areas are heavily influenced by both human activities and natural phenomena. The North Yellow Sea, a crucial maritime center, faces heightened ecological pressures due to its prominence in global maritime logistics, while the South Yellow Sea, confronted with challenges such as pollution and population growth related to its aquaculture industry, underscores the necessity for comprehensive environmental monitoring. This is particularly crucial for tracking trace metal contamination to mitigate ecological impacts and ensure the sustainability of these key areas. The triple filter automatic sampler was used to collect water samples from the surface seawater, approximately 0.5 to 1 m deep. Due to frequent wind and waves in the field, a 1.5 m long plastic rod was used to secure the sampler, preventing damage or displacement of the primary filter. Additionally, to accommodate the ocean’s twice-daily tides and ensure the primary filter stayed submerged for effective sampling, we established four monitoring points daily. Monitoring occurs twice in the morning and twice in the evening after each tide. Considering that the concentration of dissolved Pb in seawater does not change significantly every hour, fewer measurements per day result in continuous determination over more days. Temperature (T), dissolved oxygen (DO), and salinity (S) were precisely determined in the field using a CTD instrument (Qingdao, China) and a shipboard Ferrybox system (4H-JENA engineering GmbH, Germany), providing essential data for evaluating the ecological well-being of the monitored coastal regions.

2.4. Water Quality Assessment Methods

Contamination factors (C_f) were utilized to assess the pollution levels of trace metals in the offshore mariculture waters under study [11,12]. C_f denotes the pollution level for individual trace metals.

$$C_f={\displaystyle \frac{C_{heavy metal}}{C_{background}}}$$

(1)

C_{heavy metal} represents the concentration of a specific metal (i.e., Pb), and C_background refers to the background concentration of that metal in seawater. According to Kang Tian et al., the median concentration of Pb (1.08 μg/L) was selected as the background concentration [13]. Following the criteria of Hakanson [11] (i.e., C_f < 1 indicates low pollution, 1 ≤ C_f < 3 indicates moderate pollution, 3 ≤ C_f < 6 indicates considerable pollution, and C_f ≥ 6 indicates very high pollution), the pollution levels were evaluated in both study areas.

2.5. Ecological Risk Assessment

This study evaluated the ecological risk of Pb in seawater through the species sensitivity distribution (SSD) method, a commonly employed approach in ecological risk assessments [14,15]. This method, which assesses the ecological risk of particular pollutants to the entirety of the seawater ecosystem, initially involved the calculation of the predicted no effect concentration (PNEC) for Pb.

$$PNEC={\displaystyle \frac{{HC}_5}{AF}}$$

(2)

HC₅ (the 5% SSD value) represented the maximum heavy metal concentration that left 95% of species in the ecosystem unaffected [14], with the HC₅ value for Pb being 25.3 μg/L [16]. The assessment factor (AF) was the uncertainty factor, which was set at 3.

The risk quotient (RQ) [17] for heavy metal pollution was calculated as follows: RQ < 0.1 indicates low ecological risk; 0.1 ≤ RQ < 1.0 signifies moderate ecological risk; and RQ > 1.0 denotes high ecological risk.

$$RQ={\displaystyle \frac{MEC}{PNEC}}$$

(3)

In this formula, MEC (measured environmental concentration) represents the actual concentration of heavy metals in seawater.

2.6. Software Used

Origin (version 2023) software was used to generate electrochemical correlation maps, distribution maps, pollution factor maps, and ecological risk maps. The analysis of station locations was conducted using Ocean Data View (version 5.5.1), which can be accessed at https://odv.awi.de/ (accessed on 20 October 2023).

3. Results and Discussion

3.1. Reproducibility and Stability of CAEDS

The reproducibility of the field monitoring instrument was evaluated by conducting 10 consecutive tests on standard seawater samples containing 0.5 μg/L and 10 μg/L Pb²⁺. The observing peak current changes were shown in Figure 3a, in which the black dotted line represents the average value of 10 peak currents. The peak current remained constant over ten tests (RSD 4.5% for 0.5 μg/L Pb and 0.7% for 10 μg/L Pb, respectively), indicating that the instrument has strong reproducibility when analyzing real samples. Additionally, to confirm its suitability for prolonged field monitoring, the CAEDS instrument was operated continuously for five days, and analyses were conducted twice daily on the same standard samples (2 μg/L Pb). At the same time, mercury film is automatically applied every night and calibrated using standard samples. The performance assessment, as shown in Figure 3b, showed that the purple line was the concentration of the standard sample tested during the 5 days. With an RSD of 5.6% observed over the five-day continuous monitoring period, the findings substantiate the instrument’s effectiveness for long-term field detection.

