The survey and sampling were conducted in Zhanjiang Bay during four cruises in April (spring), July (summer), September (autumn), and December (winter) of 2017. A total of 26 sampling stations were set up to investigate and analyze the physical and chemical characteristics of the water body and Chl a. The locations of sampling stations are shown in Figure 1 .

Water samples from each station were collected at a depth of 0.5 m below the water surface. Organic glass water samplers were used in situ to collect surface water for measurements of nutrient concentrations and Chl a. The temperature and salinity of the seawater were measured simultaneously using a conductivity–temperature–depth sensor (CTD) (SBE911, Seabird). Water was pre-filtered with a sieve before sampling to remove impurities and most zooplankton from the water. The water samples (0.2–1.0 L) were then filtered with a GF/F membrane (interception efficiency equivalent to 0.65 μm) as soon as possible. The membrane was then folded in half, wrapped in aluminum foil, labeled, and stored at –20°C until analysis.

The nutrient concentration was measured using a Skalar automatic nutrient analyzer (Skalar-Analytical, Breda, the Netherlands), and water sample analysis was conducted according to the “Ocean Survey Specification-Observation of Seawater Chemical Elements” and the “SKALAR SAN++ Automatic Nutrient Analyzer Operation Manual”. Nitrate (NO₃-N), nitrite (NO₂-N) and ammonium (NH₄-N) concentrations were measured using the cadmium copper column reduction-diazo azo method, the diazo azo method and the indigo carmine method, with detection limits of 0.1, 0.02, 0.05μmol/L, respectively. Phosphate (PO₄-P) with the phosphorus molybdenum blue method, and silicate (SiO₃-Si) with the silicon molybdenum blue method, with detection limits of 0.1, 0.03μmol/L, respectively. The Chl a concentration in seawater was measured using the fluorescence method according to the “Marine Survey Specification-Marine Biological Survey”, with detection limits of 0.6ug/L. The filter membrane was extracted with 90% acetone and placed in a low-temperature refrigerator at –20°C for 24 h. The extraction solution was determined using a Turner Designs Fluorometer (Model 10AU). The concentration of inorganic nitrogen (DIN) is taken as the sum of the NO₃-N, NO₂-N, and NH₄-N concentrations.

The surface water temperature and salinity data are shown in Table 1 . The water temperature was in the range 16.4–34.2°C, being highest in autumn and lowest in spring: autumn (30.9°C), summer (29.4°C), spring (24.7°C), winter (18.0°C). Station Z26 is located near the site of discharge of cooling water from a thermal power plant ( Figure 2 ). The surface water temperature at this station is 3–4°C higher than the surrounding area in each season. Except for Z26, the water temperature shows little change within a single season.

The surface water salinity was in the range 17.4–29.9. Seasonal changes in discharge from the Suixi River, the monsoon climate, and replenishment by offshore water masses mean that the amount of fresh water entering Zhanjiang Bay varies seasonally. The salinity decreases from winter to autumn: winter (28.0), spring (27.3), summer (19.3), autumn (19.2). The salinity shows a gradual increase from the top of the bay to the middle, and then outside of the bay, indicating that the survey area is a transition zone from freshwater to seawater ( Figure 3 ).

To obtain the spatiotemporal distribution characteristics of environmental factors such as Chl a and nutrients, Kriging interpolation method in Surfer 13 was adopted in this study for spatial interpolation, and analyzed using linear component analysis. The spatiotemporal distribution of surface water Chl a in Zhanjiang Bay is shown in Figure 4 . Many studies have shown that under the influence of land-based pollution discharge and runoff, the Chl a concentration shows a decreasing trend from inside to outside of the bay (Justić et al., 1995a; Lucas et al., 2023). In spring and summer, the overall spatial distribution of surface Chl a in Zhanjiang Bay shows an increasing trend from inside to outside of the bay. In autumn and winter, due to the influence of aquaculture near Donghai Island, the Chl a concentration near Donghai Island is relatively high. In autumn and summer, at station Z6, the Chl a concentration are 21.15µg/L and 18.97µg/L, respectively, which are the highest concentration in the middle of the bay. The average concentration of Chl a is highest in outside of the bay in autumn, while it is highest in the middle of the bay in winter, and the average concentration from the top of the bay to outside of the bay as follows: autumn (11.44、8.68、15.23、17.63µg/L), winter (8.17、14.28、11.82、6.15µg/L).

