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Volume 40 Issue 8
Aug.  2021
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Article Contents
Abstract
1. Introduction
2. Materials and methods
3. Results and discussion
4. Conclusions
Acknowledgements
References
Article Navigation >  Acta Oceanologica Sinica  > 2021  >  40(8): 32-43
Xiaogang Chen, Jinzhou Du, Xueqing Yu, Xiaoxiong Wang. Porewater-derived dissolved inorganic carbon and nutrient fluxes in a saltmarsh of the Changjiang River Estuary[J]. Acta Oceanologica Sinica, 2021, 40(8): 32-43. doi: 10.1007/s13131-021-1797-z
Citation: Xiaogang Chen, Jinzhou Du, Xueqing Yu, Xiaoxiong Wang. Porewater-derived dissolved inorganic carbon and nutrient fluxes in a saltmarsh of the Changjiang River Estuary[J]. Acta Oceanologica Sinica, 2021, 40(8): 32-43. doi: 10.1007/s13131-021-1797-z
shu

Porewater-derived dissolved inorganic carbon and nutrient fluxes in a saltmarsh of the Changjiang River Estuary

doi: 10.1007/s13131-021-1797-z
  • 1.

    State Key Laboratory of Estuarine and Coastal Research, East China Normal University, Shanghai 200241, China

  • 2.

    Key Laboratory of Coastal Environment and Resources of Zhejiang Province, School of Engineering, Westlake University, Hangzhou 310024, China

  • 3.

    Institute of Advanced Technology, Westlake Institute for Advanced Study, Hangzhou 310024, China

  • 4.

    Institute of Eco-Chongming (IEC), Shanghai 202162, China

Funds:  The Natural Science Foundation of Shanghai under contract No. 19ZR1415300; the Zhejiang Provincial Natural Science Foundation of China under contract No. LQ21D060005; the China Postdoctoral Science Foundation under contract No. 2020M681931.
More Information
  • Corresponding author: Jinzhou Du E-mail address: jzdu@sklec.ecnu.edu.cn
  • Received Date: 2020-05-27
  • Accepted Date: 2020-07-30
    • Available Online: 2021-07-08
  • Publish Date: 2021-08-31
    Abstract
  • Saltmarshes are one of the most productive ecosystems, which contribute significantly to coastal nutrient and carbon budgets. However, limited information is available on soil nutrient and carbon losses via porewater exchange in saltmarshes. Here, porewater exchange and associated fluxes of nutrients and dissolved inorganic carbon (DIC) in the largest saltmarsh wetland (Chongming Dongtan) in the Changjiang River Estuary were quantified. Porewater exchange rate was estimated to be (37±35) cm/d during December 2017 using a radon (222Rn) mass balance model. The porewater exchange delivered 67 mmol/(m2·d), 38 mmol/(m2·d) and 2 690 mmol/(m2·d) of dissolved inorganic nitrogen (DIN), dissolved silicon (DSi) and DIC into the coastal waters, respectively. The dominant species of porewater DIN was ${\rm {NH}}_4^+ $ (>99% of DIN). However, different with those in other ecosystems, the dissolved inorganic phosphorus (DIP) concentration in saltmarsh porewater was significantly lower than that in surface water, indicating that saltmarshes seem to be a DIP sink in Chongming Dongtan. The porewater-derived DIN, DSi and DIC accounted for 12%, 5% and 18% of the riverine inputs, which are important components of coastal nutrient and carbon budgets. Furthermore, porewater-drived nutrients had obviously high N/P ratios (160–3 995), indicating that the porewater exchange process may change the nutrient characteristics of the Changjiang River Estuary and further alter the coastal ecological environment.
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  • Saltmarshes are one of the highest productive ecosystems on the earth, which have been considered to be important sources/sinks, and/or transformers of important compounds such as carbon and nutrients in the coastal landscape (Tobias and Neubauer, 2019). Atmospheric CO2 is assimilated into organic compounds (i.e., carbohydrates) by saltmarshes. Meanwhile, the carbohydrates are utilized by respiration for plant growth and reproduction (Liang et al., 2017; Tang et al., 2018). The dead plants of saltmarsh are decomposed into CO2, and other organic and inorganic matter by microorganisms, with the release of nutrients, and the remaining organic carbon is buried in the sediments (Mcleod et al., 2011; Tang et al., 2018). Some of the carbon and nutrients are then released to the coastal waters through submarine groundwater discharge (SGD) and/or porewater exchange (Charette, 2007; Santos et al., 2019; Liu et al., 2021).

