A preliminary study on dissolved iodine in the northern South China Sea Shelf

A preliminary study on dissolved iodine in the northern South China Sea Shelf

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Jianrong Lin¹^,^*

Acta Oceanologica Sinica | 2024, 43(12) : 47 - 57

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Acta Oceanologica Sinica | 2024, 43(12): 47-57

• Marine Chemistry •

A preliminary study on dissolved iodine in the northern South China Sea Shelf

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Jianrong Lin¹^,^*

Affiliations

¹ Key Laboratory of Estuarine Ecological Security and Environmental Health of Fujian Province University (Xiamen University Tan Kah Kee College), Zhangzhou 363105, China

Published: 2024-12-25 doi: 10.1007/s13131-024-2417-5

Outline

Abstract

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We investigated dissolved iodine species in seawater from the northern South China Sea Shelf. Iodide concentrations were determined by cathodic stripping square wave voltammetry, and iodate was measured by spectrophotometry. Dissolved organic iodine (DOI) was measured with reference to reduced iodide. R-TDI (R-X or rationalized-X is the concentration of X normalized to a salinity of 35, TDI represents total dissolved iodine) was in the range of 0.43–0.46 µmol/L, showing a relatively conservative behavior, while iodate, iodide, and DOI showed non-conservative behaviors. Distribution characteristics in the surface waters showed R-iodate values in the 0.28–0.32 µmol/L range and an offshore>inshore trend, while R-iodide was in the 0.11–0.19 µmol/L range and R-DOI in the 0–0.07 µmol/L range, reflecting an inshore>offshore trend for both. The vertical distribution showed the highest R-iodide concentrations in the surface waters and decreased values with depth, reaching less than 0.01 µmol/L at depths>200 m. R-iodate increased with depth with a measured peak value of 0.43 µmol/L. Seawater with high iodate/iodide ratio (up to 2.9) was found in the central upwelling region and gradually decreased to 2.0 far from this center. The relationship between R-iodide and R-iodate among all samples followed the 1:1 relationship with a slope slightly less than 1, indicating that the conversion between iodate and iodide species could not account for the observed changes. This finding also suggests that DOI may be an important participant in the mass balance. A box model was applied to calculate the input and output of iodine species, and the result showed that approximately 8% of iodate (1.50 × 10⁸ mol/a) imported to the shelf sea was reduced. Concomitantly, the amount of iodide and DOI produced in the shelf amounted to 1.07 × 10⁸ mol/a, roughly 14% higher than the input iodide.

Key words

total dissolved iodine / iodate / iodide / dissolved organic iodine / northern South China Sea Shelf

Cite this Article

Jianrong Lin. A preliminary study on dissolved iodine in the northern South China Sea Shelf[J]. Acta Oceanologica Sinica, 2024 , 43 (12) : 47 -57 . DOI: 10.1007/s13131-024-2417-5

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1 Introduction

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In the ocean, iodate (

${\mathrm{IO}}_{3}^{-} $

, oxidized form), iodide (I⁻, reduced form), and dissolved organic iodine (DOI) are the dominant iodine species, with an estimated total concentration of 0.45 µmol/L (Tsunogai and Henmi, 1971; Wong, 1991; Lee Chen, 2005; MacKeown et al., 2022; Lin, 2023). Thermodynamically, iodate is the favored form of iodine and the overwhelmingly dominant form below the oceanic mixed layer in oxygenated seawater. The existence of DOI in seawater was first reported by Truesdale (1975), and its significance has been highlighted, particularly in coastal waters (Wong and Cheng, 2001a, 2008; Lin, 2023). Research has revealed that different iodine species can be converted into one another in continental shelf seas (Wong and Zhang, 2003a; Wong et al., 2004; Wong and Cheng, 2008; Chance et al., 2010; Zhou et al., 2017; Carrano et al., 2020; MacKeown et al., 2022; Lin, 2023; Shaikh et al., 2023). For example, in the Antarctic shelf sea area, the concentration of iodide increases with increasing chlorophyll a (Chl a) concentration (Chance et al., 2010). In general, iodine speciation changes are much more pronounced in shelf waters than in the open ocean (Truesdale, 1994; Truesdale and Jones, 2000; Truesdale and Upstill-Goddard, 2003; Wong and Zhang, 2003a; Truesdale et al., 2003; Wong et al., 2004; Chance et al., 2010; Moriyasu et al., 2020; MacKeown et al., 2022; Lin, 2023; Shaikh et al., 2023). In addition to biological factors, iodine in shelf waters is controlled by physical processes, such as upwelling and internal waves (Wong et al., 2004; Chance et al., 2010; Moriyasu et al., 2020; MacKeown et al., 2022). In the euphotic layer, iodate increases and iodide decreases with depth, and [

${\mathrm{IO}}_{3}^{-} $

]/[

${\mathrm{I}}^{-} $

] increases with water depth. The [

${\mathrm{IO}}_{3}^{-} $

], [

${\mathrm{I}}^{-} $

], and [

${\mathrm{IO}}_{3}^{-} $

]/[

${\mathrm{I}}^{-} $

] molar ratio characteristics of upwelled waters are distinct from those of surface waters. Hence, iodine can be used as a tracer. As a tracer, iodine offers advantages over temperature and salinity because the efficacy of temperature and salinity can be seriously limited in shelf environments. This limitation arises because coastal waters may also have the same signatures. Furthermore, temperature can be modified by heat exchange between the sea surface and the atmosphere (Wong et al., 2004). Similarly, iodine can be used to trace internal waves.

