The Nutrient and Heavy Metal Contents in Water of Tidal Creek of the Yellow River Delta, China: Spatial Variations, Pollution Statuses, and Ecological Risks

In order to understand the spatial distribution, ecological risks, and pollution status of nutrients and heavy metals in the coastal tidal creek water of the Yellow River Delta (YRD), a total of 21 water samples were collected from 7 sample sites. The results indicated that along the coastline from northwest to southeast in the YRD, the concentrations of TN, TP, and NH4+-N in the water decreased and then increased; the Cu concentration increased, decreased, and then increased; and the Pb concentration decreased. The average TN/TP mass ratio indicated that the tidal creek water belonged to a potential phosphorus-restricted eutrophication state. The RI result indicated that Cu and Pb in the water were at low ecological risk, while the SSD and RQ results indicated that Cu in the water was at a high ecological risk level and had potential harm to aquatic organisms. Based on the single-factor method, the water quality of the tidal creek inside and outside the Yellow River Delta Nature Reserve belonged to Grade IV. Cu should be controlled to improve the water quality and reduce the ecological risk, especially in the Yellow River Delta Nature Reserve.


Introduction
Coastal tidal flats are in the important transitional regions between land and sea, with special climatic, hydrological, soil, and biological characteristics. Coastal tidal flats, influenced by both sea and land, are very fragile ecosystems [1,2]. Coastal tidal flats in the Yellow River Delta (YRD) are not only important parts of natural wetland in the YRD, but also important parts of coastal wetlands around the Bohai Sea [3]. Coastal tidal flats in the YRD provide an important transfer station and wintering habitat for bird migration in Northeast Asia and the Western Pacific Rim, which is of great significance for global biodiversity conservation [4]. Tidal creek, which provides an important channel for tidal flat water exchange, is the most significant first-level geomorphic unit on the natural silt beach [5]. Tidal creek affects biodiversity maintenance in tidal flats, especially vegetation and bird diversity [6,7]. Birds are very sensitive to the change of wetland environment, so the wetland environment is directly related to the reproduction and survival of water birds [8]. In order to better protect the biodiversity and ecological environment, the Shandong Yellow River Delta National Nature Reserve (SYNR) was established in the YRD.
With the rapid development of industry and agriculture in the YRD, more land-based pollutants are transported to the coastal water and adversely affected the coastal tidal flats, especially nutrients and heavy metal pollution [3,9,10]. As the youngest estuary delta in the world, the YRD has a wide distribution of saline-alkali land due to its low precipitationevaporation ratio and seawater influence. Therefore, farmers in the YRD increased the use the sample used for heavy metal determination was acidified with nitric acid on site to maintain the stability of the metal elements in the samples. The water samples before analysis were kept in the refrigerator at 4 °C. The basic physical and chemical properties of the water in the tidal creek were measured at each sample site. The pH, the oxidationreduction potential (ORP), the total dissolved solid matter (TDS), and the temperature of the water were measured by the HI98195 tester (Hanna, Roma, Italy).

Laboratory Analysis
The total nitrogen (TN) in the water was determined by alkaline potassium persulfate with UV spectrophotometry (Hitachi, Tokyo, Japan). According to the Chinese National Standard (HJ535-2009), ammonium nitrogen (NH4 + -N) was determined by Nessler's reagent spectrophotometry. The nitrate nitrogen (NO3 − -N) in the water was determined by ultraviolet spectrophotometry. The total phosphorus (TP) in the water was determined by ammonium molybdate spectrophotometry. According to Chinese national standards (GB7475-87, GB7485-87), Cu, Cd, and Pb in the water samples were determined by atomic absorption spectrophotometry, and As was determined by silver diethyldithiocarbamate spectrophotometry. In this study, the quality control and assurance system included reagent blanks, replicate samples, and reference materials, including cupric nitrate solution, cadmium nitrate solution, lead nitrate solution, and arsenic trioxide solution (GB7475-87, GB7485-87). The standard deviations of triplicate measurements were less than 5.0%. Recovery rates of the heavy metals were between 85% and 103%.

Potential Eutrophication Assessment
The potential eutrophication evaluation method is based on the fact that the relative excess of nutrients in seawater cannot be used by phytoplankton and represents only potential eutrophication. The substantial contribution of excess P or N to eutrophication can be shown only when sufficient maximum limiting N or P supplement is obtained in water to make the N/P value close to the Redfield value [25]. The method was mainly used to evaluate the nutrient level and limiting factors of coastal estuaries and marine waters. The classification of potential eutrophication levels is shown in Table 1 [26].

