Guangdong Province is located in the southern Nanling Mountains, along the South China Sea coast, bordering Guangxi, Hunan, Jiangxi, Fujian, Hong Kong, and Macau, and facing Hainan across the Qiongzhou Strait (Song et al., 2022). Meanwhile, Guangdong Province is located in the southeastern part of the coastal economic zone, south of the South China Sea, with a sea area of 419,300 square kilometers, making it a large marine economic province in China (Figure 1A). Located between latitude 20°09ˊ~25°31ˊ North and longitude 109°45ˊ~117°20ˊ East, it has a vast land area of 179,800km². The climate of the offshore waters of Guangdong Province belongs to the subtropical monsoon climate. The direction of the sea currents in the coastal waters of Guangdong Province changes in different seasons. Generally speaking, in summer, due to the influence of the monsoon, the currents mainly flow from south to north, while in winter, the monsoon is reversed, resulting in a corresponding change in the direction of the currents, which mainly blow from the north to the South China Sea (Figures 1C, D). In the spring and fall seasons, the alternation of the northeast monsoon and southwest monsoon brings about changes in the direction and strength of the currents, which may show a more complex distribution of flow direction and speed. However, the currents may also be affected by other factors, such as typhoons, tides, land runoff and so on (Qian et al., 2014; Wei et al., 2020). The total length of the mainland coastline is 4114km, and the marine resources are abundant. According to the Guangdong Yearbook, there are 714 species of marine organisms in Guangdong waters, including 187 species of phytoplankton, 319 species of zooplankton, 175 species of benthic and intertidal organisms, 24 species of reef-building stony corals, and 9 species of coral reef fish. Zhuhai, Guangdong Province, has 147 islands, one of the cities with more islands in China. since the 1970s, with the high economic development and population density of Guangdong, the degree of pollution of Guangdong’s offshore waters has been increasing. Among them, the Pearl River estuary is the most polluted sea area, the rising trend of pollution is more obvious, and the seawater eutrophication is serious (Ke et al., 2022).

Monitoring data from the Guangdong Provincial Department of Ecological and Environmental Protection (http://gdee.gd.gov.cn/) in 2020 for the offshore waters of Guangdong Province were the source of data for the analysis and study in this paper. Water flow seasons were sampled during the dry, wet and normal seasons in 2020 according to seasonal hydrological changes (Ministry of Environmental Protection of the People’s Republic of China, 2002). April and May during the monitoring period represent the dry season, July and August represent the wet season, and October and November are the normal seasons. In order to ensure the comparability of data among the three water seasons, monitoring data from 52 common station monitoring stations were used in this paper (Figure 1B).

After the sample were collected, they were first filtered through acetate filter membrane with a pore size of 0.45 μm. The filters need to be rinsed with pure water before use (China National Standardization Management Committee, 2007). Water samples were frozen at - 20°C prior to chemical analysis. In this study, DIN included NO 3 - N, NO 2 - N, NH 4 + N, and PO₄³–P was considered as DIP (Sun et al., 2021). In the laboratory analysis, where Chl-a, NO 3 - N, NO 2 - N, NH 4 + N, and DIP were analyzed by Spectrophotometric method (Chen et al., 2016), sodium hypobromite oxidation method, diazo-diazo method, respectively. zinc- cadmium reduction method (Wang et al., 2022; Lu et al., 2023), and phosphorus-molybdenum blue method (Murphy and Riley, 1962; Mariko et al., 2018). The detection limit of Chl-a was 0.04 mg/L; the detection limit of NO 3 - N, NO 2 - N and PO₄³–P was 0.02 μmol/L; the detection limit of NH 4 + N was 0.03 μmol/L.The relative standard deviations of the selected samples were determined by the standard colorimetric method described in the Specifications for Marine Investigations (China National Standardization Administration, 2007). The relative standard deviations (RSDs) of the selected samples were< 5% for repeated determinations (China National Standardization Administration, 2007). Details of the analytical methods, procedures and instrumentation in this study have been described in the Specifications for Marine Investigations (China National Standardization Administration Committee, 2007).