Iodide produced in the shelf sea may also be imported into the open ocean (Wong, 1995; Wong and Zhang, 2003a; Wong et al., 2004; Wong and Cheng, 2008). For East China Sea and South Atlantic Bay, the annual net output of iodide generated by iodate is around 1.0 × 10⁹ mol/a and 1.8 × 10⁹ mol/a, respectively, while other studies show no evidence of conversion from iodate to iodide (Truesdale, 1978, 1994; Truesdale and Jones, 2000), indicating further the need for more studies.

In this study, the distribution of iodine species in the northern South China Sea was studied, and a box model was used to calculate the import and export of the iodine species.

2 Study area and methods

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2.1 Study area

The South China Sea is located in the region bound within 4°–23°N, 105°–121°E and has an estimated area of 3.56 × 10⁶ km² (Wong et al., 2007). Owing to the influence of southwest winds, the northern shelf area of the South China Sea can experience wind-driven upwelling in summer, such as in East Guangdong (Wu and Li, 2003). The northern Luzon Strait is the main channel connecting the South China Sea and the West Pacific. The deep water of the West Pacific enters the South China Sea through the Luzon Strait. The residence time of deep water is about 30 years. Water balance with the input of deep water is mainly realized through the output of middle layer water (Wong et al., 2007). The Research Center for Environmental Changes (RCEC) at Academia Sinica organized the South China Sea Shelf Sea OR1-CR929 cruise from June 3 to 12, 2010. Our report covers four sections from east to west, labeled T1, T2, T3, and T4, each consisting of 12–15 stations (Fig. 1).

We divided the sea area into four areas according to bathymetry: inner-shelf (≤40 m), mid-shelf (40–90 m), outer-shelf (90–120 m), and open SCS (>120 m). The depth of the outer continental shelf is taken to be 120 m in this research. The water area and volume of each part of the continental shelf are listed in Table 1.

2.2 Sampling methods

The sampling bottles used were made from high-density polyethylene or polypropylene that needed to be pretreated before sampling. The steps were as follows: (1) soaking in reverse osmosis water for at least 24 h, (2) washing with detergent (2% micro) (Cole Parmer Company) and then rinsing with RO water, (3) washing with RO water after soaking in 10% HCl for 24 h, and (4) drying in a clean room. The filter membrane was soaked in 0.5 mol/L HCl solution for 12 h, washed with Milli-Q water to neutral, and dried in a clean room. After sampling, the bottle cap was tightened and immediately stored in a −20℃ refrigerator. The cap was confirmed to still be tight after several hours when the sample was frozen.

2.3 Measuring methods

Iodide was determined by cathodic stripping square wave voltammetry, which has a precision of ±5%. At –0.15 V (relative to standard calomel) potential, Hg initially lost electrons, and then iodine was deposited onto the suspended mercury electrode in the form of Hg₂I₂ for a certain time. The addition of Triton X-100 to the solution resulted in a significant increase in peak current, and the peak height was positively correlated with the iodide concentration. Quantitative analysis was carried out by adding different iodide concentrations in solution. Dissolved oxygen was removed by aeration with high-purity argon. Sulfite was added to the solution to remove oxygen quickly. Iodate was measured by a spectrophotometric method with a Shimadzu UV1700 spectrophotometer, which has ±3% precision. DOI was measured as reduced iodide according to the method developed by (Wong and Cheng, 1998).

Nitrate and Chl a data were supplied by RCEC. Longitudes, latitudes, salinity, and temperature were received with a GPS receiver in real time and recorded with cruise CTD data. The precision of longitude and latitude was ±0.00001° accurate, salinity precision was ±0.01, and temperature precision was ±0.001℃.

3 Results

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3.1 Surface and vertical distribution of temperature and salinity

Figure 2 shows the surface temperature and salinity. Figures 3 and 4 illustrate the sector and vertical distribution of temperature, salinity, Chl a, and nitrate for each section, respectively.

In general, surface temperature and salinity increased from the inner to the outer shelf (Figs 2 and 3b1–b4); in the inner and outer shelves, the temperature and salinity increased from about 26℃ to 26.5℃ and from 27 to 28, respectively. The freshwater signal in the west shelf (Figs 2 and 3b4) is characterized by higher temperature (~28℃) and lower salinity (~24.5) than surrounding waters. In a typical summer, the temperature and salinity of shelf waters are greatly affected by freshwater input from the Zhujiang River (Callahan et al., 2004; Cai et al., 2004).

Upwelling is observed in the eastern part of the inner shelf, which is characterized by lower temperature and higher salinity than surrounding waters. The average surface salinity is about 34.2 (non-upwelling < 33.6) and the temperature is around 24.6℃ (non-upwelling > 25.8℃) (Figs 2 and 3b2). Although temperature can serve as a good tracer in the central upwelling area, this is only true during periods of pronounced wind-driven upwelling.

The concentration of upwelling nitrate is comparable to that in other continental shelf sea areas. The South China Sea is an oligotrophic sea. The nitrate levels in the shallow water bodies above the continental shelf were near or below the detection limit during the investigation period (Figs 3c1–c4). This implies that upwelling that sourced water from shallow depths did not cause an increase in surface nutrients.