The Chl a concentration is highest in summer and lowest in spring: summer (11.6 µg/L), winter (11.5 µg/L), autumn (10.7 µg/L), spring (1.8 µg/L). The range of Chl a concentration during the survey period is 0.44–37.43 µg/L, with the lowest value occurring in spring at station Z24 near the thermal power plant, and the highest value in summer at Z11 near the bay mouth.

In spring, the concentration of Chl a in the surface water was 0.44–4.23 µg/L. The highest value was recorded at station Z09 in the bay. Except for stations Z09, Z15, and Z16, and station Z14 outside of the bay, the concentration at other stations is <3.50 µg/L. Stations Z26 to Z04 at the top of the bay yield concentrations of <1 µg/L.

In summer, the concentration of Chl a in the surface water varies from 2.36 to 37.43 µg/L. Station Z11 recorded the highest value of 37.43 µg/L. The concentrations at stations Z12, Z21, and Z22, near the bay mouth, are 35.46, 27.52, 36.40 µg/L, respectively, much higher than those at other stations. The concentrations at stations Z09, Z10, and Z17, middle of the bay, are relatively low.

In autumn, the Chl a concentration is 2.1–22.3 µg/L, with the highest value at station Z02. The concentrations at stations Z06 and Z07, near the Donghai Dam, and Z11, Z12, and Z13, near the bay mouth, are relatively high (21.15, 18.16, 20.62, 19.76, and 17.63 µg/L, respectively). The concentrations at stations Z18, Z17, and Z09 in the bay are relatively low.

In winter, the Chl a concentration is 3.6–19.1 µg/L, with the lowest value located at station Z25, at the top of the bay, and the highest value at station Z12 near the bay mouth. The top of the bay shows low concentrations, with an average value of 8.17 µg/L.

The Chl a concentration reflects the abundance of marine phytoplankton biomass (Duan et al., 2014). Generally, Chl a is influenced by environmental factors such as nutrients, temperature, salinity, light, and hydrodynamic processes (Minu et al., 2020). As a typical semi-enclosed bay, Zhanjiang Bay is jointly affected by the input of terrestrial freshwater and the transport of offshore water masses, resulting in spatial differences in nutrients, temperature, salinity, and hydrodynamic forces among different areas inside of the bay. There is a seasonal variation in the average value of Chl a in the surface water of Zhanjiang Bay, with the highest value in summer and the lowest in spring. Results of a correlation analysis of four seasonal environmental factors and Chl a in Zhanjiang Bay in 2017 are given in Table 2 .

In spring, the average concentration of Chl a in the surface water of Zhanjiang Bay is the lowest level throughout the year. Chl a is significantly positively correlated with SiO₃-Si (r = 0.772), negatively correlated with PO₄-P (r = –0.722), and negatively correlated with DIN (r = –0.409).

In summer, the average concentration of Chl a in the surface water of Zhanjiang Bay reached its highest value throughout the year. Chl a was significantly positively correlated with pH (r = 0.681), dissolved oxygen (DO) (r = 0.741), and chemical oxygen demand (COD) (r = 0.537), and negatively correlated with PO₄-P (r = –0.668).

In autumn, the correlation between Chl a concentration and various nutrient elements is not significant. Autumn is the start of the dry season for the catchment of the Suixi River in Zhanjiang Bay, with a decrease in terrestrial nutrients. In summer, phytoplankton proliferate and consume excessive nutrients, resulting in a decrease in nutrients in the water.

In winter, the average Chl a concentration in the surface water of Zhanjiang Bay is second only to that in summer. Chl a is significantly positively correlated with pH (r = 0.729), DO (r = 0.720), and COD (r = 0.601), and negatively correlated with SiO₃-Si (r = –0.665) and DIN (r = –0.558). In winter, the average Chl a concentration is still relatively high. Similar to summer, Chl a concentration is significantly positively correlated with pH, DO, and COD, all nutrients show a significant negative correlation with salinity, indicating dilution and mixing in the survey area.

