Submarine Groundwater and River Discharges Affect Carbon Cycle in a Highly Urbanized and River-Dominated Coastal Area

Riverine carbon flux to the ocean has been considered in estimating coastal carbon budgets, but submarine groundwater discharge (SGD) has long been ignored. In this paper, the effects of both SGD and river discharges on the carbon cycle were investigated in the Guangdong-HongKong-Macao Greater Bay Area (GBA), a highly urbanized and river-dominated coastal area in China. SGD-derived nitrate (NO3–), dissolved organic carbon (DOC), and dissolved inorganic carbon (DIC) fluxes were estimated using a radium model to be (0.73–16.4) × 108 g/d, (0.60–9.94) × 109 g/d, and (0.77–3.29) × 1010 g/d, respectively. SGD-derived DOC and DIC fluxes are ∼2 times as great as riverine inputs, but SGD-derived NO3– flux is one-fourth of the riverine input. The additional nitrate and carbon inputs can stimulate new primary production, enhance biological pump efficiency, and affect the balance of the carbonate system in marine water. We found that SGD in the studied system is a potential net source of atmospheric CO2 with a flux of 1.46 × 109 g C/d, and river, however, is a potential net sink of atmospheric CO2 with a flux of 3.75 × 109 g C/d during the dry winter season. Two conceptual models were proposed illustrating the major potential processes of the carbon cycle induced by SGD and river discharges. These findings from this study suggested that SGD, as important as rivers, plays a significant role in the carbon cycle and should be considered in carbon budget estimations at regional and global scales future.


INTRODUCTION
Nutrient budgets in the ocean are subject to a variety of influences such as surface rivers, submarine groundwater discharge (SGD), and the atmosphere. River is a visible pathway of biogenic substances such as nitrogen, phosphorus, and carbon from the land to the ocean, and plays a vital role in the global carbon and nitrogen cycles (Cole et al., 2007;Dai et al., 2012).
The global riverine carbon flux was estimated to be (1.9-2.7) × 10 15 g/year, in which the organic component accounts for approximately 45% (Cole et al., 2007;Cai, 2011). As the largest river in the Guangdong-HongKong-Macao Greater Bay Area (GBA), the Pearl River delivers about 1.27 × 10 13 g/year of carbon to the estuary and coastal area, in which dissolved inorganic carbon (DIC) and dissolved organic carbon (DOC) account for more than 70% (Zhang et al., 2013). Many studies were conducted to investigate the carbon cycle in the Pearl River Estuary (He et al., 2010;Su et al., 2017;Liang et al., 2020). The Guangdong-HongKong-Macao GBA, one of the significant bay development areas alongside New York, San Francisco (United States), and Tokyo (Japan), is a highly developed and urbanized region in China. With the rapid economic development and urbanization in the GBA, the regional water environment has been affected seriously by human activities. For instance, Liu et al. (2020) showed that the organic C/N ratio in the Pearl River decreased from 11.8 to 5.0 after the river passed through several big cities. Such variations in nutrient/carbon flux and composition will change the carbon cycle (e.g., stimulating new primary production and enhancing biological pump efficiency) in coastal oceans, and have a significant implication for estimating coastal carbon budgets Ye et al., 2021).
Submarine groundwater discharge, an important part of the hydrological cycle, is an invisible pathway of materials (i.e., nutrients, metals, and rare earth elements) into the ocean Johannesson et al., 2011;Kim et al., 2011;Luo and Jiao, 2016;Rodellas et al., 2017;Wang et al., 2018;Zhang et al., 2020). The latest study including more than 200 coastal sites worldwide shows that SGD delivers more nutrients (nitrogen, phosphorus, and silicon) than rivers in most (∼60%) of the coastal areas studied (Santos et al., 2021). The study finds that SGD-driven nutrients can enhance primary productivity, fish production, or coral calcification, but on the other hand, can lead to eutrophication of coastal waters, algal blooms or hypoxia events. Therefore, SGD can be regarded as an important indicator of the functioning and vulnerability of coastal ecosystems and is of great significance to the survival and maintenance of coastal societies at local and regional scales, even more at a global scale (Sawyer et al., 2016;Michael et al., 2017;Cho et al., 2018;Alorda-Kleinglass et al., 2021).
Similar to rivers, SGD also carries carbon into the oceans and plays an important role in the marine carbonate system (Cole et al., 2007;Liu et al., 2012;Stewart et al., 2015). A growing number of scientists have devoted more intensive attention to this aspect in recent years. Many studies investigated SGDderived carbon flux [e.g., total alkalinity (TA), DIC] in different systems such as mangrove, estuary, salt marsh, and embayment (Liu et al., 2014(Liu et al., , 2017Sadat-Noori et al., 2016;Chen et al., 2018;Xiao et al., 2020;Dai et al., 2021;Wu et al., 2021). In the northern South China Sea, Liu et al. (2012) and Dai et al. (2021) estimated the SGD-derived DIC fluxes in different scales and they found that SGD is an important contributor to DIC. However, the effects of SGD on the marine carbon budgets are usually ignored in most studies (Liu et al., 2018;Ye et al., 2021;Yu et al., 2021) since it usually occurs below the seawater surface and is almost invisible and highly variable. In addition, the study on the SGD-derived carbon in the GBA is very limited. Thus, understanding the contribution of both SGD and river to carbon cycle is of great significance to environmental assessment and management in the region.
The objectives of this study were to evaluate the carbon flux supplied by SGD and river, and reveal their potential impacts on the marine carbon cycles. Firstly, SGD-derived nitrate (NO 3 − ) and carbon (DOC, DIC) fluxes were estimated using radium isotopes ( 224 Ra, 223 Ra, and 228 Ra) as tracers. Then the importance of SGD and river discharges on the carbon budget and their contributions to the new production and carbon cycling were discussed. Finally, two conceptual models were proposed illustrating the major potential processes of the carbon cycle induced by SGD and river discharges.