3.2. Continuous Monitoring of Dissolved Pb at Two Fixed Locations

The continuous monitoring of Pb concentration in the surface seawater of the North and South Yellow Seas, China, was carried out utilizing a field monitoring system equipped with a triple filtration mechanism. Figure 4a shows continuous monitoring of Pb in the South Yellow Sea of China from August 13th to 30th, with observations made four times daily at a fixed location. To assess the instrument’s reliability under real-world conditions, scheduled maintenance and calibration were performed on August 16th and 20th. Throughout the monitoring period from the 16th to the 20th, the Pb concentrations remained steady and did not significantly fluctuate. The instrument maintained consistent operation and uninterrupted functionality during this duration, confirming the reliability of our long-term data collection efforts. Compared to other time frames, the elevated Pb concentrations observed in the seawater surface layer between August 13th and 15th could be attributed to typhoons and storms [18]. These weather events, accompanied by heavy precipitation, facilitate Pb emissions into the atmosphere primarily from mining activities. Subsequently, atmospheric aerosols absorb Pb and transport it to the ocean surface through rainfall [19]. Moreover, typhoons can induce substantial oceanic disturbances, such as swells and storms, potentially mobilizing Pb from seafloor sediments into the water column [20,21,22]. The heavy rainfall and storm surges associated with typhoons may augment sediment resuspension, influencing the bioavailability and transport of heavy metals [21,23].

Figure 4b shows continuous monitoring of Pb in the North Yellow Sea of China from September 13th to 30th, with observations made four times daily at a fixed location. By establishing the protocols outlined in the Haiyang study, the instrument underwent a meticulously designed stress test involving systematic shutdowns and restarts. This protocol included a five-day observational phase to evaluate the instrument’s resilience and reliability under stress conditions. Following this comprehensive assessment, the instrument’s functionality was successfully restored, ensuring the continuity of high-quality data collection. After restarting, the instrument operated normally, showing resilience to power outages during field operations. Between September 13th and 18th, a slight increase in Pb was noted, particularly starting on the 17th, possibly due to rainfall and its role in transporting atmospheric Pb to the ocean surface [24]. After the resumption of monitoring on September 24th, a noticeable decrease in Pb concentration coincided with red tide occurrence, suggesting a potential correlation [25]. The Pb concentrations remained relatively stable from September 24th to 30th, marking the conclusion of the monitoring period.

3.3. Effects of Environmental Factors on the Dissolved Pb Concentration

Variations in environmental conditions, such as temperature, salinity, and dissolved oxygen levels, significantly affect the transport and conversion of trace metals in seawater [26]. Changes in dissolved oxygen levels had a notable impact on the movement and transformation of trace metals. At the same time, adjustments in temperature and salinity could affect their distribution between seawater and sediment layers [27]. As a result, the Pearson correlation test was utilized to evaluate the correlation coefficients between environmental variables and dissolved Pb concentrations.

The continuous distribution of Pb during the monitoring period in the South Yellow Sea of China is shown in Figure 4a. The distributions of temperature (T) and dissolved oxygen (DO) and their correlations with Pb are shown in Supplementary Figure S1a,c. Temperature exhibited a significant positive linear correlation with Pb (R = 0.39, p < 0.05), with typhoon-induced tidal mixing likely contributing to increased surface seawater temperatures. Typhoon conditions, characterized by robust winds and sizable waves, induced vigorous seawater agitation, augmenting vertical turbulence. This turbulence facilitated the mixing of surface and deep waters, mobilizing seafloor trace metals, including Pb, to the surface [23,28]. A notable negative linear correlation was observed between Pb and DO (R = −0.39, p < 0.05). Typically, typhoons, accompanied by overcast and rainy conditions, diminish phytoplankton photosynthesis, consequently lowering oxygen production and DO levels [29,30]. After a typhoon, as seawater agitation subsides, Pb settles to the seafloor, decreasing surface Pb concentrations.