Particulate organic carbon in the ocean originates either from terrestrial material carried by runoff, is transported into the ocean through the atmosphere, or from in situ production in the marine environment (Savoye et al., 2012; Le et al., 2017). The Chl a concentration is the most commonly used proxy for the phytoplankton carbon biomass (Duan et al., 2014). The POC/Chl a ratio can also serve as an effective indicator of whether organic matter is derived from fresh local inputs (i.e. in situ phytoplankton production) or terrestrial particle inputs. A POC/Chl a value of >200 indicates that POC comes mainly from terrestrial transport; a POC/Chl a value of 20–200 indicates that POC is composed mainly of newly produced organic matter from phytoplankton (Chen et al., 2021). The station data show significant seasonal variations in the average concentration of Chl a and POC/Chl a in the surface seawater of Zhanjiang Bay in 2017 ( Figure 8 ). In spring, the average concentration of Chl a in surface water is the lowest, with an average POC/Chl a value of >200. In addition, there is a weak positive correlation between POC and Chl a in spring (R² = 0.15, P < 0.05; Figure 8 ), indicating that POC in Zhanjiang Bay is mainly affected by terrestrial inputs in spring. In summer, the average concentration of Chl a in surface water is the highest, with POC/Chl a values of <200 ( Figure 8 ), and there is no significant linear relationship between POC and Chl a (R² = 0.01, P > 0.05) ( Figure 9 ), indicating that the contribution of phytoplankton in situ production to POC is small, and the impact of terrestrial input is dominant. The average concentration of Chl a in surface water in autumn and winter is second only to that in summer, and the POC/Chl a values are <200 (60 in autumn and 50 in winter) ( Figure 8 ). In addition, there is a significant positive correlation between POC and Chl a (R² = 0.49 in autumn, P < 0.01; R² = 0.42 in winter, P < 0.01; Figure 9 ), indicating that in situ phytoplankton production is the main source of POC in surface water in Zhanjiang Bay during autumn and winter.

The concentration and composition of nutrients can have a significant impact on the growth of phytoplankton. Justin et al. and Dorch et al. proposed chemical dose limit standards for nutrients: if N/P < 10 and Si/N > 1, DIN is the limiting factor; if Si/P > 22 and N/P > 22, then PO₄-P is the limiting factor; if Si/P < 10 and Si/N < 1, then SiO₃-Si is the limiting factor, deviating from the corresponding ratio, and phytoplankton growth may be limited by insufficient nutrients (Dortch and Packard, 1989; Justić et al., 1995b). The average concentration of SiO₃-Si in the surface water of Zhanjiang Bay in spring is 9.6 µmol/L, being just 0.30, 0.45, and 0.53 of the average concentration in summer, autumn, and winter, respectively ( Figure 10 , Table 1 ). The Si/P value in spring is 3/1 and Si/DIN is 0.6/1, so SiO₃-Si is a key factor limiting the growth of surface phytoplankton in Zhanjiang Bay in spring, leading to a decrease in the concentration of Chl a ( Figure 10 ), which also explains the significant positive correlation between Chl a and SiO₃-Si. This result is consistent with other research results (Chen et al., 2021). In spring, the area of high Chl a in Zhanjiang Bay corresponds to the area of low PO₄-P and DIN, indicating that PO₄-P and DIN are not the main factors causing low concentrations of Chl a.

In summer, where there is abundant rainfall and high land runoff, coastal waters contain high concentrations of Chl a (Ho et al., 2010; Wang et al., 2022). Summer is the rainy season in Zhanjiang Bay, when discharge from the Suixi River is high. This brings point and surface source pollutants generated by land-based industry and agriculture into Zhanjiang Bay (Zhang et al., 2012; Shi et al., 2015). The average concentration of nutrients in the surface water of Zhanjiang Bay during summer is significantly higher than in other seasons ( Table 1 ). In addition, the high temperature and intense light in the bay favor the growth of phytoplankton (He et al., 2023), so the average concentration of Chl a reaches the highest value of the year. Phytoplankton grows vigorously during summer and produces more DO. If the average concentration of Chl a in water exceeds 10 µg/L, the pH is mainly affected by photosynthesis by algae. As the mass of algae increases, the CO₂ consumed by photosynthesis increases and the pH increases, showing a significant positive linear correlation with Chl a (Zang et al., 2011). In summer, the COD concentrations at stations Z11, Z22, and Z13, at mouth and outside of the bay, all exceeded 2 mg/L, much higher than in the middle of the bay. This may be related to the large quantity of organic pollutants discharged from the Baosteel sewage outlets and the large amount of land source pollutants discharged from the west coast of Guangdong that is carried by the current, resulting in a consistent spatiotemporal distribution of Chl a concentration and COD at the mouth and outside of the bay. In summer, the average concentrations of DIN and SiO₃-Si increased by 119% and 234% compared with spring, respectively, while the average concentration of PO₄-P only increased by 66% compared with spring. In addition, in areas with strong photosynthesis, algae proliferate extensively, and PO₄-P in the water is also consumed to a great extent.