Study Area
The GBA, with a total land area of ∼56,000 km 2 (including nine cities in Guangdong province and two Special Administrative regions: HongKong and Macao) and a coastal length of ∼1,480 km, is located in the northern South China Sea (Figure 1). There are one important estuary in southern China (Pearl River Estuary) and many important bays including Daya Bay, Dapeng Bay as well as Tolo Harbor in HongKong. The Pearl River, with an annual runoff of 3.26 × 10 11 m 3 /yr and suspended particles matter (SPM) load of 5.0 × 10 10 g/d, is the largest river into the South China Sea through eight outlets . During the dry winter season, the coastal current flows from northeast to southwest, and the Pearl River flow turns to the west. Rainfall in this region is abundant and occurs mostly (80%) during the wet summer season (from April to September).

Sample Collection and Measurements
Fieldwork was carried out during the dry winter season (January 6-13, 2020, Supplementary Table 1). The sampling sites within 30 seawater from three transects (A1, A2, and A3), 24 coastal groundwater, and 7 rivers are shown in Figure 1. Water samples for radium extraction were collected in a large volume (60 L for seawater, 30 L for river water, ∼5 L for coastal groundwater) and drained slowly (<1 L/min) through manganese-coated acrylic fiber (Mn-fiber) that absorb dissolved radium from water (Moore, 1976). Nitrate samples were filtered through 0.45 µm filters and collected with 45 ml Nalgene sampling bottles. DIC and DOC samples were filtered through 0.22 µm filter membranes, poisoned with HgCl and H 3 PO 4 solution, respectively, and stored in 45 mL sampling vials. These samples were refrigerated at 4 • C before analysis. The values of pH, oxidation-reduction potential (ORP), and salinity in water were immediately measured in situ using a multi-parameter water quality analyzer (HI9829T, HANA). 224 Ra, 223 Ra, and 228 Ra were detected by radium delayed coincidence counting system (RaDeCC; Moore, 2008). The uncertainties for radium measurements were 7% for 224 Ra and 228 Ra, and 12% for 223 Ra. Analyses for nitrate were carried out colorimetrically using a flow injection analyzer (FIA) with a detection limit of 5 µg/L (HACH QC8500, American). DOC and DIC were determined by a Total Organic Carbon/Nitrogen analyzer (Multi N/C 3100 Analyzer, Germany). The uncertainties of DOC and DIC concentration were better than 3%.