Laboratory Analysis
The total nitrogen (TN) in the water was determined by alkaline potassium persulfate with UV spectrophotometry (Hitachi, Tokyo, Japan). According to the Chinese National Standard (HJ535-2009), ammonium nitrogen (NH 4 + -N) was determined by Nessler's reagent spectrophotometry. The nitrate nitrogen (NO 3 − -N) in the water was determined by ultraviolet spectrophotometry. The total phosphorus (TP) in the water was determined by ammonium molybdate spectrophotometry. According to Chinese national standards (GB7475-87, GB7485-87), Cu, Cd, and Pb in the water samples were determined by atomic absorption spectrophotometry, and As was determined by silver diethyldithiocarbamate spectrophotometry. In this study, the quality control and assurance system included reagent blanks, replicate samples, and reference materials, including cupric nitrate solution, cadmium nitrate solution, lead nitrate solution, and arsenic trioxide solution (GB7475-87, GB7485-87). The standard deviations of triplicate measurements were less than 5.0%. Recovery rates of the heavy metals were between 85% and 103%.

Potential Eutrophication Assessment
The potential eutrophication evaluation method is based on the fact that the relative excess of nutrients in seawater cannot be used by phytoplankton and represents only potential eutrophication. The substantial contribution of excess P or N to eutrophication can be shown only when sufficient maximum limiting N or P supplement is obtained in water to make the N/P value close to the Redfield value [25]. The method was mainly used to evaluate the nutrient level and limiting factors of coastal estuaries and marine waters. The classification of potential eutrophication levels is shown in Table 1 [26]. The potential ecological risk index method (RI) considers heavy metal pollution, biotoxicity, and combined ecological risks and reflects the characteristics of bioavailability and the relative contribution ratio [27]. The RI can analyze the pollution degree of a single pollution element and determine the comprehensive ecological risk of pollutants. The calculation formula is as follows: where RI is the potential ecological risk index, E i r is the potential ecological risk coefficient of the metal i, C i is the measured concentration of heavy metals in water, and C i n is the reference concentration for heavy metal i in water. The C i n in this study was mainly based on the heavy metal concentration standard in the China National Seawater Quality Standard (GB3097-1997), the sea water quality in China can be divided into four grades according to the different use functions and protection objectives of the sea (Table 2). In this study, Sample Sites 3, 4, and 5 were all located in the SYNR, so the Grade I water quality standard was selected for these sites. Sample Site 7 was adjacent to Guangli Port, so the Grade IV water quality standard was selected for Sample Site 7. The Grade III water quality standard was selected for other sample sites. T i was the toxicity coefficient of the metal i, which mainly reflected the sensitivity of organisms to heavy metal pollution and the toxicity level of heavy metals. Hakanson gave the toxicity coefficient of metals (Table 3) [28]. Classification of E i r and RI is shown in Table 4.   The risk quotient (RQ) method was used to evaluate the water ecological risk of Cu and Pb. The calculation formula is as follows: where C is the exposure concentration of heavy metals, and PNEC is the predicted noeffect concentration. In this study, PNEC was calculated as the quotient of HC 5 and the assessment factor (AF, AF = 2) according to the AF method provided by the European Technical Guidance Document. The assessment factor was set as 2 to explain the uncertainty of missing populations and provide a preferable protection margin. The risk classification of the RQ is shown in Table 5 [29]. In this study, the toxicity data of Cu and Pb to aquatic organisms used, such as median lethal concentration (LC 50 ) and median effect concentration (EC 50 ), were collected from the USEPA ECOTOX database (http://www.epa.gov/ecotox/, accessed on 30 November 2021). The 4-7-day toxicity data was selected for algae, and the 24-96 h toxicity data were selected for crustaceans, invertebrates, fish, and amphibians. If a species had multiple toxicity data to meet the requirements, the geometric mean value was taken. According to the above screening principles, the statistical values of Cu and Pb toxicity data are shown in Table 6. Selected toxicity data were tested with the Shapiro-Wilk test (p > 0.05) and Kolmogorov-Smirnov test (p > 0.05) to construct SSD curves. The logarithm of the ascending toxicity data concentration is the abscissa of the SSD curve, and the cumulative probability is the ordinate of the SSD curve. The maximum environmental harmful concentration (HC 5 ) allowed to protect 95% and above species from being affected was finally obtained through logistic model fitting.

Single-Factor Method
The single-factor (SF) method classifies water quality according to an individual factor. The calculation formula is as follows: where Q is the comprehensive level of water quality evaluated by a single factor, Q i is the water quality level of evaluation parameter i, and Max is the worst evaluated water quality among water quality parameter i, according to the China National Seawater Quality Standard (GB3097-1997).