    SGD is defined as “all the fluid flow on continental margins from the seabed to the coastal ocean, with scale lengths of meters to kilometers, regardless of fluid composition and driving force” (Burnett et al., 2003). Some small scale processes with scale lengths of millimeters to meters such as flow- and topography-induced advection, wave pumping, ripple migration, shear flow and shallow bio-irrigation are collectively referred to as porewater exchange (Santos et al., 2012b; Hong et al., 2017). Although SGD or porewater exchange in coastal regions has received the increasing attention in recent decades (Burnett et al., 2001; Taniguchi et al., 2019), determining that the porewater exchange in coastal wetlands, especially in saltmarshes, is challenging and rarely reported because of complex and heterogeneous sediments, variability in tidal water flows and burrowing crab effects (Tait et al., 2016; Gutiérrez et al., 2018).

    Over the past decades there have been significant advances in quantifying SGD or porewater exchange using natural geochemical tracers (Taniguchi et al., 2019; Santos et al., 2021). In particular, tracing SGD or porewater exchange with radon and/or radium radioisotopes has been widely applied to most coastal systems, such as estuaries (Xu et al., 2014; Correa et al., 2020; Guo et al., 2020), bays or lagoons (Su et al., 2014; Yi et al., 2016; Sugimoto et al., 2016), mangroves (Maher et al., 2013; Tait et al., 2016; Chen et al., 2021; Wang et al., 2021), coral reefs (Wang et al., 2014; Oehler et al., 2019), sandy beaches (Tamborski et al., 2017; Chen et al., 2019), karst systems (Rodellas et al., 2017; Chen et al., 2020a), marginal seas and shelves (Gu et al., 2012; Liu et al., 2017; Tan et al., 2018). Recently, a novel method, termed “the 224Ra/228Th disequilibrium approach”, was developed to quantify benthic solute fluxes from coastal sediments (Cai et al., 2014, 2015, 2020; Hong et al., 2017; Shi et al., 2019). Up to now, the estimates of porewater exchange and associated biogenic elemtnts such as carbon and nutrient fluxes in saltmarshes is limited (Cai et al., 2003; Charette, 2007; Krest et al., 2000; Santos et al., 2019). Although these studies estimated the porewater exchange and associated carbon and nutrient fluxes in saltmarshes using radon or radium tracers, they only calculated the fluxes of dissolved inorganic nitrogen (DIN, the sum of ${\rm {NO}}_2^- $, ${\rm {NO}}_3^- $ and ${\rm {NH}}_4^+ $) without distinguishing the main component of DIN.

    Chongming Dongtan, located in the center of the Changjiang River Estuary, is a Ramsar Wetland of International Importance and National Nature Reserve (Li et al., 2014). Recent studies of Chongming Dongtan have primarily focused on biodiversity conservation, carbon sequestration capability and carbon storage capacity of wetland vegetation (e.g., Spartina alterniflora), as well as soil organic carbon content (Gan et al., 2009; Guo et al., 2009; Li et al., 2010). However, the carbon and nutrient fluxes via porewater exchange in Chongming Dongtan have not yet been estimated.

    In this study, hypothesizing that porewater exchange will be a significant pathway for carbon and nutrients exported to the coastal waters. This hypothesis was tested by performing radon (222Rn, a natural groundwater tracer) time series observation in surface waters of Chongming Dongtan wetland and focusing on both dissolved inorganic carbon (DIC) and nutrient fluxes via porewater exchange.

