Isotopic evidence for the shift of nitrate sources and active biological transformation on the western coast of Guangdong Province, South China

Sources and seasonal variations of nitrate in the coastal multiple-aquifer groundwater of Beihai, southern China

Elevated nitrate (NO3-) loadings in groundwater may cause health effects in drinking water and nutrient enrichment of aquatic ecosystems. To reveal the sources and seasonal variations of NO3- in the coastal groundwater of Beihai, southern China, we carried out hydrochemical and isotopic (δ15N-δ18O in NO3-) investigations in the summer and winter, respectively, concerning multiple-aquifer groundwater, rainwater, seawater, and surface water. The sources of the main elements present in the waters were interpreted by ionic ratios. NO3- sources were identified by combined use of the δ15N values and δ18O values or NO3-/Na+ molar ratios, with estimations of the proportional contribution by the Bayesian stable isotope mixing model. Denitrification was interpreted along the flow paths. The results show groundwater main elements are originated primarily from silicate weathering, and secondarily from anthropogenic inputs and carbonate dissolution. Its qualities are largely affected by seawater intrusion along the coastline. Because of difference in the predominant minerals within the aquifers and in scale and extent of seawater intrusion, the groundwater displays distinct ionic ratio characters. NO3- concentrations are up to 33.9 mg/L, with higher loadings in the plains relative to along the coastline. Soil N, domestic sewage, rainwater, chemical fertilizers, and algae are NO3- sources, with average proportional contributions of 0.255, 0.221, 0.207, 0.202, and 0.116, respectively. In relation to the winter, higher production of NO3- from nitrification of soil N- and algae-derived ammonium induced by higher temperatures in the summer accounts for increases in groundwater NO3- loadings. In the rural areas, elevated loadings of NO3- in the winter may be due to larger infiltration fractions of sewage. Seasonal variations of atmospheric NO3- deposition and farming may also cause the dynamics. Our results improve the understanding of sources and seasonal dynamics of NO3- in coastal groundwater.

Identifying the external N and Hg inputs to the estuary ecosystem based on the triple isotopic information (δ15NNO3, Δ17ONO3 and δ18ONO3)

The supply and sources of N and Hg in the Geum estuary of the western coast of Korea were evaluated. Triple isotope proxies (δ15NNO3, Δ17ONO3 and δ18ONO3) of NO3- combined with conservative mixing between river and ocean waters were used to improve isotope finger-printing methods. The N pool in the Geum estuary was primarily influenced by the Yellow Sea water, followed by riverine discharge (821 × 106 mol yr-1) and atmospheric deposition (51 × 106 mol yr-1). The influence of the river was found to be greater for Hg than that of the atmosphere. The triple isotope proxies revealed that the riverine and atmospheric inputs of N have been affected by septic wastes and fossil fuel burning, respectively. From the inner estuary towards offshore region, the influence of the river diminishes, thus increasing the relative impact of the atmosphere. Moreover, the isotope proxies showed a significant influence of N assimilation in February and nitrification in May.

Identification of nitrate accumulation mechanism of surface water in a mining-rural-urban agglomeration area based on multiple isotopic evidence

The accumulation of nitrate (NO3-) in surface waters resulting from mining activities and rapid urbanization has raised widespread concerns. Therefore, it is crucial to develop a nitrate transformation information system to elucidate the nitrogen cycle and ensure sustainable water quality management. In this study, we focused on the main river and subsidence area of the Huaibei mining region to monitor the temporal and spatial variations in the NO3- content. Multiple isotopes (δD, δ18O-H2O, δ15N-NO3-, δ18O-NO3-, and δ15N-NH4+) along with water chemistry indicators were employed to identify the key mechanisms responsible for nitrate accumulation (e.g., nitrification and denitrification). The NO3- concentrations in surface water ranged from 0.28 to 7.50 mg/L, with NO3- being the predominant form of nitrogen pollution. Moreover, the average NO3- levels were higher during the dry season than during the wet season. Nitrification was identified as the primary process driving NO3- accumulation in rivers and subsidence areas, which was further supported by the linear relationship between δ15N-NO3- and δ15N-NH4+. The redox conditions and the relationship between δ15N-NO3- and δ18O-NO3-, and lower isotope enrichment factor of denitrification indicated that denitrification was weakened. Phytoplankton preferentially utilized available NH4+ sources while inhibiting NO3- assimilation because of their abundance. These findings provide direct evidence regarding the mechanism underlying nitrate accumulation in mining areas, while aiding in formulating improved measures for effectively managing water environments to prevent further deterioration.