There were significant seasonal differences in Chl-a concentrations in coastal waters of Guangdong Province (P<0.01). During the survey period, the average concentration of Chl-a in the coastal waters of Guangdong Province was 11.97 ± 28.12 μg/L, with a concentration range of 0.18-218.40 μg/L.From a temporal perspective, the average concentration of Chl-a in the three water seasons was 9.99 ± 9.84 μg/L in the dry season, with a concentration range of 1.22-36.04 μg/L; The average concentration of Chl-a in the wet season was 18.28 ± 38.07 μg/L, with a concentration range of 0.18-218.40 μg/L; The average concentration of Chl-a in the normal season was 7.65 ± 27.64μg/L, and the concentration range was 0.22-202.80μg/L.Upon comparison, the average Chl-a concentration was the highest in the nearshore waters of Guangdong Province during the wet season, and its Chl-a concentration showed a trend of increasing and then decreasing from the dry season to the normal season. In terms of spatial distribution, during the dry season, the highest Chl-a concentration was found in the waters near Zhanjiang City, at Station 33, at 36.04 μg/L. This was followed by the waters near Zhuhai City (Figure 2A). During the wet season, Chl-a concentrations were higher in Zhanjiang City, Yangjiang City and the outer locations of the Pearl River Estuary, which highest Chl-a concentrations were found at Station 8 near Shenzhen City with up to 218.40 μg/L (Figure 2B). During the normal season, the waters with high Chl-a concentrations were mainly concentrated near Yangjiang City, where the highest recorded in the season occurred at Station 49. While all other areas were at low levels (Figure 2C).

There were significantly seasonal differences in DIN concentrations in coastal waters of Guangdong Province (P<0.01). During the survey period, the average concentration of dissolved inorganic nitrogen DIN in the coastal waters of Guangdong Province was 25.84 ± 35.72μmol/L, with a concentration range of 0.29-231.21μmol/L. From the perspective of time, in the three water seasons, the average concentration of DIN in the dry season was 30.68 ± 43.58μmol/L, and the concentration range was 0.29-231.21μmol/L; the average concentration of DIN in the wet season was 21.91 ± 35.45μmol/L, and the concentration range was 0.35-127.30μmol/L; the average concentration of DIN in the normal season was 24.91 ± 26.12μmol/L, and the concentration range was 2.11-99.21μmol/L. By comparison, the average concentration of DIN in the dry season was higher in the coastal waters of Guangdong Province. However, the distribution of DIN concentration from the dry season to the normal season showed a trend of first decreasing and then increasing. From the perspective of spatial distribution, the horizontal distribution of DIN concentration in all three seasons showed decreasing near-shore to far-shore, with the Pearl River port as the center and decreasing outward concentration (Figure 3). During the dry period, the high DIN concentration was mainly concentrated in the location of the Pearl River estuary, and the highest level was recorded at the Station 1 in its nearby waters (Figure 3A); while in the wet season, the DIN concentration at each station was generally lower than that in the dry season. During the period of wet water, in addition to the high DIN pollution concentration in the Pearl River Estuary, the DIN concentration in the sea near Zhuhai, Jiangmen, Yangjiang and Zhanjiang is also high compared to other stations (Figure 3B). In the normal season, the distribution of DIN concentration is basically the same as that in the wet season, but the distribution range of high DIN concentration is much smaller than that in the wet season (Figure 3C).

There were significantly seasonal differences in DIP concentrations in coastal waters of Guangdong Province (P<0.01). During the survey period, the average concentration of dissolved inorganic phosphorus DIP in the coastal waters of Guangdong Province was 0.59 ± 0.71μmol/L, with a concentration range of 0.01-3.65μmol/L. From a temporal perspective, in the three water seasons, the average concentration of DIP in the dry season was 0.58 ± 0.73μmol/L, with a concentration range of 0.09-3.23μmol/L; The average concentration of DIP during the wet season was 0.48 ± 0.65μmol/L, with the concentration range of 0.06-3.10μmol/L; the average concentration of DIP during the normal season was 0.70 ± 0.73μmol/L, with the concentration range of 0.01-3.65μmol/L. By comparison, the average concentration of DIP during the normal season was the highest in the coastal waters of Guangdong Province, and the concentration of DIP from the dry season to the normal season showed a first decrease and then increase. The DIP concentration from the dry season to the normal season showed a trend of decreasing and then increasing, and the DIP concentration in the normal was higher than the DIP concentration in the dry season. In the dry season, the high DIP concentrations were mainly concentrated in the waters near the Pearl River Estuary and Zhanjiang City (Figure 4A); while in the wet season, the DIP concentrations at all stations were generally lower than those in the dry season (Figure 4B). During the normal period, the highest value of DIP concentration appeared in the sea near Zhanjiang City, where the highest record was obtained at Station 28, up to 3.65 μmol/L, followed by the location of the Pearl River Estuary (Figure 4C).