Data Analysis
One-way ANOVA was used to analyze the effects of the sample sites on the TN, NO 3 − -N, NH 4 + -N, TP, Cd, As, Pb, and Cu in the tidal creek water. After carrying out one-way ANOVA, Fisher's protected LSD test was used to detect significant differences (p < 0.05) among the sample sites. Pearson's correlation analysis was performed to test the relationship between the TN, NO 3 − -N, NH 4 + -N, TP, Cd, As, Pb, Cu, pH, ORP, TDS, and water temperature in the water.

Spatial Distribution of Nitrogen and Phosphorus
In general, along the coastline from northwest to southeast in the YRD, the contents of TN, TP, and NH 4 + -N in the tidal creek water showed a spatial trend of decreasing and then increasing, while the contents of NO 3 − -N had no obvious spatial distribution trend ( Figure 2). The contents of TN, TP, and NH 4 + -N in the water samples from the SYNR were lower than those outside the SYNR ( Figure 2). The TN concentration in the water of the 7 sample sites ranged from 3.26 to 5.39 mg L −1 , with an average of 4.24 mg L −1 . There were significant differences in the TN concentration among different sample sites (F = 10.485, p < 0.001). Comparing the TN concentration in the water samples from the 7 sample sites, the TN concentration in Sample Site 1 was the highest, while those in Sample Site 6 were the lowest. The TN concentration in the water samples from the SYNR was lower than that in Sample Sites 1, 2, and 7 outside the SYNR ( Figure 2A). There were different spatial distributions of the TN content in the water of the tidal creek and sediments around the tidal creek in the YRD. The highest TN content was found in sediments of the tidal flats near the Yellow River estuary [30].
Water 2022, 14, x FOR PEER REVIEW 7 of 20 the TP concentration among the different sample sites (F = 24.164, p < 0.001). The TP concentration in the water of the tidal creek at Sample Site 2 was the highest, and that at Sample Site 5 was the lowest ( Figure 2B), which was lower than the TP concentration in the Yellow River estuary (0.04 mg L −1 ) [31]. The TP content in the water of sample sites in the SYNR was lower than that of the sample sites outside the SYNR ( Figure 2B). This was similar to the spatial distribution of the TN content in sediments around the tidal creek in the YRD [30].
The concentration of NO3 --N in the water samples of the 7 sample sites ranged from 0.32 to 1.95 mg L −1 , with an average of 0.64 mg L −1 . Compared with other research results, the average concentration of NO3 − -N in the tidal creek water in this study was lower than that in the eight rivers of the YRD in Laizhou Bay (before rainfall, the average NO3 − -N content was 1.37 mg L −1 ; the average content after rainfall is 1.47 mg L −1 ) [11]. There were significant differences in the NO3 − -N concentration among different sample sites (F = 40.558, p < 0.001). Comparing the NO3 − -N concentration in the water samples from the 7 sample sites, the concentration in Sample Site 1 was the highest, while that in Sample Site 3 was the lowest ( Figure 2C). The NO3 − -N content of the water samples from the SYNR was lower than that of Sample Sites 1, 2, and 7 outside the SYNR ( Figure 2C). This was different from the spatial distribution of the NO3 − -N content in the sediments around the tidal creek of the YRD, such as the NO3 − -N content in the sediments inside the SYNR was higher than that of Sample Sites 6 and 7 outside the SYNR [30].
The concentration of NH4 + -N in the tidal creek water of the 7 sample sites ranged from 0.41 to 0.68 mg L −1 (mean: 0.56 mg L −1 ). There were significant differences in NH4 + -N concentration among different sample sites (F = 2.919, p < 0.05). The NH4 + -N content in the tidal creek water in Sample Site 6 was the highest, and that in the tidal creek water in Sample Site 2 was the lowest in the YRD ( Figure 2C).  The results of this study were compared with the previous research results on nitrogen and phosphorus in the water of the YRD or the estuary of the Yellow River ( Table 7). The results showed that N and P in the coastal tidal flat water of the YRD may mainly come from local land sources. More attention should be paid to the control of nitrogen content than phosphorus content in the coastal tidal flats of the YRD. Table 7. Comparison of N and P in the water of YRD and its coastal water. The TP concentration in the water of the tidal creek at all sample sites ranged from 0.02 to 0.14 mg L −1 , with an average of 0.06 mg L −1 . There were significant differences in the TP concentration among the different sample sites (F = 24.164, p < 0.001). The TP concentration in the water of the tidal creek at Sample Site 2 was the highest, and that at Sample Site 5 was the lowest (Figure 2B), which was lower than the TP concentration in the Yellow River estuary (0.04 mg L −1 ) [31]. The TP content in the water of sample sites in the SYNR was lower than that of the sample sites outside the SYNR ( Figure 2B). This was similar to the spatial distribution of the TN content in sediments around the tidal creek in the YRD [30].
The concentration of NO 3 --N in the water samples of the 7 sample sites ranged from 0.32 to 1.95 mg L −1 , with an average of 0.64 mg L −1 . Compared with other research results, the average concentration of NO 3 − -N in the tidal creek water in this study was lower than that in the eight rivers of the YRD in Laizhou Bay (before rainfall, the average NO 3 − -N content was 1.37 mg L −1 ; the average content after rainfall is 1.47 mg L −1 ) [11]. There were significant differences in the NO 3 − -N concentration among different sample sites (F = 40.558, p < 0.001). Comparing the NO 3 − -N concentration in the water samples from the 7 sample sites, the concentration in Sample Site 1 was the highest, while that in Sample Site 3 was the lowest ( Figure 2C). The NO 3 − -N content of the water samples from the SYNR was lower than that of Sample Sites 1, 2, and 7 outside the SYNR ( Figure 2C). This was different from the spatial distribution of the NO 3 − -N content in the sediments around the tidal creek of the YRD, such as the NO 3 − -N content in the sediments inside the SYNR was higher than that of Sample Sites 6 and 7 outside the SYNR [30].
The concentration of NH 4 + -N in the tidal creek water of the 7 sample sites ranged from 0.41 to 0.68 mg L −1 (mean: 0.56 mg L −1 ). There were significant differences in NH 4 + -N concentration among different sample sites (F = 2.919, p < 0.05). The NH 4 + -N content in the tidal creek water in Sample Site 6 was the highest, and that in the tidal creek water in Sample Site 2 was the lowest in the YRD ( Figure 2C).
The results of this study were compared with the previous research results on nitrogen and phosphorus in the water of the YRD or the estuary of the Yellow River ( Table 7). The results showed that N and P in the coastal tidal flat water of the YRD may mainly come from local land sources. More attention should be paid to the control of nitrogen content than phosphorus content in the coastal tidal flats of the YRD. 3.2. TN/TP Mass Ratio P and N are the primary limiting nutrients for the production of aquatic algae and also the important factors affecting water eutrophication [34]. The TN/TP mass ratio, as an important factor, affects the explosive growth of algae and is an important indicator of the production cycle and quantity of planktonic algae in water. The TN/TP mass ratio varies from 37 to 185 (mean: 96) in the tidal creek water of the 7 sample sites of the YRD. The TN/TP mass ratio (mean: 134) of Sample Sites 3, 4, and 5 in the tidal creek water inside the SYNR was higher than that (mean: 68) of Sample Sites 1, 2, 6, and 7 outside the SYNR. It is noteworthy that the TN/TP mass ratio in the water of Sample Site 5 was the highest in the sample sites of the YRD (Figure 3). Compared with previous research results (Table 7), the TN/TP mass ratio in the water of the YRD has increased over the past decade.
Potential eutrophication evaluation analysis (Table 1) [26] showed that the tidal creek water of the seven sample sites in the YRD was in phosphorus-restricted potential eutrophication state. When the ratio was between 10 and 25, there was a linear correlation between N and P concentration and algae growth, which was suitable for algae growth and prone to eutrophication [35]. In recent years, eutrophication was a serious environmental problem in the Bohai Sea [36], so the Chinese government has pushed the Action Plan for the Environmental Comprehensive Management and Control of the Bohai Sea (https://www.mee.gov.cn/xxgk2018/xxgk/xxgk15/201812/t20181211_684274.html, accessed on 11 December 2018). When controlling nitrogen source input, the coastal tidal creek water of the YRD, the TN/TP mass ratio should also be concerned to prevent the ratio in water from reaching the eutrophication critical point.