Submarine Groundwater Discharge-Derived Solute Fluxes Based on 228 Ra Tracing Model
Radium isotopes ( 224 Ra, 223 Ra, 228 Ra, and 226 Ra), with the halflife from 3.6 days to 1,600 years, are widely used to trace SGD fluxes at different scales because of their enrichment in SGD (Rodellas et al., 2015;Wang et al., 2015;Zhang et al., 2016). Garcia-Orellana et al. (2021) summarized the application of radium isotopes tracing SGD and associated solute fluxes. Three models, radium mass balance model, radium endmember mixing model, and eddy diffusive mixing model, are usually applied to estimate SGD fluxes. The basic strategies of these models to estimate SGD is to determine first the radium flux supplied by SGD, and subsequently convert it into SGD flux by characterizing the groundwater radium endmember. Radium mass balance model which considers all the potential radium sources and sinks is the most widely used Wang et al., 2018).
The eddy diffusive mixing model used in this study is suitable for open coastal systems.
The long-lived 228 Ra was used to assess SGD flux into the GBA. The total input of 228 Ra in the system is mainly controlled by SGD and river discharges; the input from sediments is negligible due to the long half-life of 228 Ra. Thus SGD-derived 228 Ra (F Ra−228 SGD ) can be calculated by subtracting riverine 228 Ra (F Ra−228 R ) from the total input (F Ra−228 Total ) (Eq. 1a). The 228 Ra flux attributed to river was calculated as the product of the activity of 228 Ra (C Ra−228 R−i ) in the river and the corresponding river discharge (Q R−i ) (Eq. 1b). The total 228 Ra input to the system was balanced by the offshore export of 228 Ra, which was calculated as the product of the eddy diffusion coefficient (K h ), the offshore gradient of 228 Ra (g Ra−228 Sea ), and the cross-sectional area (A) of the system (Eq. 1c).
where K h can be determined by an advection-diffusion model of short-lived 224 Ra and 223 Ra; n is the number of rivers in the region. The 224 Ra and 223 Ra distribution can be described as follows if ignoring the advection (Hancock et al., 2006;Wang et al., 2021): where C Ra Sea is 224 Ra or 223 Ra activity in seawater (dpm/100 L), x and D are offshore distance and seawater depth (m), respectively; l is the decay constant of 224 Ra or 223 Ra (d −1 ), and B is the sedimentary radium flux [dpm/(m 2 d)]. The boundary conditions, the radium fluxes across the coastline (x = 0) to be F 0 and at the shelf edge (x = x e ) to be lim x→x e ∂C Ra Sea ∂x = 0, were given to solve this equation. Integration of Eq. 2a with respect to x yields: The values of K h can be determined when we obtained optimal agreement between measured and modeled radium activities ( 224 Ra and 223 Ra) for each transect. SGD-derived nitrate or carbon flux (F N ) is commonly calculated by multiplying the SGD flux (Q SGD ) by the concentration of nitrate or carbon in coastal groundwater . It can be represented by the following equation: where C N and C Ra−228 GW are solute (nitrate, DOC, and DIC) concentration and 228 Ra activity in coastal groundwater, respectively.
The common method to estimate SGD-derived solute (nitrate and carbon) fluxes is to calculate first the SGD flux by dividing SGD-associated radium flux by radium activity in groundwater (i.e., radium endmember), and then multiply the SGD flux by solute concentration in groundwater (i.e., solute endmember). During this process, the determination of the two endmembers is usually major source of uncertainty for the final SGD-derived solute fluxes because of the spatial and temporal variability of radium and solutes in groundwater (Michael et al., 2011;Waska et al., 2019;Wang et al., 2020). To reduce the uncertainty of SGD-derived nitrate/carbon flux estimations, in this study, the term C N /C Ra−228 GW in Eq. 3 was regarded as a unified parameter to discuss later.