There was a significant difference in DIN/DIP in seasonal coastal waters of Guangdong Province (P<0.05). During the survey period, the DIN/DIP in the nearshore waters of Guangdong Province fluctuated between 2.05 and 259.47 with a mean value of 43.77 ± 41.01. The mean value of DIN/DIP in the coastal waters of Guangdong Province decreased from 49.52 ± 41.50 in the dry season to 37.13 ± 42.38 in the wet season, and then increased significantly to 44.65 ± 38.91 in the normal season. The DIN/DIP ranged from 3.05 to 259.47 during the dry season. The DIN/DIP ranged from 2.05 to 231.02 during the wet season and from 15.73 to 238.04 during the normal season. Spatially, there were some differences in DIN/DIP in the nearshore waters of Guangdong (Figure 5). During the dry season, the proportion of DIN/DIP distribution was larger near the Pearl River Estuary and smaller away from the estuary. The highest value of DIN/DIP was found at the monitoring station near Shenzhen, which was Station 7, with a high value of 259.47. (Figure 5A). During the period of wet water, the range with higher DIN/DIP ratio was the sea area near Zhuhai city (Figure 5B). During the normal season, the range of higher DIN/DIP ratio is different from the dry and wet seasons, it occurs far from the Pearl River Estuary, and the highest record occurs near Shenzhen, specifically at Station 2, where the DIN/DIP is 238. 04. (Figure 5C).

There were significantly seasonal differences in EI in coastal waters of Guangdong Province (P<0.05). During the survey period, the average EI of the three water periods in the nearshore waters of Guangdong was 4.16 ± 10.90, and the minimum and maximum values of EI were 0.00 and 82.51, respectively. The EI of the nearshore waters of Guangdong showed a decreasing trend from the dry season to the normal season. During the dry season, the range of EI in the coastal waters of Guangdong was 0.00-82.51, with a mean value of 5.93 ± 15.44; During the wet season, the range of EI in the coastal waters of Guangdong was 0.00-34.77, with a mean value of 3.93 ± 8.93; During the normal season, the range of EI in the coastal waters of Guangdong was 0.00-31.87, with a mean value of 2.63 ± 6.09; spatially, the EI in the offshore waters of Guangdong showed significant differences (Figure 6). During the dry season, the EI was higher near the Pearl River Estuary, and the further away from the Pearl River Estuary, the lower the EI was (Figure 6A). During the wet season, its EI is smaller than that in the dry season. Except for the stations near the Pearl River Estuary, the EI of the waters near Zhanjiang City, Yangjiang City, Zhongshan City, Shenzhen City and Zhuhai City is relatively large (Figure 6B). Normal water period, its highest EI value appeared in the sea near Zhanjiang City, which is the Station 28 to obtain the highest record of this water period, up to 31.87, followed by higher EI in the sea near Zhongshan City and Zhuhai City (Figure 6C).

The Spearman correlation coefficients showed that the influencing factors of DIN, DIP and EI showed some seasonal variations in the nearshore waters of Guangdong Province (Figure 7). In the dry and wet water periods, DIN, DIP, EI and Chl-a were significantly positively correlated (P<0.01); while in the normal season (Figure 7C), EI and Chl-a were positively (Figures 7A, B) correlated (P<0.05), but DIN, DIP and Chl-a were not significantly correlated. In the three water periods, DIN, DIP, and EI were significantly negatively correlated with pH (P<0.01). In the wet season, DIN was significantly positively correlated with DO (P<0.05), except for the other water periods, the concentrations of DIN and DO were not correlated. Meanwhile, there was no significant correlation between DIP, EI and DO. DIN, DIP and EI were positively correlated with COD in all three water periods (P<0.05). The concentrations of DIN, DIP and EI were significantly positively correlated in all three water periods (P<0.01). In addition (Figure 7D), DIN, DIP and EI were significantly and positively correlated with suspended matter in all three water periods (P<0.01). DIN, DIP, and EI were positively correlated with petroleum during the dry and normal water periods (P<0.05). In addition, DIN/DIP was significantly and positively correlated with Chl-a in all three water periods (P<0.01) (Figure 7D).