The temperature, salinity, nutrients, and chlorophyll of each section reflect the oceanographic setting of each site (Fig. 4). For example, along Section T2, Stations 22 and 23 (near Dongsha Islands), strong internal waves are formed due to the bathymetry, resulting in deep water upwelling (Fig. 4b2). At Stations 22 and 23, from depths of 20–120 m, the average temperature and salinity are (21.13 ± 3.92)℃ and (21.05 ± 3.47)℃, 34.47 ± 0.14 and 34.52 ± 0.13, respectively. At the nearby Stations (21, 24), the temperature and salinity progressed from (22.93 ± 2.38)℃ and (22.56 ± 3.38)℃, 34.33 ± 0.18 and 34.50 ± 0.12, respectively. Although the difference in salinity is not significant, the average temperature difference between them is as high as 1.8℃. Lower-temperature water bodies are caused by the rise of deep water. Second, there is enhanced water column mixing near the shore; in Station 17, the surface salinity exceeds 34.2, which is about 0.6 higher than the average of the inland shelf. At the same time, the temperature drops to about 24.6℃ (Figs 3b2, 4a2 and 4b2).

The most obvious feature of Section T3 is the low salinity of surface water, to a depth of 20 m, at Stations 52–54 (Fig. 4b3). The average salinity of the three stations below 20 m is 33.633 ± 0.107, while at the adjacent Station 25, the average salinity of the station below 20 m is 34.375 ± 0.071, a difference of more than 0.7. This discrepancy may be caused by precipitation.

The most notable feature of Section T4 is that several stations on the inland shelf are affected by freshwater input (Figs 4a4, b4, c4 and d4). The lowest salinity value appeared at Station 40. A front appeared at Stations 39 and 40: within a range of 7.6 km, salinity increased from 28.7 to above 33. This is the largest salinity variation observed during the voyage, which may be attributed to the westward transport of Zhujiang River freshwater. Previous studies have found that about one-third of Zhujiang River freshwater moves westward in summer (Pan et al., 2006; Su, 2004). High nitrate and chlorophyll concentrations reaching 14.9 µmol/L and 2.89 μg/L, respectively, are found, likely influenced by Zhujiang River water input. Nitrate concentration can reach >100 times the value of continental shelf water (Dai et al., 2008). A notable feature of Section T4 is that the inner shelf area shows high nitrate and chlorophyll values due to the impact of freshwater input, with the highest values of 14 µmol/L and 1.6 μg/L (Figs 3c1 and d1), respectively.

In general, temperature and salinity increase from the continental shelf toward the deep ocean basin, while nutrient and chlorophyll concentrations rapidly decrease. This represented a typical summer when the northern South China Sea is greatly affected by freshwater input from Zhujiang River. Moreover, the shelf mixed layer is generally in an oligotrophic state, which is consistent with the results of Yuan (2005).

3.2 DI in the northern South China Sea

The variation of R-TDI and R-total inorganic iodine (TII) in the continental shelf area is not significant, only about (0.43 ± 0.03) µmol/L (Figs 5a1–a4), indicating that the proportion of exchanging species from dissolved to other phases is very small. Other external input sources, such as atmospheric deposition, river water, or sediment interstitial water, can also be ignored.

Contrary to the R-TDI distribution, the changes in surface

${\mathrm{IO}}_{3}^{-} $

and

${\mathrm{I}}^{-} $

concentrations are significant (Figs 5b1–b4). The surface

${\mathrm{IO}}_{3}^{-} $

(R-

${\mathrm{IO}}_{3}^{-} $

) shows a trend of low near shore and high far shore, with

${\mathrm{IO}}_{3}^{-} $

ranging from 0.257 µmol/L to 0.319 µmol/L. On the contrary, the surface

${\mathrm{I}}^{-} $

(R-

${\mathrm{I}}^{-} $

) shows a trend of low far shore and low near shore, with

${\mathrm{I}}^{-} $

ranging from 0.11 µmol/L to 0.21 µmol/L.

${\mathrm{I}}^{-} $

variation is greater than that of the continental shelf

${\mathrm{I}}^{-} $

along the coast of the United States (Wong and Zhang, 1992; Wong, 1995).

Iodide is inversely correlated with iodate, and the variation in the ratio of

${\mathrm{IO}}_{3}^{-} $

${\mathrm{I}}^{-} $

is also significant (Figs 5a1–a4). The minimum of 1.3 appears at the area of freshwater input, while the maximum of 2.9 appears at the upwelling area.

The vertical distribution of the mixing layer is similar to that of the surface layer, with

${\mathrm{I}}^{-} $

concentration increasing while

${\mathrm{IO}}_{3}^{-} $

decreases from the outer shelf to the inner shelf. Below the mixing layer, R-TDI exhibits a conservative behavior (Fig. 6). From the open SCS (40–120 m), outer shelf (40–120 m), middle shelf (40–90 m) to inner shelf (0–40 m), the average concentration of R-

${\mathrm{IO}}_{3}^{-} $

decreases from 0.356 µmol/L to 0.283 µmol/L, while R-

${\mathrm{I}}^{-} $

and R-DOI increase from 0.065 µmol/L to 0.138 µmol/L and from 0.018 µmol/L to 0.025 µmol/L, respectively. In particular, during the transport of seawater from the outer shelf to the inner shelf, dissolved iodine changes due to enhanced biological effects.

The results are similar to those of the shelf sea of England and East China Sea (Truesdale and Jones, 2000; Wong and Zhang, 2003a) but much smaller than that of the continental shelf sea in the southeastern United States, where the proportion of

${\mathrm{IO}}_{3}^{-} $

converted into other species is about 60%–80% (Wong and Zhang, 1992; Wong, 1995). This is the main difference between middle/high and low-latitude shelf marginal seas (Wong and Zhang, 1992; Wong, 1995; Truesdale and Jones, 2000; Wong and Zhang, 2003a).