Radium Isotopes
The spatial distributions of radium ( 224 Ra, 223 Ra, and 228 Ra) and salinity in seawater and coastal groundwater are shown in Figure 2. In general, the high radium activity and low salinity occurred in the nearshore area, whereas the low radium activity and high salinity occurred in the offshore area. Seawater salinity ranged from 14.49 to 34.99 PSU (Practical Salinity Unit) and it was very low in the western nearshore area resulting from the Pearl River freshwater dilution ( Figure 2D). The radium activity of seawater ranged from 11.72 to 113.72 dpm/100 L for 224 Ra, from 0.15 to 4.76 dpm/100 L for 223 Ra, and from 8.04 to 80.41 dpm/100 L for 228 Ra (Supplementary Table 1). Coastal groundwater, however, had very high radium activity within 224 Ra: 0.81-119.2 dpm/100 L, 223 Ra: 0.05-4.10 dpm/100 L, and 228 Ra: 0.17-34.46 dpm/100 L (Supplementary Table 2). As shown in Figure 3, 224 Ra had a strong linear relationship with 223 Ra and 228 Ra in both coastal groundwater and seawater. The coastal groundwater with high Ra activities and ratios ( 224 Ra/ 223 Ra and 224 Ra/ 228 Ra) was the main source of radium in the ocean.

Nitrate and Carbon
Solute concentrations of coastal groundwater ranged from 0.005 to 3.17 mg/L with a median of 0.19 mg/L for NO 3 − , from 1.00 to 7.83 mg/L with a median of 1.34 mg/L for DOC, and from 4.58 to 31.49 mg/L with a median of 13.56 mg/L for DIC. For the river water samples, solute concentrations ranged from 0.69 to 4.36 mg/L with a median of 1.22 mg/L for NO 3 − , from 1.08 to 1.82 mg/L with a median of 1.23 mg/L for DOC, and 9.50 mg/L for DIC (Figure 4 and Supplementary Table 3). The highest concentration of NO 3 − (4.36 mg/L) and DOC (1.82 mg/L) occurred in the Dan'ao River (R7) which suffered serious environmental pollution from high-intensity human activities . Solute concentrations in both coastal groundwater and river water are ranked as DIC>DOC>NO 3 − . Nitrate concentrations in coastal groundwater are less than that in river water, while carbon contents in coastal groundwater are slightly greater than that in river water (Figure 4).

Eddy Diffusion Coefficient
In this study, the eddy diffusion coefficient was estimated by using Eq. 2. All the values used in the Eq. 2 are shown in Table 1. Based on the observed data, seawater depth (D) can be described by a linear relationship with offshore distance (x): D = 5.84 × 10 −4 x for A1, D = 3.58 × 10 −4 x for A2, and D = 5.43 × 10 −4 x for A3 ( Table 1). The optimal agreement between measured and modeled radium activities, as illustrated in Figure 5, was obtained within the application of the short-lived 224    values of K h derived from 223 Ra are less than that from 224 Ra which usually presents an accurate estimation for K h (Li and Cai, 2011;Wang et al., 2021).