Nitrate budget of a terrestrial-to-marine continuum in South China: Insights from isotopes and a Markov chain Monte Carlo model

Anthropogenic nitrate (NO3-) production has been increasing and is exported to the ocean via river networks, causing eutrophication and ecological damage. While studies have focused on river NO3- pollution, what has been lacking is the quantification of the sources of NO3- in coastal rivers. This study applied the dual isotopes (δ15N/δ18O-NO3-) to quantify the sources and their fluxes of NO3- in two inflow rivers of the Qinzhou Bay. By adding our results to the NO3- source apportionment in Qinzhou Bay, we, for the first time, established the NO3- budgets of the terrestrial-to-marine continuum in both high- and low-flow seasons. We quantitatively showed the direct and indirect roles (e.g., the stimulation of nitrification by sewage ammonium-NH4+) of terrestrial sources in driving the high NO3- loading in the estuary. The results highlighted the necessity to consider coastal rivers and estuary as a whole, which could shed light on the effective reduction of NO3- pollution in coastal environments.

Incorporating marine particulate carbon into machine learning for accurate estimation of coastal chlorophyll-a

Accurate predictions of coastal ocean chlorophyll-a (Chl-a) concentrations are necessary for dynamic water quality monitoring, with eutrophication as a critical factor. Prior studies that used the driven-data method have typically overlooked the relationship between Chl-a and marine particulate carbon. To address this gap, marine particulate carbon was incorporated into machine learning (ML) and deep learning (DL) models to estimate Chl-a concentrations in the Yang Jiang coastal ocean of China. Incorporating particulate organic carbon (POC) and particulate inorganic carbon (PIC) as predictors can lead to successful Chl-a estimation. The Gaussian process regression (GPR) model significantly outperforming the DL model in terms of stability and robustness. A lower POC/Chl-a ratio was observed in coastal areas, in contrast to the higher ratios detected in the southern regions of the study area. This study highlights the efficacy of the GPR model for estimating Chl-a and the importance of considering POC in modeling Chl-a concentrations.

Source apportionment of nitrate in the Pearl River Estuary using δ15N and δ18O values and isotope mixing model

The mitigation of eutrophication in the Pearl River Estuary (PRE) has encountered numerous challenges in regards to source control. Herein, the isotope mixing model (SIAR) was used to quantify the primary nitrate sources in the PRE. The results showed that the nitrate levels were significantly higher in the high-flow season than in the low-flow season. Meanwhile, we found the most important nitrate sources were manure and sewage during the high-flow season, with a contribution ratio of 47 % in the low salt area (LSA) and 29 % in the high salt area (HSA). During the low-flow season, the primary nitrate sources were identified as reduced nitrogen fertilizer in the LSA and manure and sewage in the HSA, which accounted for 52 % and 44 %, respectively. Furthermore, we also suggest that a feasible measure might be to control the pollution caused in the PRE by manure and sewage as well as reduced nitrogen fertilizer.