Spatial Distribution of Heavy Metals
The average concentration of Cu in the tidal creek water of the 7 sample sites was 60.00 µg L −1 , the average concentration of Pb was 6.96 µg L −1 , and the average concentrations of As and Cd in the water of the 7 sample sites were less than 0.50 µg L −1 and 0.09 µg L −1 in the YRD, respectively. The order of the average concentration of Cu, Pb, Cd, and As in the tidal creek water was Cu > Pb > Cd > As, which was similar to the order in the tidal flat sediments around the tidal creek [15]. In the SYNR, the average concentrations of Cu and Pb in the tidal creek water were 65.56 µg L −1 and 7.03 µg L −1 , respectively; outside the SYNR, the average concentrations of Cu and Pb in the tidal creek water were 59.16 µg L −1 and 6.90 µg L −1 , respectively. Compared with the concentrations of heavy metals in the rivers, estuaries, and coastal waters around the Bohai Sea, Bohai Bay, and Laizhou Bay (Table 8), the concentrations of Cu and Pb decreased from the inland waters to the sea. Therefore, it was inferred that Cu and Pb in the water of the tidal creek in the YRD mainly came from land. The average concentration of As in the water of the coastal tidal creek of the YRD was lower than that in the Bohai Sea and coastal Bohai Sea, while the average concentration of Cd was close to that in the Bohai Sea and coastal Bohai Sea (Table 8). Therefore, it was inferred that As and Cd in the coastal tidal creek water of the YRD were affected by both marine and land pressures.