Submarine Groundwater Discharge-Derived Nitrate and Carbon Fluxes Based on Radium Model
The total 228 Ra flux to the system (Eq. 1c) was calculated to be 6.13 × 10 12 dpm/d based on the derived K h (Table 1) and the linear gradient of 228 Ra ( Figure 6A) at three transects. The riverine 228 Ra flux (Eq. 1b) was estimated to be 1.68 × 10 12 dpm/d, which includes dissolved in the water and desorbed from SPM (Supplementary Table 4). Thus SGDderived solute fluxes (Eq. 3) were calculated to be (0.73-16.4) × 10 8 g/d with a median of 2.06 × 10 8 g/d for NO 3 − , (0.60-9.94) × 10 9 g/d with a median of 2.25 × 10 9 g/d for DOC, and (0.77-3.29) × 10 10 g/d with a median of 1.66 × 10 10 g/d for DIC. Here the ranges of NO 3 − / 228 Ra, DOC/ 228 Ra, and DIC/ 228 Ra ratios between the first and third quartiles of the coastal groundwater dataset ( Figure 6B) were used in Eq. 3. A previous study suggested that the sources of SGD-derived nitrate in the GBA were natural soil and anthropogenic discharge . SGD is a mixture of terrestrial freshwater and circulated seawater. The circulated seawater, accounting for a large part of SGD, is important for the delivery of solutes from land to the ocean.

Riverine Nitrate and Carbon Fluxes
The nitrate and carbon fluxes via rivers into the GBA were calculated as the product of the concentrations of nitrate and carbon in the river and the corresponding river discharge. In the GBA, the Pearl River is the dominant river with a flux of 8.92 × 10 8 m 3 /d and a mean concentration of 1.01 mg/L for NO 3 − , 1.18 mg/L for DOC, and 9.50 mg/L for DIC. Thus, the riverine NO 3 − , DOC, and DIC inputs were estimated to be 8.82 × 10 8 g/d, 1.05 × 10 9 g/d, and 8.48 × 10 9 g/d, respectively. Numerous studies have well-investigated cycles of nitrogen and carbon and their seasonality within the Pearl River and adjacent waters (Lu et al., 2009;He et al., 2010;Liu et al., 2020;Xuan et al., 2020;Ye et al., 2021). Liu et al. (2020) investigated the spatial distribution of organic carbon and nitrogen in the Pearl River. They found that both DOC and total organic nitrogen (TON) were increased from upstream to downstream, with an increase of five times for TON and two times for DOC in the dry season. The nitrogen and carbon fluxes from the current study are consistent with previous findings. Wang et al. (2021) quantified nitrate fluxes from SGD and rivers into the GBA in the wet summer season with values of (5.28 ± 0.73) × 10 8 g/d and 1.32 × 10 9 g/d, respectively. It can be found that nitrate fluxes from SGD and river to the GBA were greater in the wet summer season than in the dry winter season. This is the less discharge of river and groundwater during the dry season, which can be supported by the seawater salinity and pH distributions in the seawater. Figure 7 shows seawater salinity and pH in the dry and wet seasons. Both salinity and pH were less in the wet summer season than in the dry winter season, which indicated directly the decrease of terrestrial freshwater including river water and fresh groundwater.

Variability of Nitrate Fluxes and K h
The eddy diffusion coefficients for the GBA in the summer season were estimated by Wang et al. (2021) using the same method. The summer eddy coefficient was 21.04 km 2 /d for A1 transect, 88.4 km 2 /d for A2 transect, and 11.76 km 2 /d for A3 transect . The winter eddy coefficient determined in this study is about 4-10 times greater than the summer eddy coefficient. During the winter, the strong current will enhance horizontal mixing and result in relatively higher values for eddy mixing. Among three transects (A1-A3), A2 has the highest values of eddy diffusion coefficient during both summer and winter seasons. Transect A2 is the longest one and is more influenced by terrestrial freshwater (Pearl River). The force of Pearl River discharge could enhance horizontal mixing and thus create a relatively higher value of eddy diffusion coefficient. In addition, differences in scale can produce some of the variances in the eddy diffusion coefficient. The eddy diffusion term usually becomes larger at greater length scales (Moore, 2000).