Distribution and sources of microplastics in the Beibu Gulf using in-situ filtration technique

The Beibu Gulf is a vital link between China and the ASEAN nations, and microplastic contamination is rising due to fast growth, coastal life, fisheries, and mariculture. The abundance, distribution, and source analyses were conducted at 25 sample points for this study. According to this study, the average MPs was 0.25 ± 0.05 items/m³, ranging from 0.01 items/m³ to 0.89 items/m³. Fibers, white, cellulose, and 0.33-1 mm were abundant in shape, color, composition, and size, respectively. Multi-statistics-based source analysis indicated land-based inputs (packing materials, textile materials, fisheries, and mariculture) were dominant in the Beibu Gulf. In this study, we also acknowledged a comprehensive comparison and convenience between plankton pumps and other conventional designs to collect microplastic samples from water. We suggested that using a uniform design could elevate the data quality of microplastics.

Increasing intrusion of high salinity water alters the mariculture activities in Zhanjiang Bay during the past two decades identified by dual water isotopes

The decrease of river runoff caused by the intensified human activities (e.g. artificial dams) and increasing intrusion of high salinity water in the coastal bays have become a worldwide environmental problem. However, the mixing can hardly be identified by traditional method with temperature and salinity due to the complicated water sources in the coastal area. Thus, it is difficult to quantify the impact of intrusion of high salinity water on coastal ecological environment. Here, seasonal dual water isotopes (δD and δ¹⁸O), hydrographic parameters, and nutrients were investigated in a typical semi-enclosed mariculture bay in South China Sea (SCS), to quantify the intrusion of high salinity water and its impact on the water environment. The results showed that salinity in the bay has increased significantly (18%) over the past two decades due to the decrease of runoff and dredging activity. Zhanjiang Bay is mainly affected by the seawater from the SCS in outer bay, and the seawater from the outer bay (89%) was significantly higher than that of freshwater (7%) in summer, despite the increase in freshwater input from the river during this period. In winter, the intrusion of high salinity water increased (accounting for 94%) due to the decrease of runoff input. However, the contribution of groundwater was similar in summer (4%) and winter (5%). The estimation results from the relationship of δ¹⁸O-salinity and δD-salinity showed that the intrusion of high salinity water has increased significantly for the past two decades (increased by 23%). This resulted in the area suitable for oyster breeding is decreasing, and the oyster breeding activities have been gradually moving to the inner bay. Moreover, the nutrients in Zhanjiang Bay were mainly originated from freshwater input in summer (54%-90%), while it changed to the SCS input from the outer bay in winter (40%-97%). This study suggests that the intrusion of high salinity water significantly increases the salinity, and seriously retains the pollutants of freshwater in the bay, which poses a great threat to the oyster breeding activities in the semi-enclosed bay.

Impact of intensive mariculture activities on microplastic pollution in a typical semi-enclosed bay: Zhanjiang Bay

Microplastic (MP) was investigated in Zhanjiang Bay, a semi-enclosed bay in south China and famous for considerable mariculture industry, to evaluate whether mariculture activities accelerated MP pollution. The MP abundances ranged from 0 to 2.65 n/m³ (number/m³), showing seasonal variances with higher levels in May and September and lower levels in January. In the inner part of the bay, a significantly high MP abundance and predominance of foam were found during the oyster breeding period, and pollution sources were prone to be single and extensive. This suggested that MPs were strongly influenced by the intensive plastic products for oyster culturing, especially during breeding. Moreover, plastic cages used for culturing were the main source of MPs in the central part of the bay. By conducting statistical analysis for eight representative bays, the economic growth, social development, agriculture structure, and aquaculture development were supposed to influence the local MP pollution level.

Pollution assessment and sources of dissolved heavy metals in coastal water of a highly urbanized coastal area: The role of groundwater discharge