Figure 4b shows the continuous distribution of Pb during the monitoring period in the North Yellow Sea of China. The distributions of temperature (T) and salinity (S) and their correlations with Pb are shown in Supplementary Figure S1b,d. A notable negative correlation was observed between Pb and salinity (R = −0.84, p < 0.05). Following red tide outbreaks, prolonged rainfall decreases salinity and introduces trace metals to the ocean’s surface layer [31]. This phenomenon can potentially impact the growth and distribution of red tide organisms, as changes in salinity and increases in heavy metals disrupt the balance of marine ecosystems [32]. Simultaneously, the decrease in salinity could have led to the formation of a stratified saline layer, facilitating the aggregation of red tide organisms. After 18th September, the temperature decreased to an optimal range for phytoplankton growth (T = 24.71 ± 0.13 °C) [33], resulting in the aggregation of phytoplankton and increased absorption of Pb, leading to a significant positive correlation between Pb and temperature (R = 0.77, p < 0.05). Bilgrami et al. [34] noted that Pb does not have toxic effects on phytoplankton below specific concentrations. Phytoplankton cell walls absorb trace metal ions from water during growth. Moreover, phytoplankton cell walls contain functional groups such as carboxyl and hydroxyl groups, which can be complex with metal ions, thereby enhancing metal adsorption [35]. Following a red tide event, trace metals settle to the seafloor with algal debris. A decreased phytoplankton population leads to reduced photosynthesis, CO₂ uptake, and pH while increasing carbonic acid [36]. Environmental factors significantly influence the behavior of trace metals such as Pb in seawater. These changes can alter metal solubility and bioavailability, ultimately impacting marine ecosystem health.

3.4. Comparison with Other Coastal Waters

The dissolved Pb concentrations in the South Yellow Sea ranged from 0.29 to 1.57 μg/L, while those in the North region ranged from 0.32 to 2.28 μg/L, with concentrations generally higher in the North region. This variation could be attributed to increased human activity and economic development in the North Yellow Sea region, which was characterized by industries, in contrast to the primarily fisheries-focused southern region. Increased urbanization and industrialization have led to increased domestic and industrial wastewater, including sewage and ship effluents containing Pb-based paint, potentially increasing Pb levels in seawater. Analysis of the dissolved trace metal concentrations in both regions compared to those in other coastal areas or bays (

Table 1. Comparison of dissolved Pb concentrations in seawater from the study area with those in coastal regions of China and the world (μg/L).

Region	Pb Concentration (μg/L)	Reference
Yellow River Estuary, China	5.61 (0.42–13.3)	[38]
Laoshan Bay, China	0.81 (0.16–9.13)	[40]
Laizhou Bay, China	1.91 (1.29–2.87)	[39]
Dalian Bay, China	0.3 (0.2–0.7)	[37]
Yantai Offshore Area, China	0.13–1.06	[24]
Macrotidal Ennore Creek, India	13.89	[41]
Southwest coast of the Bay of Bengal, Bangladesh	5.48	[42]
South Yellow Sea, China	0.6 (0.29–1.57)	This study
North Yellow Sea, China	0.9 (0.32–2.28)	This study

The values inside parentheses indicate the range of dissolved Pb concentrations, while those outside parentheses denote the average concentration.

) revealed that the total dissolved Pb concentrations were more significant than those in Dalian Bay and Yantai offshore [24,37]. They were lower than those in the Yellow River estuary and Laizhou Bay [38,39]. In the North Yellow Sea, the Pb concentrations were similar to those in Laoshan Bay [40]. Furthermore, when contrasted with international waters, Pb concentrations in the coastal regions of the North and South Yellow Sea were lower than those in India’s Macrotidal Enore Creek and the Southwest Coast of the Bay of Bengal [41,42].

In contrast to the local research domain, disparities in Pb levels emerged when comparing the study site with neighboring bays or marine zones, despite all regions falling within the Yellow and Bohai Seas. These differences are attributed to the higher population density and economic development level in the Bohai Sea vicinity compared to the coastal areas of the North Yellow Sea. The Bohai Sea zone boasted a dense population primarily involved in manufacturing, while fishing predominantly characterized the coastal waters of the North Yellow Sea. Heightened human activities and industrial growth resulted in a notable surge in domestic and industrial effluents, encompassing urban contamination and vessel operations (particularly by older ships utilizing Pb-based paints), potentially augmenting Pb discharge into the marine environment. Moreover, as it courses through its estuary and Laizhou Bay, the Yellow River plays a pivotal role in transporting dissolved metals from the land to the sea. Consequently, the Yellow River likely transports substantial industrial and agricultural discharge from densely inhabited downstream regions into these waters, contributing to heightened Pb levels. Compared with overseas research areas, industrial contamination and domestic wastewater discharge issues were pronounced near India’s Macro Enore Creek and the southwest coast of the Bay of Bengal. Local transportation modes such as small, motorized tricycles and boats are prevalent, and their exhaust emissions exacerbate Pb pollution in aquatic ecosystems.