    2.1 Study area

    Chongming Dongtan is the largest and youngest saltmarsh in the Changjiang River Estuary, located at the eastern end of Chongming Island, China (Fig. 1a). The climate of this region is classified as a subtropical ocean climate with a mean annual temperature of 15.3°C. The mean annual precipitation is 1 120 mm, with 60% of rainfall occurring from May to September (Hu et al., 2012). Tides in this region are irregular and semidiurnal, with a range of 2.0–3.1 m (Yang, 1999). The tidal flat has a large vegetation area (27.5 km2) dominated by native Phragmites australis, and S. alterniflora (Ge et al., 2015). Chongming Dongtan was designated a national nature reserve in 2005 because of its importance for bird conservation (Ma et al., 2009). More recently, Chongming Island has been strategically positioned by the Shanghai Overall Planning Office to be developed into a world-class eco-island and has received great attention and cooperation and involvement from international organizations (e.g., the United Nations Environmental Programs) (Luo et al., 2018).

    Figure  1.  Study site location (a) and sampling stations (b) in Chongming Dongtan. Zero and five indicate water depths of 0 m and 5 m.
    DownLoad: Full-Size Img PowerPoint

    2.2 Experimental design and laboratory analysis

    Field investigations were performed in Chongming Dongtan wetland in December 2017. To assess how tidal fluctuations drive porewater exchange, a time series station was deployed for 24 h (two complete tidal cycles) at the surface waters of Chongming Dongtan wetland (Fig. 1b). Continuous 222Rn monitoring in coastal waters was conducted using a RAD7 detector connected with an air-water exchanger. Salinity, temperature, and dissolved oxygen (DO) were measured in the surface water every hour (Multi 3430 WTW, Weilheim, Germany).

    Discrete samples for DIC, nutrients and 226Ra were collected every 3 h. DIC and nutrient samples were filtered through the 0.45 μm cellulose acetate filters into polyethylene bottles. Then the water samples were poisoned with saturated HgCl2 and kept away from light. DIC samples were analyzed using the CM140 Total Inorganic Carbon Analyzer (UIC Inc., Rockdale, IL, USA). Nutrient samples were analyzed using an auto-analyzer (Model: Skalar SANplus) (Liu et al., 2009). DIN represents the sum of ${\rm {NO}}_2^- $, ${\rm {NO}}_3^- $ and ${\rm {NH}}_4^+ $. Dissolved inorganic phosphorus (DIP) and dissolved silicon (DSi) are the concentrations of ${\rm {PO}}_4^{3-} $ and Si(OH)4, respectively. 226Ra samples were collected by filtering surface water through MnO2-impregnated acrylic fiber (Moore and Arnold, 1996). The fibers were then analyzed using a RAD7 detector (Kim et al., 2001).

    Porewater samples in the saltmarsh intertidal zone were collected from a push-point piezometer using a peristaltic pump (Fig. 1b). Crab burrows can strongly enhance porewater exchange in coastal wetlands (e.g., saltmarshes and mangroves) (Xin et al., 2009; Tait et al., 2016; Xiao et al., 2021). Based on the possible depths of crab burrows, the porewater depths (i.e., porewater end-member) were 0.4–1.5 m in this study. These depths are consistent with previous studies (0.3–1.5 m) (Maher et al., 2013; Tait et al., 2016). Temperature, salinity and DO were measured directly in the field using a digital multi parameter meter (Multi 3430 WTW, Weilheim, Germany). 222Rn samples were collected in 250 mL glass bottles using the overflow method and were immediately analyzed using a RAD7 detector (Chen et al., 2018a, 2018b). DIC and nutrient samples were stored and analyzed as described above.

    To determine the diffusive flux of 222Rn from sediments, triplicate about 100 g sediments were collected at the time series station and incubated with 400−500 mL of radium-free water for 30 days. The 222Rn activities in equilibrated water were then analyzed using a RAD7 detector (Corbett et al., 1998).