Heavy metal concentrations and physicochemical parameters in coastal waters were measured to analyze the spatial distribution characteristics, pollution degrees, and sources of heavy metals in the heavily urbanized Guangdong-Hong Kong-Macao Greater Bay Area (GBA) in China. Heavy metal concentrations in the eastern GBA were higher than those in the west, and the levels of Pb and Zn in seawater were higher than those in groundwater and river water. Both the pollution factors and comprehensive water quality index demonstrated that seawater was not contaminated with As, Cd, Cr, and Ni, whereas low to considerable levels of contamination of Pb and Zn were observed in the central and eastern sections of the GBA. Multiple statistical analyses suggested that the Pb and Zn contaminations in seawater were probably derived from atmospheric deposition and human activities, and the excess amounts of As, Cd, Cu, Ni, and Zn in groundwater were attributed to anthropogenic activities. The heavy metal fluxes from submarine groundwater discharge (SGD) were comparable to, or even greater than, those from local rivers. Therefore, SGD is a significant invisible contributor of heavy metals into the coastal ocean that has often been overlooked in comparison to other visible pollution sources. This study suggests that SGD should be considered in the assessment of heavy metal pollution and future water quality management protocols in marine ecosystems.

Coastal currents regulate the distribution of the particulate organic matter in western Guangdong offshore waters as evidenced by carbon and nitrogen isotopes

The δ¹³C, δ¹⁵N and C/N ratio of the particulate organic matter (POM) in western Guangdong waters were determined to evaluate the impacts of the coastal currents on the POM in spring and summer. The predominance of photosynthetic organic matter in the nearshore was triggered by nutrients brought by the coastal currents in spring and summer, while the proportion of terrestrial organic matter in the offshore was very high in spring but low in summer. In spring, the weaker and narrower coastal currents carried insufficient nutrients (phosphate deficiency) to the offshore and prohibited phytoplankton production. This scenario contributes to the dominance of terrestrial organic matter transported by the cyclonic circulation beyond the coastal currents in the offshore in spring. The Bayesian mixing model reveals that the proportion of terrestrial organic matter (with 75.8% of C₃ plants) in the offshore was higher in spring than in summer (with 33.7% of C₃ plants).

Seasonal Sources and Cycling of Nitrogen Revealed by Stable Isotopes in the Northeastern Beibu Gulf, China

Isotope measurements were performed on dissolved nitrate (NO3−) and ammonium (NH4+) in the coastal waters of the northeastern Beibu Gulf, China, to investigate the seasonal nitrate sources and their biogeochemical processes, which are due to the rapid development of local industrialisation and urbanisation. The high N/P ratio observed in the coastal bay during both fall and spring suggests that P is a limiting nutrient, which in turn indicates that increasing P causes conditions favourable for algal blooms. Higher nutrient concentrations and δ15N-NO3− and δ15N-NH4+ values were found in the nearshore area in the fall, suggesting that nutrients originated mainly from land-based pollution. A Bayesian isotope mixing model was used to calculate the contribution of potential NO3− sources and the results showed that in the nearshore area, NO3− originated mainly from manure and sewage (58%). In the spring, however, in addition to the impact of urban sewage effluents, the exchange of sediment and water was another important factor causing higher nutrient concentrations and positive NO3− isotopes in the nearshore area. There were lower concentrations of nutrients and an increase in δ15N-NO3− and δ15N-NH4+ values in the offshore area in the fall, and the NO3− loss in the surface water was mainly caused by the process of assimilation. However, the exchange of sediment and water was the dominant factor causing higher nutrient concentrations (except for NO3−) and positive dual nitrate isotopes but lower NO3− concentration in the offshore area during the spring. Overall, isotope analysis of NO3− and NH4+ helps to illustrate the major sources of the former and their biological transformation in the northeastern Beibu Gulf.

Carbon and nitrogen isotopic compositions of particulate organic matter in the upwelling zone off the east coast of Hainan Island, China

The isotopic compositions (δ¹³C and δ¹⁵N) and C/N ratios of suspended particulate organic matter (POM) were investigated off the east coast of Hainan Island in the South China Sea during summer. Coastal upwelling influenced the nearshore stations of transects S2 and S3, and higher δ¹³C and δ¹⁵N values suggested that coastal upwelling played a significant role in determining the POM sources. The POM at the nearshore area of transect S1 was controlled by the coastal current and freshwater discharge. Additionally, organic matter may be transported to the offshore area via tidal movements in transects S1 and S3. Based on the stable isotope analysis in an R model, the marine organic matter contribution in the upwelling area (19%) was higher than that in the other areas (transect S1 and the offshore area) (7%). The δ¹³C and δ¹⁵N values and C/N ratios reflect the carbon and nitrogen sources and their cycling in the upwelling zone off the east coast of Hainan Island.