The temperature, salinity, and dissolved iodine profiles at four deep-water stations on the South China Sea slope are shown in Fig. 7. The average concentrations of R-TDI in the mixed layer (0–40 m) or euphotic layer (40–130 m) are (0.437 ± 0.016) µmol/L and (0.442 ± 0.020) µmol/L, respectively. At 2000 m depth, the value is (0.465 ± 0.010) µmol/L. Overall, the R-TDI only has a slight change in all depths.

The concentration of dissolved iodine in different forms varies greatly with depth. The iodide in the mixed layer (0–40 m) is (0.127 ± 0.015) µmol/L (Table 2), which decreases rapidly with increasing depth. When the depth is >200 m, the iodide falls below the detection limit. The iodate in the mixed layer is (0.296 ± 0.011) µmol/L, which increases to above (0.422 ± 0.005) µmol/L when the depth >200 m, slowly increasing with depth and remaining almost constant below 1000 m. These results are similar to the results of open oceans such as the Atlantic and Pacific (Tsunogai and Henmi, 1971; Wong, 1991; Wong and Cheng, 2001a), where iodine exchange mainly occurs within the euphotic layer.

4 Discussion

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4.1 Tracing upwelling

Upwelling promotes high

${\mathrm{IO}}_{3}^{-} $

and low

${\mathrm{I}}^{-} $

concentrations, leading to high

${\mathrm{IO}}_{3}^{-} $

${\mathrm{I}}^{-} $

ratios. Therefore, iodine can be used to study upwelling. Compared with iodine systems, salinity and temperature have the following limitations: after the upwelling enters the surface, the sea surface temperature in the upwelling area and other sea areas will quickly converge due to the heat exchange between the sea surface and the atmosphere. Surface water salinity is significantly different from upwelled water. When the salinity of the surface water and the upswelled water is similar, it is difficult to use salinity as an indicator of upwelling (Wong and Zhang, 2003a; Wong et al., 2004). Choosing

${\mathrm{IO}}_{3}^{-} $

${\mathrm{I}}^{-} $

ratios can overcome these shortcomings; it is not only a supplement to traditional upwelling indicators but can also be used as the primary indicator when temperature and salinity are insensitive to upwelling.

The

${\mathrm{IO}}_{3}^{-} $

${\mathrm{I}}^{-} $

ratios of the mixed layer in the northern shelf of the South China Sea during the cruise was in the range of 1.2–3 (the uncertainty of the ratio is ~0.05) (Table 2). In the western shelf due to freshwater input, which resulted in the lowest surface

${\mathrm{IO}}_{3}^{-} $

${\mathrm{I}}^{-} $

ratio reaching 1.2, the ratio range in most sea areas was above 2, with the highest ratio of 3.0 occurring in the upwelling center of the eastern shelf, similar to the research results in the East China Sea (Wong and Zhang, 2003a). The ratio of

${\mathrm{IO}}_{3}^{-} $

${\mathrm{I}}^{-} $

can provide information on where water rises from. The ratio of

${\mathrm{IO}}_{3}^{-} $

${\mathrm{I}}^{-} $

at 40 m and 50 m is 3 and 3.6, respectively, and the average value of

${\mathrm{IO}}_{3}^{-} $

${\mathrm{I}}^{-} $

at 0–40 m is about 2.5. In addition, the continental shelf area is 5.9, while the ratio outside the shelf is around 10 (Table 2). Therefore, it is reasonable to believe that the upwelling in the South China Sea inner shelf should be around 40 m.

4.2 Iodine species conversion in the northern South China Sea

Analyzing the distribution of iodine at deep-water stations in the South China Sea can help scholars understand the similarities and differences in iodine distribution between the South China Sea and the open ocean (To counteract the effects of physical changes, iodine is normalized to a salinity of 35).

Figure 8a shows that the relationship between

${\mathrm{IO}}_{3}^{-} $

and

${\mathrm{I}}^{-} $

in the South China Sea is not entirely in a 1:1 ratio. The data in the inner shelf show greater deviation, which may be due to the higher productivity of the inner shelf, where more iodine exist in the form of organic matter. The impact of river inputs may also be greater. The linear relationship between the iodate and iodide+DOI is found to be more in line with −1:1 (Fig. 8b). As a product of iodate, DOI cannot be neglected.

The relationship between R-TII and R-TDI (Fig. 9) also clearly illustrates the conversion relationship of iodine. R-TII represents all dissolved inorganic iodine, while R-TDI represents all dissolved iodine. If DOI can be ignored, the concentration of total inorganic iodine should be equivalent to the concentration of TDI. However, a significant difference is observed between the surface water of the inner and middle shelves. After taking into account the error (the area between the two dashed lines is the range covered by the analysis error, with an error margin of ±0.02 µmol/L), it can be seen that some data with lower concentrations still fall outside the 1:1 relationship. Similarly, the deviation of the inner shelf is relatively large, which once again proves that although the DOI concentration is low, ignoring it will bring significant errors to the conversion products of iodate (Wong and Zhang, 2003a, 2003b).

4.3 Iodine species output

In general, iodate-rich and DOI-poor water from the open South China Sea intruded into the northern South China Sea shelf (Table 2), while iodide and DOI rich water was transported from the shelf into the open South China Sea. The reduction of iodate could have been carried out by the biologically mediated reduction (Bluhm et al., 2011; Moriyasu et al., 2020; MacKeown et al., 2022) or caused by natural light (Wong and Cheng, 2001a). In this study, a box model was applied to calculate the inter-conversion of the iodine species.