The data of nutrient content, DIN/DIP and EI are listed in this paper. However, in order to further elucidate the pollution levels of nutrients in the nearshore waters of Guangdong Province, it is necessary to compare the results of this study with those of other studies (Table 1). The DIN concentrations in the nearshore waters of Guangdong Province were higher than those previously reported in Hainan Island, Kaohsiung Harbor, Laizhou Bay, Qinzhou Gang, Weizhou Island, and Lianyungang (Chen et al., 2016; Ning et al., 2020; Zhang P, et al., 2020; Lu et al., 2022; Wang et al., 2022; Zhang M, et al., 2022), but lower than those in the Changjiang River, Mississippi River (Turner et al., 2007; Cui and Xian, 2018). However, the DIP concentrations were higher than those in Laizhou Bay, Weizhou Island and Lianyungang (Ning et al., 2020; Wang et al., 2022; Zhang M, et al., 2022), but lower than those in Kaohsiung Harbor, Qinzhou Gang, the Changjiang River and Mississippi River (Turner et al., 2007; Chen et al., 2016; Cui and Xian, 2018; Lu et al., 2022). The DIN/DIP was higher than the previously investigated Qinzhou Gang, Weizhou Island, Lianyungang, and Mississippi River (Turner et al., 2007; Ning et al., 2020; Lu et al., 2022; Wang et al., 2022), but lower than Laizhou Bay and the Changjiang River (Cui and Xian, 2018; Zhang M, et al., 2022). EI is higher than previously reported for Hainan Island, Kaohsiung Harbor Laizhou Bay, Lianyungang (Chen et al., 2016; Zhang P, et al., 2020; Wang et al., 2022; Zhang M, et al., 2022) and lower than the Changjiang River (Cui and Xian, 2018). In addition, the results showed that the DIN pollution level in offshore waters of Guangdong Province in 2020 was higher compared with other river estuaries, and the pollution level of DIP concentration is not low. And in general, the pollution level of DIN is higher than that of DIP. Therefore, DIN/DIP is much higher than 16 and higher than the DIN/DIP values of most other rivers. Besides, the mean value of EI is much lower than the Changjiang River and higher than other estuaries.

According to the survey results, compared with the seawater quality standard, the percentage of the number of monitoring stations with DIN concentration exceeding the Grade IV seawater quality standard (DIN ≤ 35.71μmol/L) to the total number of monitoring stations was 28.85%, 19.23% and 23.08% in the dry season, wet season and normal season respectively (Figure 8A). In general, the level of DIN pollution in offshore waters of Guangdong Province is small. Mainly, the DIN concentration of most monitoring stations in the coastal waters of the Pearl River Estuary cannot reach the seawater quality standard of Grade IV or above (Shen et al., 2022), indicating that the coastal area of the Pearl River Estuary is seriously polluted. With the rapid socio-economic development, human activities such as land-based pollutant discharges, marine aquaculture and port shipping along the Pearl River Estuary are increasingly expanding, making the rivers along the Pearl River Estuary carry more nutrients. As a result, DIN concentrations at several monitoring stations in the area exceeded seawater quality standards (Li et al., 2020; Qian et al., 2022). In addition, DIN concentrations were highest during the dry season. Compared to the dry season, the average DIN concentrations in both the wet and normal season were significantly lower, because July and August are the rainy season, a high typhoon season, and constantly affected by the intrusion of rainwater, and the resuspended nutrients are quickly diluted.In addition, the temperature in the coastal water gradually increases and there is sufficient light, which is the peak period for algal growth during summer. As the temperature meets the conditions for rapid algal growth, the algae can absorb a large amount of nutrients from the water body while growing, resulting in a relatively low average DIN concentration during the wet period (Ke et al., 2022).

Compared with the national seawater quality standard, the percentage of the number of monitoring stations with DIP concentration exceeding the Grade IV seawater quality standard (DIP ≤ 1.45μmol/L) to the total number of monitoring stations was 7.69%, 9.62% and 7.69% in the dry season, wet season and normal season respectively (Figure 8B). In general, the level of DIP pollution in the offshore waters of Guangdong Province is small, and the DIP concentrations at most monitoring stations meet the seawater quality standard of Grade IV or higher, but there are still individual stations that do not meet the Grade IV standard. The average concentration of DIP is lowest during the wet season, which may be related to the frequent rainy season in July and August, when rainwater replenishes the sea and dilutes the concentration of DIP, or it may be caused by phytoplankton absorbing large amounts of nutrients from the seawater after heavy typhoon rains (Herbeck et al., 2011). The peak of DIP concentration occurs during the normal water period, which is due to the input of fresh water, domestic sewage and industrial wastewater from rivers with greater influence.