Zhanjiang Bay is located in the subtropical zone, with strong winter light and temperatures that remain high, which favor the growth of phytoplankton (He et al., 2023). On the other hand, winter is the dry season in the catchment of the Suixi River, which is a time of reducing river discharge. To maintain water balance in the bay, more water from outside of the bay enters the inside of the bay and the nutrient-rich coastal flow provides a substantial amount of nutrients, resulting in a high concentration of surface Chl a and the proliferation of algae in Zhanjiang Bay during winter. This leads to the consumption of nutrients in the water, a decrease in the concentration of SiO₃-Si and DIN, and a significant negative correlation between Chl a and each of SiO₃-Si and DIN. In the mixing area of freshwater and saltwater, the nutrient concentration is controlled by the mixing process. The nutrient mixing line can be obtained by connecting the freshwater and seawater endpoints on nutrient-salinity relationship diagram, as shown in Figure 11 . A two end-member model shows that from top of the bay to the outside of the bay, SiO₃-Si and DIN are depleted, while PO₄-P is added ( Figure 11 ). In recent years, the land-based emissions of PO₄-P have been relatively high in Zhanjiang Bay (He et al., 2023). Therefore, although the phytoplankton grows vigorously in winter, the PO₄-P concentration remains high after nutrient consumption. DIN: P and Si: P are both far less than 22 ( Table 1 ), which also indicates that PO₄-P concentration is not a limiting factor for phytoplankton production in winter. Therefore, climate conditions may be the dominant factor affecting winter Chl a.

Many studies have shown that POC in the nearshore area of the northern South China Sea is affected by the monsoon. During spring and summer, the water temperature rapidly increases due to the influence of the southwest monsoon, accelerating the stratification of seawater, weakening the vertical movement of upper and lower water masses, shallowing the ocean mixed layer, and making it difficult for deep nutrients to be carried to the surface, thereby limiting the in situ production of POC (Chen, 2005). In addition, due to the influence of the humid and hot airflow of the southwest monsoon, spring and summer are the rainy seasons in Zhanjiang Bay, with strong terrestrial erosion by rainwater, so more land-based materials are brought into the nearshore area (Zhang et al., 2021). In autumn and winter, influenced by the northeast monsoon, the surface water temperature decreases, the vertical mixing of seawater strengthens, and the ocean mixed layer deepens. Deep nutrients are more easily carried to the surface, thereby promoting the growth of marine phytoplankton (Chen and Chen, 2006), which consistent with the data of this study.

The Chl a concentration in the surface water of Zhanjiang Bay in all seasons shows a decreasing trend from the top to the middle, and then to the mouth and outside of the bay. This is contrary to the spatial pattern of nutrients, indicating that the latter are not the main factor affecting Chl a. At the top of the bay, Z26 is located at the discharge outlet of the power plant, Z25 is located at the oyster breeding site, Z24 is located at the dock of the thermal power plant, and Z01–Z04 is located at the intersection of the waterway. The marine environment at the top of the bay is greatly affected by human activity, and at the same time, the bay is rich in nutrients due to the input of land-based runoff. However, there is a large area of aquaculture at the top of the bay and middle of the bay, especially the large-scale cultivation of oysters, where feeding and biological excretion have an effect. The concentration of suspended particulate matter at the top of the bay is relatively high, the transparency of the water body is reduced, the DO content is low, and photosynthesis is limited. In addition, the predation effect of oysters results in a relatively low Chl a. The inlet at the top of Zhanjiang Bay is narrow, with low water flow velocity and slow water exchange. The middle of the bay is more open, with greater water exchange. The mouth and outside of the bay are affected by tides that exchange large volumes of water (Wang et al., 2021). Many studies have shown that the coastal currents in western Guangdong directly affect runoff transport in the western Pearl River waters, and have an important impact on material transport, water quality, environmental factors, and fisheries and aquaculture in western Guangdong and even in the Beibu Gulf (Chen et al., 2021). The transparency of surface water has a great impact on the growth of phytoplankton (Wijewardene et al., 2021). The turbidity of the water at the mouth of Zhanjiang Bay and outside of the bay is low, and the transmittance is high (Li et al., 2020). The addition of eutrophic material from the Pearl River, carried by the coastal current, favors the growth of algae (Jian et al., 2022). Therefore, the concentration of Chl a is relatively high at the mouth and outside of the bay. In spring, weaker and narrower coastal currents transport insufficient nutrients to the nearshore, limiting the production of phytoplankton (Chen et al., 2021). Consequently, the concentration of Chl a is the lowest in spring. In summary, in addition to factors such as terrestrial nutrient transport, aquaculture, and climate, the geographical characteristics of Zhanjiang Bay may also result in a close relationship between Chl a and hydrodynamic conditions inside of the bay, as well as water masses outside of the bay.