When water enters the continental shelf from the open sea area outside the shelf, the concentration of TDI remains basically unchanged while iodate decreases and iodide+DOI increases (Table 2). The conversion of iodate to iodide takes several months (Jickells et al., 1988; Wong, 1995). Iodide life can reach 1–20 years (Wong, 2001), while the half-life of DOI may only be a few hours (Wong and Cheng, 2001b). Nonetheless, there is a high possibility of recalcitrant DOI that resists degradation (Wong and Cheng, 1998), and the residence time of this part of the DOI is relatively long. There have been multiple reports on iodide transport (Wong, 1995; Wong and Zhang, 2003a; Wong et al., 2004; Wong and Cheng, 2008), yet DOI has not been reported.

Assuming that iodine on the northern shelf of the South China Sea is in a steady state, a box model is used to calculate the conversion and output of iodate, iodide, and DOI on the shelf (Fig. 10). According to the hydrological data, the upper mixed layer in the northern South China Sea during summer is about 40 m (Table 2), similar to the results in other studies (Cai et al., 2004; Wong et al., 2007).

The box model mainly considers the conversion of iodine species in the mixed layer of the northern shelf of the South China Sea (0–40 m). Its input and output include upwelling, advection input of the mixed layer outside the shelf (0–40 m), atmospheric deposition and river input, advection, and evaporation output of the mixed layer of the shelf (0–40 m) (Fig. 10).

For the South China Sea Shelf (0–40 m), the water flux is as follows:

(1)

$ {F}_{\mathrm{R}} + {F}_{\mathrm{P}} + {F}_{\mathrm{V}} +{F}_{\mathrm{SH}}= {F}_{ \mathrm{HS}} +{F}_{\mathrm{E}}, $

where F_R represents the water volume of the river, F_P represents wet deposition, F_V represents the water volume of upwelling, F_SH represents the water volume entering the shelf area outside the shelf, F_HS represents the water volume entering the shelf area from outside the shelf, and F_E represents the water volume evaporated from the shelf area.

In the model, input and output terms such as river inputs, evaporation, and subsidence on the northern shelf of the South China Sea can be ignored.

Equation (1) can be simplified as

(2)

$ {F}_{\mathrm{V}} +{F}_{\mathrm{SH}}= {F}_{\mathrm{HS}}. $

For the salt flux (S) on the South China Sea Shelf sea,

(3)

$ {{S}}_{\mathrm{D}} {F}_{\mathrm{V}} +{S}_{\mathrm{S}} {F}_{\mathrm{SH}}= {F}_{\mathrm{HS}} {S}_{\mathrm{H}}, $

where S_D represents salt flux of upwelling, S_S represents salt flux of the mixed layer outside the shelf, and S_H represents salt flux of the mixed layer on the shelf.

The iodate, iodide, and DOI fluxes in the South China Sea Shelf sea are as follows:

(4)

$ {A}_{\mathrm{D}} {F}_{\mathrm{V}} + {A}_{\mathrm{S}} {F}_{\mathrm{SH}}= {F}_{ \mathrm{HS}} {A}_{ \mathrm{H}} +{C_A} $

(5)

$ {I}_{\mathrm{D}} {F}_{\mathrm{V}} +{I}_{\mathrm{S}} {F}_{\mathrm{SH}}+ {{P}}_{\mathrm{I}}= {F}_{\mathrm{HS}} {I}_{\mathrm{H}} $

(6)

$ {O}_{\mathrm{D}} {F}_{\mathrm{V}} +{O}_{\mathrm{S}} {F}_{\mathrm{SH}} +{{P}}_{\mathrm{O}}={F}_{\mathrm{HS}} {O}_{\mathrm{H}} $

where A_D, I_D, and O_D represent the iodate, iodide, and DOI concentration in the upwelling; A_S, I_S, and O_S represent the three species’ concentrations of the mixed layer outside the shelf; A_H, I_H, and O_H represent the concentrations of the mixed layer on the shelf sea; C_A is the iodate converted from the continental shelf; and P_I or P_O are the iodide or DOI converted from the continental shelf.

To distinguish the degree of influence of the inner, middle, and outer shelves on the conversion of iodine species, the shelves are divided into three parts (inner, middle, and outer parts). The parts outside the shelf area and the water volume of each part of the shelf were also calculated (Table 2). The weighted average concentration of each parameter (with water volume as the weight) was carried out for the mixed layer and below the mixed layer in each part, and the weighted average concentration and water volume of dissolved iodine in the mixed layer of the shelf were calculated separately. The results are listed in Table 3.

The volume of water in the whole shelf mixing layer is 8074 km³, residence time is 1.3 a, input (or output) volume of water is 6210 km³/a, and volume of shelf upwelling is 3107 km³, which accounts for about 50% of the annual input water (same as output). The annual input and output fluxes of salt are (210.7 ± 1.9) Pg and (212.6 ± 0.9) Pg, respectively. As the difference between them is (1.9 ± 2.0) Pg, it can still be considered as the basic balance of salt input and output.

To the shelf mixing layer, iodate input and output are 19.1 × 10⁸ mol/a and 17.6 × 10⁸ mol/a, respectively (Tables 3 and 4). The difference is 1.5 × 10⁸ mol/a, which is about 8% of the total input iodate converted to iodide and DOI, far less than that of the East China Sea (~20%) and the U.S. coastal shelf (~28%). Furthermore, iodide input and output are 6.7 × 10⁸ mol/a and 7.7 × 10⁸ mol/a, respectively (Tables 3 and 4), and the difference is 1.00 × 10⁸ mol/a. DOI input and output are 1.11 × 10⁸ mol/a and 1.18 × 10⁸ mol/a, respectively (Tables 3 and 4), and the difference is 0.07 × 10⁸ mol/a, hence, the total difference is 1.07 × 10⁸ mol/a, indicating an increase of approximately 14%.