3.5. Water Quality Assessment and Risk Assessment

The dissolved lead (Pb) concentrations in the surface seawater of the North and South Yellow Seas were evaluated according to Chinese seawater quality standards. The pollution factor (C_f) values for dissolved Pb were documented in

Table 2. C_f of dissolved Pb and the pollution levels assessment throughout the monitoring period in surface water of the South and North Yellow Seas, China.

	Date	Cf	The Pollution Levels
Surface seawater of the South Yellow Sea, China	13 August 2023	0.94 (0.90–0.98)	Low
	14 August 2023	1.07 (1.03–1.18)	Moderate
	15 August 2023	1.26 (1.17–1.46)	Moderate
	17 August 2023	0.59 (0.58–0.65)	Low
	18 August 2023	0.48 (0.46–0.51)	Low
	19 August 2023	0.50 (0.46–0.55)	Low
	21 August 2023	0.53 (0.49–0.56)	Low
	22 August 2023	0.40 (0.37–0.42)	Low
	23 August 2023	0.39 (0.33–0.43)	Low
	24 August 2023	0.53 (0.49–0.55)	Low
	25 August 2023	0.39 (0.33–0.46)	Low
	26 August 2023	0.37 (0.34–0.45)	Low
	27 August 2023	0.33 (0.28–0.40)	Low
	28 August 2023	0.28 (0.27–0.29)	Low
	29 August 2023	0.48 (0.46–0.52)	Low
	30 August 2023	0.39 (0.34–0.46)	Low
Surface seawater of the North Yellow Sea, China	Date	Cf	The pollution levels
	13 September 2023	1.22 (1.21–1.24)	Moderate
	14 September 2023	1.34 (1.29–1.39)	Moderate
	15 September 2023	1.40 (1.39–1.42)	Moderate
	16 September 2023	1.46 (1.24–1.56)	Moderate
	17 September 2023	1.65 (1.27–1.86)	Moderate
	18 September 2023	1.71 (1.25–2.03)	Moderate
	24 September 2023	0.43 (0.41–0.44)	Low
	25 September 2023	0.42 (0.38–0.50)	Low
	26 September 2023	0.42 (0.35–0.49)	Low
	27 September 2023	0.38 (0.32–0.51)	Low
	28 September 2023	0.43 (0.42–0.46)	Low
	29 September 2023	0.47 (0.40–0.54)	Low
	30 September 2023	0.45 (0.41–0.53)	Low
	Grade-one seawater quality standard (Pb μg/L)	≤1.00
	Grade-two seawater quality standard (Pb μg/L)	1.00–5.00
	Grade-three seawater quality standard (Pb μg/L)	5.00–10.00
	Low pollution (Pb)	Cf < 1
	Moderate pollution (Pb)	1 ≤ Cf < 3
	Considerable pollution (Pb)	3 ≤ Cf < 6
	High pollution (Pb)	Cf ≥ 6

The values in parentheses represent the range of C_f, while the values outside parentheses represent the average C_f.

. From August 13th to August 30th, the C_f values for Pb in the South Yellow Sea ranged between 0.27 and 1.46, indicating that the pollution levels ranged from low to moderate. Concurrently, from September 13th to September 30th, C_f values in the North Yellow Sea varied from 0.32 to 2.03, suggesting low-to-moderate pollution levels, with calculations incorporating standard deviation. Pb pollution was marginally greater in the North Yellow Sea than in the South, yet it remained within China’s primary-to-secondary water quality standards. These findings suggested the presence of Pb pollution pressure in the coastal areas of both the North and South Yellow Seas in China.

Table 3. RQ of dissolved Pb and the ecological risk level assessment throughout the monitoring period in the surface seawater of the South and North Yellow Seas, China.

