Human activities have altered global nutrient cycling and have significantly changed marine systems. This is evidenced by the significant changes in nitrogen and phosphorus availability. The Maowei Sea (MWS) is the largest oyster culture bay in southwest China. From August 2018 to May 2019, the spatial and temporal nutrient concentrations and fluxes in MWS using system-wide scale seasonal data were assessed from river estuaries and adjacent coastal waters. The annual average concentrations of total nitrogen (TN) and total phosphorus (TP) in the three estuaries of Maolingjiang River (MLJR), Dalanjiang River (DLJR) and Qinjiang River (QJR) were 3.00 mg/L and 0.183 mg/L, respectively. Therein, the highest TN and TP concentrations were in DLJR, the lowest TN concentration was in MLJR, and the lowest TP concentration was in QJR. DIN and DIP were the main forms of TN and TP, accounting for 80.9% and 59.4%, respectively. The main form of DIN in MLJR and QJR was NO₃⁻, accounting for 86.8% and 84.4%, respectively, while the main form of DIN in DLJR was NH₄⁺, accounting for 55.9%. The annual flux of pollutants discharged into MWS from the three estuaries is 10,409.52 t for TN and 556.21 t for TP. The month with the largest contribution to the annual load was July, accounting for 29.2% and 24.2% of TN and TP, respectively, and the fluxes of TN and TP were significantly different among the three seasons (p < 0.05). The annual average concentrations in the surface waters of the MWS were 1.07 mg/L for TN and 0.129 mg/L for TP, and there were significant differences (p < 0.05) in the concentrations of TN and TP among the three seasons. The annual average N/P ratios of the river water and seawater were 43 and 18, respectively, which were higher than the Redfield ratio (N/P = 16), indicating that the growth of phytoplankton in MWS may be limited by phosphorus. Eutrophication owing to nutrient pollution in the three estuaries may be persistent in adjacent coastal waters, and land–ocean integrated mitigation measures should be taken to effectively improve the water quality in the river estuary and coastal water.

Steroids are environmental endocrine disruptors that are discharged from vertebrates and are also byproducts of aquaculture. They have strong endocrine disrupting effects and are extremely harmful to the environment. The pollution of steroids in Beihai Bay was assessed through analyzing sources from rivers entering the bay. Six different types of steroids were detected in seagoing rivers, seagoing discharge outlets, and marine aquaculture farms, ranging from 0.12 (methyltestosterone) to 2.88 ng/L (estrone), from 0.11 (cortisol) to 5.41 ng/L (6a-methylprednisone (Dragon)), and from 0.13 (estradiol) to 2.51 ng/L (nandrolone), respectively. Moreover, 5 steroids were detected in 13 of the 19 seawater monitoring stations, accounting for 68.4% of the samples, and their concentrations ranged from 0.18 (methyltestosterone) to 4.04 ng/L (estrone). Furthermore, 7 steroids were detected in 15 of the 19 sediment monitoring stations, accounting for 78.9% of the samples, with concentrations ranging from 26 (estrone) to 776 ng/kg(androsterone). Thus, the main source of marine steroids were the discharging rivers and pollution sources entering the sea. An ecological risk assessment indicated that estrone and methyltestosterone were at high risk in this region; 17β estradiol (E2β) was medium risk, and other steroids were of low or no risk. This study provides a scientific basis for ecological risk assessment and control.

Land-source inputs into coastal water have increased remarkably in recent years, resulting in the deterioration of water quality, eutrophication, and algae blooms. However, we have limited understanding of spatiotemporal nutrient patterns, stoichiometry, and eutrophication assessment in Tieshan Bay coastal water at present. To investigate the rapid development of the coastal areas in Tieshan Bay in the South China Sea, nutrients and other physicochemical parameters were observed in Tieshan Bay during the normal season (April), wet season (July), and dry season (October) in 2021. The results showed that the average concentrations of dissolved inorganic nitrogen (DIN), dissolved inorganic phosphorus (DIP), and chemical oxygen demand (COD) in Tieshan Bay are 0.071 ± 0.115 mg/L, 0.008 ± 0.013 mg/L, and 0.71 ± 0.219 mg/L, respectively. DIN/DIP ratio ranges from 9.1–69.3, with an average value of 19.9 ± 19.2, which exceeds the Redfield value, behaving P limitations. In addition, the mean eutrophication index (EI) was low in Tieshan Bay, with an average value of 0.5 ± 1.5. Moreover, the hotspot coastal water with high DIN, DIP, and COD concentrations was located in the upper half of Tieshan Bay in all seasons. In addition to the DIN, DIP, and COD contributions to EI, the average contribution rates of DIN, DIP, and COD are 26.6%, 8.8%, and 64.6%, respectively, which leads to the largest contribution of COD to EI. Furthermore, the average comprehensive index (CI) of organic pollution in Tieshan Bay surface seawater ranged from −1 to 5.6. The seawater near Hepu in S8 station has organic pollution in wet and dry seasons, and Tieshan Bay’s middle region also has slight organic pollution. Additionally, the DIN, DIP, and COD had significant relationships with salinity (p < 0.05), suggesting that coastal water quality is affected by land-based sources input. To achieve the seawater quality target and mitigate regional eutrophication, it is critical to implement land-based source management across the river-bay-coastal water continuum.