The model results show that iodate is mainly converted to iodide and a small fraction can be converted to DOI (Table 4), confirming that a portion of the net increase in DOI on the shelf can be exported.

The consumption rate of iodate on the northern shelf of the South China Sea is equivalent to 0.06 nmol/d, the production rate of iodide is equivalent to 0.04 nmol/d, and the production rate of DOI is 0.003 nmol/d. By comparison, in the East China Sea, the consumption rate of iodate is 0.10 nmol/d, and the production rate of iodide is 0.06 nmol/d (Wong et al., 2004). Therefore, the conversion efficiency of iodate may be higher in high-productivity marine areas. The annual average net primary productivity of the South China Sea and East China Sea shelf areas are 0.36 ± 0.15 g/(m²·a)(in terms of C) and 0.96 ± 0.38 g/(m²·a)(in terms of C), respectively(Gong et al., 2000; Lee Chen, 2005), where iodide and DOI are mainly produced by organisms. The production rate of iodide and consumption rate of iodate are approximately 5–6 nmol/d at South Atlantic Bight. The rates between different shelves can reach up to several tens of times. While the mechanism may be related to algal species, light intensity, or temperature, the exact mechanism still needs to be studied further.

5 Conclusions

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TDI concentration is (0.43 ± 0.03) µmol/L and shows conservative behavior on the northern shelf of the South China Sea. The concentration range of iodide in the surface continental shelf is 0.11–0.20 µmol/L (~30% of TDI), following a pattern of near shore>far shore (not including upwelling). DOI concentration is 0–0.07 µmol/L (~10% of TDI, near shore>far shore).

Iodide concentration decreases with increasing depth and has dropped below 0.01 µmol/L by 200 m; on the contrary, iodate concentration increases with depth and reaches over 0.43 µmol/L at 200 m. Correspondingly,

${\mathrm{IO}}_{3}^{-} $

${\mathrm{I}}^{-} $

is 2.9 in the central area of the upwelling, gradually decreasing outward to 2.4. In other areas, the ratio is about 2, and the ratio can trace the upwelling scope.

The relationship between iodate and iodide is generally linear, with the slope slightly less than 1. This indicates that while DOI is only a small proportion, it should still not be neglected.

The results of the box model not only confirm that iodide produced can output but that DOI may also output. Iodate input to the shelf is approximately 19.10 × 10⁸ mol/a, output is 17.60 × 10⁸ mol/a, and amount of reduced iodate is 1.50 × 10⁸ mol/a, accounting for approximately 8% of the total input. Meanwhile, the total flux of iodide and DOI input to the South China Sea shelf is 7.81 × 10⁸ mol/a and the total output flux of iodide and DOI is 8.88 × 10⁸ mol/a, showing an increased amount of 1.07 × 10⁸ mol/a, which is 14% compared to the input.

This study aimed to investigate the iodine system in low-latitude marginal sea and active sites for iodine species conversion. However, compared to the mid-to-high-latitude marginal seas, the conversion efficiency of iodate is lower.

Acknowledgements

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The author would like to thank MEL and RCEC for triggering this research and supplying the convenience of experiment, and the editors/anonymous reviewers for their insightful comments and suggestions.

Funding

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The Zhangzhou National Science Foundation of Fujian under contract No.JJ2020J29.

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Year 2024 volume 43 Issue 12

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doi: 10.1007/s13131-024-2417-5

Receive Date：2024-02-20
Online Date：2025-11-19
Published：2024-12-25

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Received：2024-02-20
Accepted：2024-05-07

Funding

The Zhangzhou National Science Foundation of Fujian under contract No.JJ2020J29.

Affiliations

¹ Key Laboratory of Estuarine Ecological Security and Environmental Health of Fujian Province University (Xiamen University Tan Kah Kee College), Zhangzhou 363105, China

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* jrlin@xmu.edu.cn

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表12种不同金属材料的力学参数

科 Family	属数 Number of genus	种数 Number of species	占总种数比例 Percentage of total species (%)	属 Genus	种数 Number of species	占总种数比例 Percentage of total species (%)
鹅膏菌科Amanitaceae	2	11	5.26	鹅膏菌属 Amanita	10	4.78
小菇科 Mycenaceae	2	12	5.74	丝盖伞属 Inocybe	5	2.39
多孔菌科 Polyporaceae	8	14	6.70	蜡蘑属 Laccaria	5	2.39
红菇科 Russulaceae	3	23	11.00	小皮伞属 Marasmius	6	2.87
				小菇属 Mycena	11	5.26
				光柄菇属 Pluteus	5	2.39
				红菇属 Russula	17	8.13
				栓菌属 Trametes	5	2.39

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Table 1. Sub-areas of the northern South China Sea Shelf

Sub-area	Bathymetry/m	Water depth/m	Area/km²	Volume/km³
Inner shelf	0 < bathymetry ≤ 40	0 – 40	64 981	1124
Middle shelf	40 < bathymetry ≤ 90	0 – 40	67 763	2711
Middle shelf	40 < bathymetry ≤ 90	40 – 90	67 783	1509
Outer shelf	90 < bathymetry ≤ 120	0 – 40	28 319	1133
Outer shelf	90 < bathymetry ≤ 120	40 – 120	28 319	1597
Total	Water depth ≤ 120		161 083	8074