Spatial Distribution of Heavy Metals
The average concentration of Cu in the tidal creek water of the 7 sample sites was 60.00 µg L −1 , the average concentration of Pb was 6.96 µg L −1 , and the average concentrations of As and Cd in the water of the 7 sample sites were less than 0.50 µg L −1 and 0.09 µg L −1 in the YRD, respectively. The order of the average concentration of Cu, Pb, Cd, and As in the tidal creek water was Cu > Pb > Cd > As, which was similar to the order in the tidal flat sediments around the tidal creek [15]. In the SYNR, the average concentrations of Cu and Pb in the tidal creek water were 65.56 µg L −1 and 7.03 µg L −1 , respectively; outside the SYNR, the average concentrations of Cu and Pb in the tidal creek water were 59.16 µg L −1 and 6.90 µg L −1 , respectively. Compared with the concentrations of heavy metals in the rivers, estuaries, and coastal waters around the Bohai Sea, Bohai Bay, and Laizhou Bay (Table 8), the concentrations of Cu and Pb decreased from the inland waters to the sea. Therefore, it was inferred that Cu and Pb in the water of the tidal creek in the YRD mainly came from land. The average concentration of As in the water of the coastal tidal creek of the YRD was lower than that in the Bohai Sea and coastal Bohai Sea, while the average concentration of Cd was close to that in the Bohai Sea and coastal Bohai Sea (Table 8). Therefore, it was inferred that As and Cd in the coastal tidal creek water of the YRD were affected by both marine and land pressures. One-way ANOVA showed that there were significant differences in Cu (F = 17.333, p < 0.001) and Pb (F = 28.772, p < 0.001) in the tidal creek water in the different sample sites. As the contents of Cd and As in the tidal creek water of the different sample sites were below the detection limit, the contents of these two metals in the tidal creek water of the different sample sites were not analyzed by ANOVA. The spatial distribution characteristics of Cu and Pb concentrations in the tidal creek water of the different sample sites were different ( Figure 4). The concentration of Cu was the lowest in the water of Sample Site 6 and was significantly lower than that of other sample sites (Figure 4). The spatial distribution characteristics of Cu concentration in the water of the seven sample sites were similar to that in the sediment around the tidal creek in the YRD [15]. This may be related to the relatively long-term and stable pressure of Cu pollution on the water of the coastal tidal creek and the sediment of the coastal tidal flats. One-way ANOVA showed that there were significant differences in Cu (F = 17.333, p < 0.001) and Pb (F = 28.772, p < 0.001) in the tidal creek water in the different sample sites. As the contents of Cd and As in the tidal creek water of the different sample sites were below the detection limit, the contents of these two metals in the tidal creek water of the different sample sites were not analyzed by ANOVA. The spatial distribution characteristics of Cu and Pb concentrations in the tidal creek water of the different sample sites were different (Figure 4). The concentration of Cu was the lowest in the water of Sample Site 6 and was significantly lower than that of other sample sites (Figure 4). The spatial distribution characteristics of Cu concentration in the water of the seven sample sites were similar to that in the sediment around the tidal creek in the YRD [15]. This may be related to the relatively long-term and stable pressure of Cu pollution on the water of the coastal tidal creek and the sediment of the coastal tidal flats. The spatial distribution of Pb concentration in the water of the coastal tidal creek was higher in the north and lower in the south in the YRD (Figure 4). Previous studies have shown that Pb in the sediments of the Yellow River estuary and its northwest coastal area was dominated by anthropogenic sources, while Pb in the sediments of the coastal area south of the Yellow River estuary was affected by both natural and anthropogenic sources [42,43]. Although Pb pollution sources were different in different areas of the YRD, there was no significant difference in Pb content in the sediments of the coastal tidal flats of Bohai Bay and Laizhou Bay [15]. The results of this study show that the Pb concentration in the water of the coastal tidal creek of the YRD was different. Meanwhile, according to The spatial distribution of Pb concentration in the water of the coastal tidal creek was higher in the north and lower in the south in the YRD (Figure 4). Previous studies have shown that Pb in the sediments of the Yellow River estuary and its northwest coastal area was dominated by anthropogenic sources, while Pb in the sediments of the coastal area south of the Yellow River estuary was affected by both natural and anthropogenic sources [42,43]. Although Pb pollution sources were different in different areas of the YRD, there was no significant difference in Pb content in the sediments of the coastal tidal flats of Bohai Bay and Laizhou Bay [15]. The results of this study show that the Pb concentration in the water of the coastal tidal creek of the YRD was different. Meanwhile, according to Table 4, the source of Pb in the water in the northern tidal flat of the YRD was probably mainly from the rivers or near the shore, which caused Pb pollution pressure may be greater than that from the Yellow River to tidal creek water in the YRD. The Pb pollution pressure of the tidal creek water of Bohai Bay in the north of the YRD may be increasing, which deserves further attention.