Nutrients function as essential biological substrates for coastal phytoplankton growth and serve as pivotal indicators in marine environmental monitoring. The intensification of land-based nutrient sources inputs has exacerbated eutrophication in Chinese coastal water, while mechanistic understanding of differential self-purification processes among distinct land-based source nutrients (river source, domestic source, aquaculture source, and industrial source) remains limited, constraining accurate assessment of bay’s self-purification capacity. This study conducted incubation experiments in Tieshan Bay (TSB) during Summer (June 2023) and winter (January 2024), systematically analyzing the self-purification process of nutrients and associated environmental drivers. Distinct source-specific patterns emerged: river inputs exhibited maximal dissolved inorganic nitrogen (DIN) 1.390 ± 0.74 mg/L, whereas industrial discharges showed peak dissolved inorganic phosphorus (DIP) 4.88 ± 1.45 mg/L. Chlorophyll a (Chl-a) concentrations varied markedly across sources, ranging from 34.97 ± 23.37 μg/L (domestic source) to 86.63 ± 77.08 μg/L (river source). First-order kinetics demonstrated significant source differentiation (p < 0.05). River-derived DIN exhibited the highest attenuation coefficient (−0.3244 ± 0.17 d⁻¹), contrasting with industrial-sourced DIP showing maximum depletion (−0.4332 ± 0.20 d⁻¹). Correlation analysis indicated that summer was significantly associated with the impacts of three key control factors pH, dissolved oxygen, and turbidity on nutrient dynamics (p < 0.05), whereas winter exhibited a stronger dependence on salinity. These parameters collectively may modulate microbial degradation pathways and particulate matter adsorption capacities. These findings establish quantitative thresholds for coastal nutrient buffering mechanisms, highlighting the necessity for source-specific eutrophication mitigation frameworks. The differential self-purification efficiencies underscore the importance of calibrating pollution control strategies according to both anthropogenic discharge characteristics and regional hydrochemical resilience, which is of key importance for ensuring the traceability and control of land-based sources of pollution into the sea and the scientific utilization of the self-purification capacity of the bay water body.

Introduction: Elevated nitrogen and phosphorus inputs from human activities have driven coastal eutrophication. Although Qinzhou Bay has long been influenced by anthropogenic disturbances, long-term systematic studies on its nutrient dynamics remain scarce. Methods: Based on long-term monitoring data spanning 22 years (2003-2024), this study systematically elucidates the interannual trends, spatiotemporal distribution patterns, and eutrophication status of nutrients in Qinzhou Bay. Results: The concentrations of dissolved inorganic nitrogen (DIN) and dissolved inorganic phosphate (DIP) showed an overall upward trend amid interannual fluctuations, yet were characterized by markedly distinct, phase-specific dynamics. The average DIN concentration was at its lowest during 2003-2005, increased sharply over the subsequent seven years, then exhibited a clear declining trend from 2014 to 2022. In the most recent two years, however, the concentration rose again, reaching the peak level for the entire period. The average DIP concentration remained at the lowest level from 2003 to 2014, followed by a marked increase during 2015-2020. Notably, a substantial rise in DIP concentration occurred again in the recent period from 2021 to 2024. Spatially, the distributions of nitrogen and phosphorus were highly overlapping, both showing a decreasing gradient from the inner to the outer bay, with a more pronounced gradient for nitrogen. Principal component analysis confirmed that terrestrial input is the primary driver controlling nutrient distribution. Eutrophication assessment indicated that the bay-head area remained in a eutrophic state for 18 of the past twenty-two years. Discussion: These regular changes in nutrients are closely related to the historical development of China's economy and environmental governance. This study provides a long-term case for understanding the historical evolution of nutrients in key bays, and complements the comparative understanding with existing long-term studies.

As an important port in the Beibu Gulf of the South China Sea, Tieshangang Bay is potentially at risk of PTE pollution, yet systematic research integrating multi-hydrological period data remains limited. By applying pollution indices (C_f, WQI, I_geo, RI) combined with PCA, and PMF, we investigated PTE distribution characteristics, risk assessment, and source apportionment across different hydrological seasons. The results indicate that average PTE concentrations in surface seawater meet Class II standards of the Sea Water Quality Standard, with Zn and As showing relatively high concentrations compared to other PTEs. High-concentration areas were mainly located in the inner and middle bay. In sediments, concentrations of Zn and Cr were relatively high, with values generally higher inside the bay than outside. Both C_f and WQI values for seawater PTEs were below 1, indicating an overall low pollution risk. However, Cd and Hg in sediments presented a moderate potential ecological risk. Source apportionment revealed that seawater PTEs primarily originated from an industrial–aquaculture composite source (44.60%), while sediment PTEs were mainly attributed to composite terrestrial inputs (53.16%). These findings provide a scientific basis for PTE pollution management and sustainable development in Tieshangang Bay.