Table 2. Average concentration of compositions in the mixed layer (0–40 m) and below (40–120 m) in the northern South China Sea Shelf conducted in June 2010

	Inner Shelf (0−40 m Ave.)		Mid-shelf (0−40 m Ave.)	Outer-shelf (0−40 m Ave.)	Open SCS (0−40 m Ave.)	SCS (0−40 m Ave.)
	Upwelling	Area (no upwelling)	Mid-shelf (0−40 m Ave.)	Outer-shelf (0−40 m Ave.)	Open SCS (0−40 m Ave.)	SCS (0−40 m Ave.)
T/℃	24.417 ± 0.261	25.868 ± 1.152	25.686 ± 1.480	26.439 ± 1.368	25.899 ± 1.380	26.676 ± 1.344
S	34.221 ± 0.051	33.601 ± 0.767	34.027 ± 0.168	34.021 ± 0.206	33.929 ± 0.312	34.061 ± 0.252
${\mathrm{IO}}_{3}^{-} $/(μmol·L⁻¹)	0.01 ± 0.02	0.85 ± 2.87	0.048 ± 0.107	0.006 ± 0.022	0.22 ± 0.71	0.06 ± 0.143
Chl a/(μg·L⁻¹)	0.565 ± 0.191	0.733 ± 0.699	0.411 ± 0.299	0.195 ± 0.129	0.44 ± 0.35	0.304 ± 0.191
${\mathrm{I}}^{-} $/(μmol·L⁻¹)	0.115 ± 0.007	0.131 ± 0.020	0.120 ± 0.015	0.127 ± 0.015	0.124 ± 0.016	0.123 ± 0.014
R-${\mathrm{I}}^{-} $/(μmol·L⁻¹)	0.118 ± 0.007	0.138 ± 0.023	0.124 ± 0.016	0.131 ± 0.016	0.128 ± 0.017	0.127 ± 0.015
${\mathrm{IO}}_{3}^{-} $/(μmol·L⁻¹)	0.306 ± 0.008	0.271 ± 0.018	0.287 ± 0.014	0.286 ± 0.011	0.283 ± 0.014	0.288 ± 0.011
R-${\mathrm{IO}}_{3}^{-} $/(μmol·L⁻¹)	0.313 ± 0.008	0.283 ± 0.016	0.295 ± 0.013	0.294 ± 0.010	0.292 ± 0.015	0.296 ± 0.011
DOI/(μmol·L⁻¹)	0.025 ± 0.027	0.025 ± 0.029	0.019 ± 0.022	0.015 ± 0.022	0.019 ± 0.024	0.014 ± 0.014
R-DOI/(μmol·L⁻¹)	0.025 ± 0.027	0.025 ± 0.030	0.020 ± 0.023	0.016 ± 0.024	0.020 ± 0.024	0.014 ± 0.014
TDI/(μmol·L⁻¹)	0.446 ± 0.019	0.427 ± 0.021	0.426 ± 0.023	0.428 ± 0.020	0.427 ± 0.022	0.424 ± 0.016
R-TDI/(μmol·L⁻¹)	0.456 ± 0.020	0.447 ± 0.016	0.439 ± 0.023	0.442 ± 0.022	0.440 ± 0.023	0.437 ± 0.016
${\mathrm{IO}}_{3}^{-}/{\mathrm{I}}^{-} $	2.7 ± 0.2	2.1 ± 0.3	2.6 ± 0.4	2.4 ± 0.4	2.5 ± 0.3	2.4 ± 0.4

	Mid-shelf (40−90 m Ave.)		Outer-shelf (40−90 m Ave.)		Shelf (0−120 m Ave.)	Open SCS (40−120 m Ave.)
T/℃	22.502 ± 1.106		20.590 ± 2.348		21.519 ± 1.744	20.734 ± 2.346
S	34.340 ± 0.130		34.494 ± 0.118		34.419 ± 0.124	34.498 ± 0.124
${\mathrm{IO}}_{3}^{-} $/(μmol·L⁻¹)	1.27 ± 1.19		4.20 ± 4.10		2.74 ± 4.27	4.79 ± 3.30
Chl a/(μg·L⁻¹)	0.557 ± 0.233		0.307 ± 0.199		0.432 ± 0.306	0.214 ± 0.128
${\mathrm{I}}^{-} $/(μmol·L⁻¹)	0.111 ± 0.010		0.074 ± 0.036		0.093 ± 0.037	0.064 ± 0.032
R-${\mathrm{I}}^{-} $/(μmol·L⁻¹)	0.113 ± 0.011		0.075 ± 0.037		0.095 ± 0.037	0.065 ± 0.033
${\mathrm{IO}}_{3}^{-} $/(μmol·L⁻¹)	0.311 ± 0.024		0.342 ± 0.049		0.327 ± 0.055	0.351 ± 0.033
R-${\mathrm{IO}}_{3}^{-} $/(μmol·L⁻¹)	0.318 ± 0.023		0.345 ± 0.048		0.332 ± 0.048	0.356 ± 0.032
DOI/(μmol·L⁻¹)	0.025 ± 0.015		0.018 ± 0.018		0.022 ± 0.023	0.020 ± 0.021
R-DOI/(μmol·L⁻¹)	0.026 ± 0.015		0.018 ± 0.018		0.022 ± 0.023	0.022 ± 0.022
TDI/(μmol·L⁻¹)	0.447 ± 0.017		0.434 ± 0.021		0.441 ± 0.027	0.434 ± 0.019
R-TDI/(μmol·L⁻¹)	0.456 ± 0.016		0.440 ± 0.020		0.448 ± 0.026	0.442 ± 0.020
${\mathrm{IO}}_{3}^{-}/{\mathrm{I}}^{-} $	2.8 ± 0.4		7.1 ± 6.3		5.0 ± 6.3	7.6 ± 4.8

Note: Inner-shelf stations: 8, 10, 12, 13, 15, 36, 38, 40, 42, 44, 46. Upwelling stations: 10, 12. Mid-shelf stations: 6, 17, 32, 34, 48. Outer-shelf stations: 4, 19, 21, 28, 30, 42, 50. Open SCS stations: 2, 23, 26, 54.