The deep challenge of nitrate pollution in river water of China

Nitrate pollution of surface water has attracted global attention, and the issue is becoming increasingly significant in China. To identify the pollution status, sources, and potential non-carcinogenic health risks of nitrate in China's river water, nitrate data from 71 major rivers from 30 provinces were systematically collected. The spatial distribution of nitrate concentrations in river water was analyzed, and the main nitrate pollution sources were revealed based on the presence of nitrogen and oxygen isotopes of nitrate. The results show that approximately 7.83% of samples in China exceeded the national drinking water standard for nitrate (45 mg/L). The concentrations of nitrate in Mudan River (Linkou County), Haihe (Beijing), and Yangtze River estuary (Shanghai) exceed 90 mg/L, which indicates severe pollution. The characteristic values of δ¹⁵N and δ¹⁸O of river water in China range from -23.5‰ to 26.99‰ and - 12.7‰ to 83.5‰, indicate many sources including inorganic fertilizer, soil nitrogen, wastewater or manure. The primary sources of nitrate in river water of Northeast, Northwest, Southwest, and South China were manure, septic waste, inorganic fertilizer, and soil organic matter nitrification. Manure and septic waste were the major source of nitrate in Central, East, and North China. Correlation analysis revealed that the nitrate concentrations of surface water has a positive relationship with GDP, nitrogen fertilizer application usage, wastewater discharge, and population in China. Non-carcinogenic risk of nitrate was identified in 80% of the regions in China, and potential moderate non-carcinogenic risk areas are Shanghai, Beijing, and Shaanxi. It is urgent to solve the problem of pollution and prevent the further pollution of China's river water. Though the new "10-point Water Plan" issued by the Chinese government solved previous problems, it will take decades to control and repair polluted surface water.

Radium and nitrogen isotopes tracing fluxes and sources of submarine groundwater discharge driven nitrate in an urbanized coastal area

The quantitative evaluations of nutrients delivered by submarine groundwater discharge (SGD) have been widely conducted worldwide, but sources of nutrients in the discharged submarine groundwater remain unclear. Identifying these sources of nutrients is essential to the protection and management of marine ecological environments. This study aims to evaluate the magnitudes of SGD and the associated nitrate in the Guangdong-Hongkong-Macao Greater Bay Area (GHM Greater Bay Area), China, and identify the sources of SGD-driven nitrate in this region using radioactive radium (Ra) isotopes (²²³Ra, ²²⁴Ra, and ²²⁸Ra) and stable nitrogen (N) and oxygen (O) isotope composition of nitrate (δ¹⁵N-NO₃^- and δ¹⁸O-NO₃^-). The results of the Ra mixing model show that the estimated SGD and the associated nitrate fluxes into the Greater Bay Area are (9.15 ± 1.26) × 10⁸ m³/d and (3.77 ± 0.52) × 10⁷ mol/d, respectively, both of which are comparable to the contributions from the Pearl River. Combing NO₃^- dual isotopic signatures of sampled coastal groundwater and five kinds of potential nitrate sources, we found that ammonium (NH₄⁺) fertilizer and natural soil N are the two main sources of nitrate in discharged submarine groundwater and rivers. No anthropogenic inputs from manure or sewage waste were identified. This study provides significant insights into the establishment of effective management strategies for controlling SGD-nutrients into the bay and protecting the marine ecological environment.