	Date	RQ	The Ecological Risk Level
Surface seawater of the South Yellow Sea, China	13 August 2023	0.12 (0.12–0.13)	Moderate
	14 August 2023	0.14 (0.13–0.15)	Moderate
	15 August 2023	0.16 (0.15–0.19)	Moderate
	17 August 2023	0.08 (0.07–0.08)	Low
	18 August 2023	0.06 (0.06–0.07)	Low
	19 August 2023	0.06 (0.06–0.07)	Low
	21 August 2023	0.07 (0.06–0.07)	Low
	22 August 2023	0.05	Low
	23 August 2023	0.05 (0.04–0.05)	Low
	24 August 2023	0.07 (0.06–0.07)	Low
	25 August 2023	0.05 (0.04–0.06)	Low
	26 August 2023	0.05 (0.04–0.06)	Low
	27 August 2023	0.04 (0.04–0.05)	Low
	28 August 2023	0.04 (0.03–0.04)	Low
	29 August 2023	0.06 (0.06–0.07)	Low
	30 August 2023	0.05 (0.04–0.06)	Low
Surface seawater of the North Yellow Sea, China	Date	RQ	The ecological risk level
	13 September 2023	0.16	Moderate
	14 September 2023	0.17 (0.17–0.18)	Moderate
	15 September 2023	0.18	Moderate
	16 September 2023	0.19 (0.16–0.20)	Moderate
	17 September 2023	0.21 (0.16–0.24)	Moderate
	18 September 2023	0.22 (0.16–0.26)	Moderate
	24 September 2023	0.05 (0.05–0.06)	Low
	25 September 2023	0.05 (0.05–0.06)	Low
	26 September 2023	0.05 (0.05–0.06)	Low
	27 September 2023	0.05 (0.04–0.07)	Low
	28 September 2023	0.06 (0.05–0.06)	Low
	29 September 2023	0.06 (0.05–0.07)	Low
	30 September 2023	0.06 (0.05–0.07)	Low
	Low ecological risk (Pb)	<0.1
	Moderate ecological risk (Pb)	0.1–1.0
	High ecological risk (Pb)	>1.0

The values in parentheses represent the range of RQ, while the values outside parentheses represent the average RQ.

shows the risk quotient (RQ) for Pb in two specific regions, highlighting the minor impact of storm disturbances on the ecological risk of Pb within these areas. Moreover, red tides in the North Yellow Sea influenced Pb concentrations in seawater. After these red tide events, a decrease in Pb levels was noted, providing valuable insights into the management of seawater Pb pollution. The table illustrates that the North Yellow Sea exhibited greater ecological risk than the South Yellow Sea. Nevertheless, the mean daily RQ for both regions remained below 1 throughout the monitoring phase, indicating a low-to-moderate ecological risk. Despite intense human activities near the North Yellow Sea contributing to Pb pollution, periodic natural phenomena such as red tides and storms played a more pivotal role in shaping water quality. Consequently, undertaking comprehensive research and management efforts that focus on these natural factors has emerged as imperative for preserving the health and stability of the marine environment.

3.6. Limitations and Future Research Directions

This study offered initial insights into the distribution and behaviors of Pb in the Yellow Sea, yet it faced several significant limitations. First, the limited sample size and geographical scope might prevent these findings from reflecting broader oceanic conditions. Additionally, while the study observed typhoons’ impact on Pb behaviors, detailed investigations were still required to elucidate precisely how such natural factors influence Pb dynamics. Assessing trace metal contamination was crucial for devising effective pollution management strategies. However, comprehensive environmental assessments require further research to evaluate the effects of additional heavy metals. To address these limitations, future research should be expanded to include other marine areas, adopt advanced monitoring technologies, employ sophisticated models with long-term data for more accurate simulations of natural interactions, and incorporate additional environmental factors like turbidity, redox potential, and dissolved oxygen. Moreover, future studies should develop an integrated system for the simultaneous monitoring of various heavy metals and enhance monitoring frequency to better evaluate the long-term effects of heavy metal pollution on marine biodiversity and human health. Based on these data, more effective strategies could be formulated for marine pollution management and environmental protection.

4. Conclusions

In this study, a set of CAEDS systems was set up for ongoing measurement of dissolved Pb levels in the coastal regions of the North and South Yellow Seas, China. This research significantly revealed the influence of natural elements such as typhoons on the movement and alteration of Pb. Although further investigation is required to understand precisely how these natural factors influence Pb behavior, this study lays the groundwork for future research. Using this method to monitor Pb concentrations in seawater would provide essential insights into the biogeochemical cycles and distribution of Pb across China’s marine ecosystems. Alongside our assessments of ecological risk and water quality, we found that Pb levels in South Yellow Sea coastal waters ranged from low to moderate pollution levels according to Chinese standards. This underscores the need for ongoing monitoring and tailored pollution control measures to mitigate the risk of Pb contamination to both marine organisms and human health. This pioneering research enhances our understanding of Pb pollution dynamics in marine environments and establishes a firm foundation for developing impactful public health and environmental protection policies. In summary, CAEDS achieves continuous, automated monitoring of Pb in seawater and is characterized by relatively high stability, thereby establishing a robust basis for field-based monitoring. This study filled the knowledge gap concerning the migration and transformation processes of Pb in coastal seawater and its interactions with natural elements such as typhoons and phytoplankton, offering fresh perspectives and insights.