Potential Ecological Risk Index Method (RI)
The results of ecological risk assessment of heavy metals showed that Cu (E i r = 31) and Pb (E i r = 17) of the tidal creek water in the seven sample sites in the YRD were all low risk. However, the ecological risk index (E i r ) of Cu and Pb in the tidal creek water of different sample sites was different ( Figure 5). The E i r of Cu in the water of Sample Sites 3, 4, and 5 inside the SYNR was of medium risk, while the E i r of Cu in the water of sample sites outside the SYNR was of low risk ( Figure 5A). The E i r of Pd in the tidal creek water in Sample Site 4 was of moderate risk, while that in other sites was of low risk ( Figure 5B). sites outside the SYNR was of low risk ( Figure 5A). The of Pd in the tidal creek water in Sample Site 4 was of moderate risk, while that in other sites was of low risk ( Figure 5B).
The potential ecological risk index (RI) of Cu and Pb in the coastal tidal creek water of the 7 sample sites of the YRD ranged from 4 to 121 (mean value: 48), indicating low risk ( Figure 5C). The RI of Cu and Pb in the water of sample sites in the SYNR was higher than that of other sample sites outside the SYNR. Although the ecological risk of heavy metals in the tidal creek water of the YRD was low, it was worth paying attention to Cu in the coastal tidal creek of the YRD, especially Cu pollution in the water of the coastal tidal creek of the SYNR. The E r i of Pb B The SSD curves of Cu and Pb in the water of the coastal tidal creek of the YRD are shown in Figure 6. Cu had greater toxicity to aquatic organisms, so it was necessary to keep the Cu The potential ecological risk index (RI) of Cu and Pb in the coastal tidal creek water of the 7 sample sites of the YRD ranged from 4 to 121 (mean value: 48), indicating low risk ( Figure 5C). The RI of Cu and Pb in the water of sample sites in the SYNR was higher than that of other sample sites outside the SYNR. Although the ecological risk of heavy metals in the tidal creek water of the YRD was low, it was worth paying attention to Cu in the coastal tidal creek of the YRD, especially Cu pollution in the water of the coastal tidal creek of the SYNR.

Risk Quotient (RQ)
The SSD curves of Cu and Pb in the water of the coastal tidal creek of the YRD are shown in Figure 6. Cu had greater toxicity to aquatic organisms, so it was necessary to keep the Cu concentration in the water below 6.83 µg L −1 in order to protect 95% of aquatic organisms ( Table 9). The average concentration of Cu in the tidal creek water was 60.00 µg L −1 , nearly 9 times higher than this threshold, which was at a high ecological risk level and will greatly adversely affect aquatic organisms. The aquatic organisms have low sensitivity to Pb, and the short-term water quality standard for the protection of Pb in the tidal creek water of the YRD was 47.88 µg L −1 (Table 9). Wang et al. showed that the short-term water quality standard for Pb in freshwater in China was 63.92 µg L −1 [44]. The concentration of Pb in this study did not exceed the standard. Compared with Cu, Pb has a low level of ecological risk. Therefore, there were differences in the prevention and control of heavy metal pollution in the tidal creek water of the YRD. In the YRD, attention should be paid to the prevention and control of Cu pollution in the water of the coastal tidal creek.  The Cu concentration in the water of different sample sites had exceeded its water ecological risk threshold. The RQ of Cu in each site was at a high ecological risk level and had potential harm to aquatic organisms ( Figure 7A). Among the seven sample sites, the RQ of Cu at Sample Site 4 was the highest. However, the RQ of Pb of the sample sites  The Cu concentration in the water of different sample sites had exceeded its water ecological risk threshold. The RQ of Cu in each site was at a high ecological risk level and had potential harm to aquatic organisms ( Figure 7A). Among the seven sample sites, the RQ of Cu at Sample Site 4 was the highest. However, the RQ of Pb of the sample sites located near and south of the Yellow River Estuary was at a low ecological risk level, and that of the other sample sites was at a moderate ecological risk level ( Figure 7B). High-intensity human activities were an important factor for the high ecological risk of Cu pollution in the water of the coastal tidal creek of the YRD. The pollution status of Pb was lower than that of Cu, while it should also be paid attention to it will amplify the risk due to biological enrichment. There is uncertainty in the characterization of water ecological risk, which to a variety of environmental factors and human factors, including sample colle analysis errors. Migrating waterbirds arrive in the YRD every October. There study carried out a tidal creek water survey in September 2017 to analyze bir quality at that time. Thus, these results only represent the ecological risk durin vey period because of sampling time limitations. Sampling should be carried ou times to fully analyze and understand the ecological risks of N, P, and heavy me tidal creek water of the study area.