Assessing Seasonal Nitrate Contamination by Nitrate Dual Isotopes in a Monsoon-Controlled Bay with Intensive Human Activities in South China

Nitrate (NO₃⁻) dual isotope analysis was performed in Zhanjiang Bay, which is a closed bay with intensive human activities in South China, to investigate seasonal changes in the main NO₃⁻ sources and their biogeochemical processes in the monsoon-controlled climate. The relatively low N/P ratios in Zhanjiang Bay suggests that nitrogen (N) is a limiting nutrient, which indicates that the increase of N is favorable for phytoplankton proliferation. However, a sufficient amount of ammonium was found in our study area owing to intensive human activities, which can support biological processes. Thus, less NO₃⁻ biological processes were found, indicating that NO₃⁻ isotopic characteristics may reveal details of the mixing from various sources. The Bayesian mixing model showed that NO₃⁻ in the upper bay originated from manure (43%), soil N (30%), N fertilizer (17%), and N precipitation (10%) during winter, which reflects the local human activities; while NO₃^- sources during summer were mainly N fertilizer (36%), soil N (32%), and manure (31%), indicating the source as the runoff from the upper river basin. Our results suggest that nitrate dual-isotope was very useful for tracing the main NO₃⁻ sources in the condition of the sufficient ammonium, and runoff exerted an important impact on the shift in NO₃⁻ sources between both the local source and the source from the upper river basin during the two seasons in this monsoon-controlled bay.

Typhoon Merbok induced upwelling impact on material transport in the coastal northern South China Sea

Typhoons frequently affect the South China Sea (SCS), playing an important role in changing the coastal marine system. To determine which process has the greatest impact on material transport in the coastal marine area during a typhoon, the sea temperature, salinity, concentration of nutrients, chlorophyll-a, total suspended matter, and δ¹³C of particulate organic carbon (δ¹³C-POC) in the water column of coastal Northern SCS (NSCS) were measured during two cruises in June 2017, in the pre- and post-typhoon Merbok periods. The results show that all parameters changed significantly between the two periods. During the pre-typhoon period, stratification of nutrients and physicochemical parameters, combined to high nutrient concentrations, high temperature, and low salinity in the water column of the nearshore area, suggests that the nearshore area is influenced by the river diluted water originated in the coastal cities adjacent to our study area. In the offshore area, mineralization may be responsible for the high nutrient concentration in the bottom water. However, during the post-typhoon, the stratification of nutrients is less significant and their distribution more homogenous in the whole water column of the nearshore area. In the upper water, the nutrient concentration increased and the temperature decreased significantly. These results suggest that the enhanced vertical mixing induced by the typhoon was the dominant process in changing the nutrient distribution pattern in the coastal NSCS.

Analysis of Dissolved Oxygen and Nutrients in Zhanjiang Bay and the Adjacent Sea Area in Spring

As a semi-closed bay with narrow bay mouths, the distribution of nutrients in Zhanjiang Bay was different from bays with open bay mouths and rivers with large flows. It is important to study the water quality of Zhanjiang Bay to determine the impact of human activities on this semi-closed bay. Based on field survey data in spring, the spatial distribution of nutrients and other physico-chemical parameters was investigated, in order to study the geochemical characteristics of nutrients in semi-closed bays. Higher nutrient concentrations were observed in the inner and outer bays, but lower concentrations were observed at the bay mouth. With other analyses of physico-chemical parameters, the higher nutrient concentrations in the inner bay originated mainly from the diluted freshwater input from local developments and rivers. With the strong flow that exists along the western coast of Guangdong Province, the higher dissolved inorganic nitrogen (DIN) and SiO3–Si concentrations along the outer bay may be influenced by discharge from local cities in western Guangdong Province. There was stronger phytoplankton assimilation at the bay mouth, which resulted in reduced nutrient concentrations in this area. Although the hydrographic characteristics between the inner bay and outer bay were significantly different, the distribution of chlorophyll-a (Chla) levels was similar. However, we found significantly low dissolved oxygen (DO) and high apparent oxygen utilisation (AOU) consumption levels in the inner bay, and high DO and low AOU levels in the outer bay, which suggested that decomposition was more important than photosynthesis in the closed bay, even in spring during the phytoplankton bloom.