Effects of Submarine Groundwater Discharge and River on Coastal Carbon System
Regarding carbon flux, it can be found that SGD-derived DOC and DIC fluxes are ∼2 times as great as riverine input, but SGD-derived NO 3 − flux is one-fourth of the riverine input. Anthropogenic input of nitrogen is the primary determinant of NO 3 − accumulation in the Pearl River (Xuan et al., 2020). DIC is the dominant form of carbon in SGD, and DOC concentration is below one-tenth of DIC in most groundwater samples (except for two sites, Figure 8A). The relationship between DOC and the residual NO 3 − is shown that DOC concentrations decreased while dissolved NO 3 − concentrations increased ( Figure 8B). This observation revealed that DOC could be consumed by biological production when they transport from land to sea. Waska et al. (2021) also found that the subterranean estuary is a net sink of terrestrial organic carbon. SGD-derived DIC and DOC have been reported in some coastal systems worldwide    such as Okatee Estuary, South Carolina, United States (Moore et al., 2006), Southwest Florida Shelf, United States (Liu et al., 2014), and Northern South China Sea (Tan et al., 2018). These findings suggested that SGD is a dominant source of DIC and DOC to coastal waters and may contribute significantly to coastal carbon budgets.
Submarine groundwater discharge-derived and riverine NO 3 − into the GBA can stimulate new primary production and enhance biological pump efficiency. Using the Redfield ratio of C/N = 106:16 to derive primary production in terms of carbon uptake, estimated potential primary production by SGD and river were 1.17 × 10 9 g C/d and 5.01 × 10 9 g C/d, which were 7.1 and 59.1% of SGD-derived and riverine DIC, respectively. The new primary production rates supplied by SGD showed a similar range to the study in the Daya Bay [i.e., 54-73 mg C/(m 2 d)], an important bay in the GBA .
Submarine groundwater discharge-derived and riverine DOC, once entering the coastal sea within the environment of sufficient inorganic nitrogen, could be easily consumed by biological production and thus cannot be stored as inert organic carbon. If assuming that 80% of the flux of SGD-derived and riverine DOC  will be remineralized, the remineralization process can produce CO 2 of 1.80 × 10 9 g C/d and 0.84 × 10 9 g C/d, respectively. The remineralization of SGD-derived DOC usually occurs at the bottom of the ocean which will potentially result in hypoxia and acidification at the bottom water body. Combined with the processes of nitrate and DOC, SGD is a potential net source of atmospheric CO 2 with a flux of 0.63 × 10 9 g C/d, while river is a potential net sink of atmospheric CO 2 with a flux of 4.17 × 10 9 g C/d. Previous studies in the Pearl River Estuary (Wu et al., 2017) and other river-dominated systems (Bauer et al., 2013;Guo et al., 2015) have often observed the net production of DOC in the river plume. The input of land DIC mainly affects the balance of the carbonate system in marine water. Compared with seawater, SGD and river water have lower pH values, which suggests that much more CO 2 could be dissolved in SGD and river water. Charbonnier et al. (2022) also found that terrestrial groundwater contains a lot of CO 2 and is an important contributor to the DIC flux to the coastal ocean. When SGD and river discharge to the ocean, the balance of the carbonate system may be changed resulting in enhancement of calcium carbonate or CO 2 exchange with the atmosphere. If assuming that the free CO 2 in SGDderived and riverine DIC can escape to the atmosphere from water body. Based on pH values (Supplementary Table 2), the free CO 2 was estimated to be 5% of DIC. Thus, SGD-derived and riverine DIC would potentially supply atmospheric CO 2 of 0.83 × 10 9 g C/d and 0.42 × 10 9 g C/d, respectively.
Indeed, marine carbon cycles are influenced by the combination of terrigenous NO 3 − , DOC, and DIC. Based on the above discussion, we conclude that SGD in the studied system is a potential net source of atmospheric CO 2 with a flux of 1.46 × 10 9 g C/d and river is a potential net sink of atmospheric CO 2 with a flux of 3.75 × 10 9 g C/d. Their coupled effect on ocean suggests that SGD and river would potentially enhance CO 2 exchange flux from atmospheric to ocean in the dry winter season and they play a significant role in the carbon cycles of GBA.