DIN and DIP were significantly negatively correlated with pH in all seasons (P<0.01). With the rapid development of industry in coastal areas, it has led to a great increase in the amount of CO₂ emitted into the atmosphere. The seawater absorbed a large amount of CO₂ from the air, which led to a decrease in the pH of seawater and the phenomenon of ocean acidification (Queré et al., 2014; Stocker, 2014; Rees et al., 2017). However, DIN and DIP were significantly and positively correlated with suspended matter in all seasons (P<0.01), which may be due to the fact that suspended matter in organic pollutants from land-based runoff carries a large amount of DIN and DIP. DIN and DIP were significantly and positively correlated with Chl-a in the dry and wet seasons (P<0.01), which may be due to the input of pollutants from land-based sources, promoting the rapid growth of phytoplankton (Han et al., 2023). DIN, DIP and COD are important indicators for evaluating coastal water quality (Ministry of Environmental Protection of the People’s Republic of China, 1997) (Tu et al., 2022). In addition, DIN and DIP were significantly and positively correlated with COD in each season (P<0.01) because surface runoff in each season carried a large amount of pollutants of terrestrial origin, which provided a large amount of inorganic and nutrients for a large number of phytoplankton in the water column. DIN and DIP were significantly and positively correlated (P<0.01) with petroleum species during the dry and normal season, probably because DIP in surface seawater has the same source and fate as petroleum hydrocarbons.

Nutrients are important for the growth of phytoplankton. Phytoplankton in the ocean take up nutrients from seawater in a certain ratio, and this constant ratio becomes the Redfield (Turner et al., 2003). Under normal conditions, N:P is about 16. During the survey period, DIN/DIP values ranged from 2.05 to 259.47, with a mean value of 43.77 ± 41.01, which is much higher than the Redfield ratio. And DIN/DIP was significantly and positively correlated with Chl-a (p<0.01). High Chl-a concentrations can reflect the changes of nutrient content in marine ecosystems, and the extremely strong correlation between DIN/DIP and Chl-a indicates that the imbalance of nutrient ratios in coastal waters of Guangdong Province has led to the occurrence of harmful algal blooms, thus exacerbating the eutrophication phenomenon (Schlüter et al., 2000; Qiao et al., 2017). The DIN/DIP ratios in the dry and normal seasons were higher than those in the wet season, which was caused by the higher DIN concentration and relatively lower DIP concentration in the dry and normal season. As can be seen from Figure 5, the DIN/DIP values at the location of the Pearl River Estuary were significantly higher, up to 259.47, indicating a serious imbalance in the nutrient composition of the offshore waters of the Pearl River Estuary, and the phenomenon of nutrient imbalance indicated that the discharge of industrial wastewater, domestic sewage and agricultural fertilizers near the Pearl River Estuary had a dynamic effect on the DIN/DIP values of the coastal waters. The eutrophication of nearshore water bodies in Guangdong Province is likely to continue due to the continued disturbance of nutrient ratios in these bodies (Wang et al., 2021).

With rapid socio-economic development and rapid population growth, industrialization and urbanization have accelerated, human pursuit of material culture has been elevated and the scope of activities has been expanded, leading to a gradual increase in the discharge of nutrients imported from outside into water bodies. Under the implementation of the 14th Five-Year Plan for Marine Ecological Protection of Guangdong Province (Department of Ecological Environment in Guangdong Province, 2022), land-based pollutant inputs are reduced to improve water quality according to the characteristics of the near-shore waters of Guangdong Province.Combining the concentrations of Chl-a, DIN, DIP, and the DIN/DIP stoichiometry results, we found that the eutrophication of coastal waters in Guangdong Province is low, but the eutrophication problem is still prominent in local waters, especially in the Pearl River Estuary and the waters near Zhanjiang City (Ke et al., 2023). To reduce the eutrophication in coastal waters of Guangdong Province, firstly, numerical nutrient criteria for estuaries and adjacent coastal waters should be developed (Wu et al., 2010; Huo et al., 2018; Yang et al., 2019; Xie et al., 2021; Zhang P, et al., 2022). The application of nutrient standards is not only an effective measure to prevent eutrophication of water bodies, but also a scientific basis for comprehensive monitoring, evaluation and management of nutrients in estuaries. The government should strengthen the comprehensive control of eutrophic waters and the management related to the discharge of river pollutants into the sea. In the time scale, we need further monitor nutrients and Chl-a response in the ocean for a long time. Besides, considering the spatial and temporal differences of EI, it is necessary to investigate and analyze Chl-a and nutrients according to their distribution characteristics at different locations and times, and to implement suitable management measures.In addition, the publicity of river pollution prevention and control should be strengthened to raise citizens’ awareness of coastal environmental protection participation. Integrated land and sea management of coastal water quality should be introduced in the future to control river and coastal water quality efficiently (Zeng-Lin et al., 2012; Gao et al., 2022).