Correlation Analysis
Correlation analysis showed that the TN concentration was significantly p correlated with the TP concentration and the NO3 − -N concentration, while the TN tration was significantly negatively correlated with the NH4 + -N concentration, the water temperature. The TP concentration was positively correlated with th tration of TN, NO3 − -N, and Pb and negatively correlated with the NH4 + -N conc the ORP, the TDS, and the water temperature; the concentration of Cu was posit related with the TDS and negatively correlated with the concentration of NH4 + -N pH in the tidal creek water (Table 10). The concentration of Pb had a significantl correlation with TP and a significant negative correlation with the NH4 + -N conc and the water temperature (Table 10). The results showed that TN, TP, and sources of the tidal creek water were similar, and Pb and TP sources of the ti water were similar. There is uncertainty in the characterization of water ecological risk, which is related to a variety of environmental factors and human factors, including sample collection and analysis errors. Migrating waterbirds arrive in the YRD every October. Therefore, this study carried out a tidal creek water survey in September 2017 to analyze bird habitat quality at that time. Thus, these results only represent the ecological risk during the survey period because of sampling time limitations. Sampling should be carried out several times to fully analyze and understand the ecological risks of N, P, and heavy metals in the tidal creek water of the study area.

Correlation Analysis
Correlation analysis showed that the TN concentration was significantly positively correlated with the TP concentration and the NO 3 − -N concentration, while the TN concentration was significantly negatively correlated with the NH 4 + -N concentration, pH, and the water temperature. The TP concentration was positively correlated with the concentration of TN, NO 3 − -N, and Pb and negatively correlated with the NH 4 + -N concentration, the ORP, the TDS, and the water temperature; the concentration of Cu was positively correlated with the TDS and negatively correlated with the concentration of NH 4 + -N and the pH in the tidal creek water (Table 10). The concentration of Pb had a significantly positive correlation with TP and a significant negative correlation with the NH 4 + -N concentration and the water temperature (Table 10). The results showed that TN, TP, and NO 3 − -N sources of the tidal creek water were similar, and Pb and TP sources of the tidal creek water were similar.
The concentrations of TN, TP, Cu, and Pb in the tidal creek water of the YRD were mainly affected by the pH, TDS, and water temperature. The pH value was one of the important environmental factors affecting the content of heavy metals in seawater [45]. Correlation analysis results (Table 10) showed that the pH was significantly negatively correlated with the Cu concentrations in the water and negatively correlated with the Pb concentration in the water. With the increase in the pH of the coastal tidal creek water in the YRD, the concentrations of Cu and Pb decreased, which was similar to the research results in the Bohai Sea and Yellow Sea [37]. With the increase in the TDS in the tidal creek water of the YRD, the concentration of TP in the water decreased, while the concentration of Cu in the water increased in the tidal creek water (Table 10). The higher the water temperature was, the lower the concentration of TN, TP, and Pb. With the increase in water temperature, the concentrations of TN, TP, and Pb in the water decreased (Table 10).

Water Quality Analysis
According to the China National Marine Water Quality Standard (GB3097-1997) ( Table 2), the concentrations of Cd and As in the water of all the samples in the YRD reached Grade I, and pH values reached Grades I and II, while the average concentration of Cu reached Grades III and IV, and the concentration of Pb reached Grade III. There were differences in the water quality of the tidal creeks between different sample sites. The Cu concentrations in the water of Sample Site 6 met Grade I, while that in the other sample sites all met Grades III and IV ( Figure 5). The Pb concentrations in the water of Sample Sites 2 and 4 were in Grade III, and the other sample sites were in Grade II. The sample sites in Bohai Bay in the north of the YRD may be more influenced by the proximity of Dongying ports and economic development zones. The significant difference of Cu concentration in the water between Sample Sites 6 and 7 in Laizhou Bay in the south of the YRD may be because Sample Site 7 was closer to a copper enterprise and Guangli Port.
Water quality of collected tidal creek inside and outside the SYNR was analyzed based on the SF method; the water quality of the tidal creek inside and outside the SYNR belongs to Grade IV. The Cu concentrations in the water samples taken from the SYNR were all in Grades III and IV, while 33% of the water samples taken from outside the SYNR reached Grade II, and 67% of the water samples were in Grades III and IV ( Figure 8A). The Pb concentrations in 33% of all the water samples from the tidal creek reached Grade II, and 33% of all the water samples from the tidal creek reached Grade III in the SYNR. The Pb concentrations in 42% of all the water samples from the tidal creek reached Grade II, 42% of all the water samples the from the tidal creek reached Grade III, and 17% of all the water samples from the tidal creek reached Grade IV outside the SYNR ( Figure 8B). In all the water samples of the tidal creek in the SYNR, the pH of 100% samples reached Grade II, while the pH of 83% samples reached Grade II, and 17% samples reached Grade III, in all the water samples of the tidal creek outside the SYNR. Therefore, Cu in the tidal creek water of the SYNR was the key prevention and control target.