Conceptual Model for Carbon Process
Submarine groundwater discharge and river water usually have different physical and chemical characteristics, such as the long water residence times, strong water-rock interaction, and complex biogeochemical processes in groundwater flow. Therefore, the composition and structure of chemical materials in river water and groundwater discharged into the ocean are quite different. Generally, the discharged river water has high oxygen and nutrient concentrations and forms freshwater plumes on the ocean surface. Groundwater discharged into the sea from the seabed with a low flow rate, on the other hand, has a low oxygen content and high nutrient concentrations (Slomp and Van Cappellen, 2004;Kim et al., 2017;Waska et al., 2021). Thus they present different processes in marine carbon cycles. Here two conceptual models, as shown in Figure 9, were proposed to illustrate the major potential processes of the carbon cycle induced by SGD and river discharges. Figure 9A shows the contributions of SGD and river discharges to the carbon system on the nearshore scale with shallow oceans. In this case, the bottom water can easily exchange with upper water and air. Thus CO 2 produced from SGD processes (e.g., remineralization of SGD-derived DOC) can escape to the atmosphere. On the other hand, the consumption of CO 2 in SGD processes (e.g., new primary production by SGD-derived nitrate) can achieve from the atmosphere. Other products from SGD can participate in other processes of the carbon cycle in shallow seawater. Overall, a significant amount of nitrate, DOC, and DIC stimulate phytoplankton blooms in the upper water and consume oxygen in the bottom water, resulting in coastal hypoxia (Su et al., 2017). Figure 9B shows the contributions of SGD and river discharges to the carbon system on the shelf scale with deep oceans. In this case, the bottom water is in a reducing environment and hardly exchange with upper water and air. Thus the high-carbon water masses usually occur in the bottom resulting from the input of SGD-derived nitrate and carbon. The carbon in these high-carbon water masses deposited or moved into deeper waters and finally deposited and sealed. This demonstrated that SGD can be regarded as a sink of carbon in a sense. In the upper water, only a fraction of the organic carbon produced by the river was transported to the deep ocean and stored there.

CONCLUSION
In this study, both SGD and river-derived nitrate and carbon fluxes and their impacts on the carbon cycle were investigated in the GBA, China. SGD-derived NO 3 − , DOC, and DIC fluxes were estimated to be (0.73-16.4) × 10 8 g/d, (0.60-9.94) × 10 9 g/d, and (0.77-3.29) × 10 10 g/d, respectively. The riverine NO 3 − , DOC, and DIC inputs were estimated to be 8.82 × 10 8 g/d, 1.05 × 10 9 g/d, and 8.48 × 10 9 g/d, respectively. These additional fluxes are of significance to stimulate new primary production and affect the balance of the carbonate system in marine water. We found that SGD is a potential net source of atmospheric CO 2 with a flux of 1.46 × 10 9 g C/d in the dry winter season of the study area. However, river is a potential net sink of atmospheric CO 2 with a flux of 3.75 × 10 9 g C/d. The findings of this study suggest that SGD, as important as rivers, plays a significant role in the carbon cycles but has long been ignored in the coastal carbon budgets because of its invisible and highly variable. In this study, we just considered the dissolved form of nitrogen and carbon and ignored the particulate form. A few studies have shown that particulate form is an important portion of total carbon flux (Huang et al., 2017). Overall, both SGD and river should be considered in assessing carbon budgets at regional and global scales future.

DATA AVAILABILITY STATEMENT
The original contributions presented in the study are included in the article/Supplementary Material, further inquiries can be directed to the corresponding authors.

AUTHOR CONTRIBUTIONS
XW designed the study and wrote the original draft preparation. YZ contributed to the manuscript writing, sample measurements, and data analysis. CZ reviewed and edited the manuscript and acquired funding. MHL, SY, and MQL conducted the fieldwork and sample measurements. HL contributed to the study design and data analysis. All authors approved the submitted version.