Table 3. Average concentration of the compositions and fluxes in the box model in the northern South China Sea in June 2010

		Water		Salt		${\mathrm{I}}^-_{\;} $		${{\mathrm{IO}}_{3}^{-}} $		DOI
		Volume/ km³	Flux/ (km³·a⁻¹)	Salinity	Salt flux/ (10⁵ g·a⁻¹)	Concentration/ (μmol·L⁻¹)	Flux/ (10⁸ mol·a⁻¹)	Concentration/ (μmol·L⁻¹)	Flux/ (10⁸ mol·a⁻¹)	Concentration/ (μmol·L⁻¹)	Flux/ (10⁸ mol·a⁻¹)
Output	HS (0−40 m)	8 074	6 211	33.929 ± 0.312	210.7 ± 1.9	0.124 ± 0.016	7.70 ± 0.99	0.283 ± 0.014	17.57 ± 0.87	0.019 ± 0.024	1.18 ± 1.49
Input	Upwelling F_V (40−120 m)	3 107	3 107	34.417 ± 0.176	106.9 ± 0.5	0.093 ± 0.037	2.89 ± 1.15	0.327 ± 0.055	10.16 ± 1.71	0.022 ± 0.023	0.68 ± 0.72
	Advection SH (0−40 m)	−	3 104	34.061 ± 0.252	105.7 ± 0.8	0.123 ± 0.014	3.82 ± 0.43	0.288 ± 0.011	8.94 ± 0.34	0.014 ± 0.014	0.43 ± 0.43
	Total	−	6 211	−	212.6 ± 0.9	−	6.71 ± 1.23	−	19.10 ± 1.74	−	1.11 ± 0.84
Difference (Output−Input)		−	0	−	(−1.9) ± 2.0	−	0.99 ± 1.58	−	(−1.53) ± 1.95	−	0.07 ± 1.71

Note: − represents no data.

Table 4. Average concentration of the compositions and fluxes in the box model in the northern South China Sea in June 2010. Iodine input, output, and conversion ratio in the northern South China Sea.

Note: Upwelling input: flux input from 40 m to 90 m on the shelf to the mixed layer on the shelf; translation input: flux input from the continental slope to the mixed layer of the continental shelf.

Fig. 1. Sampling sites in the northern South China Sea in June 2010 from east to west: T1, T2, T3, and T4.

Fig. 2. Surface distribution of temperature and salinity in June 2010 cruise.

Fig. 3. Sector distribution of water depth (a1–a4), temperature and salinity (b1–b4), nitrate concentration (c1–c4) and Chl a concentration (d1–d4) in the northern South China Sea Shelf-sea surface water in June 2010. The stations between the dashed line represent the inner shelf, middle shelf, outer shelf, and open SCS, respectively.

Fig. 4. Vertical distribution of temperature (a1–a4), salinity (b1–b4), nitrate concentration (c1–c4), and Chl a concentration (d1–d4) of Sections T1–T4 in the northern South China Sea in June 2010.

Fig. 5. Sector distribution of R-Iodide, R-Iodate, R-DOI, and R-TDI in four sections in the cruise conducted in June. Stations between the dashed line represent inner shelf, middle shelf, outer shelf, and open SCS, respectively. R-X represents the concentration that has been normalized to salinity 35.

Fig. 6. Sectional distribution of R-Iodide, R-Iodate, R-DOI, and R-TDI and the iodate/iodide molar ratio in four sections in the cruise conducted in June 2010. R-X represents the normalized concentration to salinity 35.

Fig. 7. Vertical distribution of R-Iodate, R-Iodide, R-DOI, and R-TDI in four stations during the cruise conducted in June 2010. R-X represents the concentration has been normalized to salinity 35.

Fig. 8. Relationship between R-Iodate vs. R-Iodide (a) and R-Iodate vs. R-(Iodide+DOI) (b). The dashed lines represent the boundaries of the model-prescribed linear bands. R-X represents that the concentration has been normalized to salinity 35.

Fig. 9. Relationship between R-TII and R-TDI of all samples (a) or the samples from less than 40 m (b) in the cruise conducted in June 2010. The dashed lines represent the boundaries of the model-prescribed linear bands. R-X represents that the concentration has been normalized to salinity 35.

Fig. 10. Box model of iodine flux in the northern South China Sea Shelf. F, S, A, I, and O represent water flux (km³/a), salinity flux (10¹⁵ g/a), and the concentrations of iodate, iodide, and DOI (μmol/L), respectively. The subscripts R, H, D, S, P, and E represent river, shelf (0–40 m), shelf (40–120 m), open SCS (0–40 m), deposition, and evaporation fluxes, respectively. C_A, P_I, and P_O represent the consumption of iodate and the production of iodide and DOI in the shelf (10⁸ mol/a).

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