Conclusions
This study provided valuable evidence on the concentrations, spatial dist ecological risk assessment of nutrients and four heavy metals, and water quali water of the coastal tidal creek in the YRD, China. The average concentrations of NH4 + -N, NO3 − -N, Cu, and Pb in the tidal creek water were 4.24 mg L −1 , 0.06 mg mg L −1 , 0.56 mg L −1 , 60.00 µg L −1 , and 6.96 µg L −1 , respectively; the concentration o Cd in the water of all sample sites was less than 0.50 µg L −1 and 0.09 µg L −1 , resp The concentrations of TN, TP, and NH4 + -N in the tidal creek water in the SYNR we than that outside the SYNR, while the concentrations of NO33 − -N had no obviou The spatial distribution characteristics of different heavy metal contents in the tid water were different. Along the coastline from northwest to southeast in the YRD content of water in different sample sites increased, decreased, and then increa Pb content in the tidal creek water was significantly higher in the north of the Y that near and to the south of the Yellow River estuary.
The TN/TP mass ratio of the tidal creek water in the sample sites of the YR longed to the potential phosphorus-restricted eutrophication state. The RI result i that Cu and Pb in the tidal creek water were at low ecological risk, while the RQ

Conclusions
This study provided valuable evidence on the concentrations, spatial distribution, ecological risk assessment of nutrients and four heavy metals, and water quality in the water of the coastal tidal creek in the YRD, China. The average concentrations of TN, TP, NH 4 + -N, NO 3 − -N, Cu, and Pb in the tidal creek water were 4.24 mg L −1 , 0.06 mg L −1 , 0.64 mg L −1 , 0.56 mg L −1 , 60.00 µg L −1 , and 6.96 µg L −1 , respectively; the concentration of As and Cd in the water of all sample sites was less than 0.50 µg L −1 and 0.09 µg L −1 , respectively. The concentrations of TN, TP, and NH 4 + -N in the tidal creek water in the SYNR were lower than that outside the SYNR, while the concentrations of NO 33 − -N had no obvious trend. The spatial distribution characteristics of different heavy metal contents in the tidal creek water were different. Along the coastline from northwest to southeast in the YRD, the Cu content of water in different sample sites increased, decreased, and then increased. The Pb content in the tidal creek water was significantly higher in the north of the YRD than that near and to the south of the Yellow River estuary.
The TN/TP mass ratio of the tidal creek water in the sample sites of the YRD all belonged to the potential phosphorus-restricted eutrophication state. The RI result indicated that Cu and Pb in the tidal creek water were at low ecological risk, while the RQ results showed that Cu in the water of the coastal tidal creek was at a high ecological risk level and had potential harm to aquatic organisms. In particular, the E i r of Cu in the water of the coastal tidal creek was of medium ecological risk in the SYNR. It was worth paying attention to Cu in the coastal tidal creek in the SYNR. Correlation analysis showed that TN, TP, and NO 3 − -N sources were similar, and Pb and TP sources were similar. TDS, pH, and water temperature, as the important factors, affected the concentrations of TN, TP, Cu, and Pb in the tidal creek water of the YRD. According to the China National Seawater Quality Standard (GB3097-1997), the concentrations of Cd and As in the water of all the samples in the YRD reached Grade I, and the pH reached Grades I and II, while the concentration of Cu reached Grades III and IV, and the concentration of Pb reached Grade III. The SF method result indicated the water quality of the tidal creek inside and outside the SYNR belongs to Grade IV. In particular, Cu in the tidal creek water was the key prevention and control target to improve water quality in the SYNR.

Data Availability Statement:
The data presented in this study are available on request from the corresponding authors.

Conflicts of Interest:
The authors declare no conflict of interest.