    2.3 222Rn mass balance model

    A 222Rn mass balance model originally developed by Burnett and Dulaiova (2003) is used to quantify porewater exchange. This model has been widely applied to coastal systems (Tse and Jiao, 2008; Zhang et al., 2016; Chen et al., 2018a, 2018b; Wang et al., 2019). Based on this model, the 222Rn flux via porewater exchange (FPWE) is estimated after all other 222Rn sources and sinks are estimated. The other 222Rn sources include incoming surface water during flood tide (Fin), 226Ra decay (FRa-226) and 222Rn diffusion from sediments (Fsed), while the 222Rn sinks include outgoing surface waters during ebb tide (Fout), atmospheric evasion (Fatm), 222Rn decay (Fdec) and mixing loss with offshore waters (Fmix). Thus, the 222Rn mass balance model in the water column can be written as follows (unit of each item in equation is Bq/(m2·h)):

    $$ {\text{∆}}F{{= F}}_{\text{PWE}}+{{F}}_{\text{in}}+{{F}}_{\text{Ra-226}}+{{F}}_{\text{sed}}-{{F}}_{\text{out}}-{{F}}_{\text{atm}}-{{F}}_{\text{dec}}-{{F}}_{\text{mix}}\text{,} $$ (1)

    in which,

    $$ {\text{∆}}F =\frac{{{I}}_{{t}+{\text{∆}}t}-{{I}}_{{t}}}{{\text{∆}}t}\text{,} $$ (2)

    where ΔF is the variation of the excess 222Rn inventory between two successive periods; I is the 222Rn inventory (Bq/m2) defined as the product of 222Rn activity in the water (Cw, Bq/m3) and water depth; and Δt is the time interval (1 h in this study).

    2.3.1   Tidal effects

    Tidal effects on the variation of 222Rn activity in coastal water cannot be ignored (Tse and Jiao, 2008; Chen et al., 2018a, 2018b), which can be described by the following equations (Zhang et al., 2016):

    $$ {{F}}_{\text{in}}\text=\frac{{{h}}_{{t}+{\text{∆}}t}{-{h}}_{{t}}}{\text{∆}t}{[b}\overline{{{C}}_{{{\rm{w}}}}}+{(1}-{b)}{{C}}_{\text{off}}\text{]}\text{,} $$ (3)
    $$ {{F}}_{\text{out}}=\frac{{{h}}_{{t}}{-{h}}_{{t}+{\text{∆}}t}}{\text{∆}t}{{C}}_{{{\rm{w}}}}{\text{,}} $$ (4)

    where ht is the water depth (h, m) at time t; $ \overline {{C_{\rm{w}}}} $ and Coff are the mean 222Rn activity (Bq/m3) in surface water and offshore water; Cw is the 222Rn activity (Bq/m3) over a time interval at time series station; b is the return flow factor, which can be determined based on a radium mixing model (Moore et al., 2006). The raw data of radium are from Zhang (2007) and Gu et al. (2012).

    2.3.2   Atmospheric evasion

    Atmospheric evasion of 222Rn can be calculated by the following equation (Maclntryee et al., 1995):

    $$ {{F}}_{\text{atm}}{= k(}{{C}}_{{{\rm{w}}}}-\mathrm{\alpha }{{C}}_{\text{air}}\text{)}\text{,} $$ (5)

    where Cair and Cw are the 222Rn activities in the air and surface water, respectively; α is partition coefficient, which is described by the Fritz Weigel equation (Burnett and Dulaiova, 2003); Gas transfer velocity (k) is calculated as follows (Maclntryee et al., 1995):

    $$ {{{k}}_{{\rm{600}}}}{\rm{ = }}\left\{ {\begin{aligned} & {{\rm{0}}{\rm{.45}}{{{\mu }}^{{\rm{1}}{\rm{.6}}}}{{\left(\frac{{{{{S}}_{{c}}}}}{{{\rm{600}}}}\right)}^{ - 0.5}}{{(\mu > 3}}{\rm{.6}}\;{\rm{m/s)}}}\\ & {{\rm{0}}{\rm{.45}}{{{\mu }}^{{\rm{1}}{\rm{.6}}}}{{\left(\frac{{{{{S}}_{{c}}}}}{{{\rm{600}}}}\right)}^{ - 0.666\;7}}{{(\mu }} \leqslant {\rm{3}}{\rm{.6}}\;{\rm{m/s)}}} \end{aligned}} \right.{\text{,}} $$ (6)

    where μ is the wind speed; Sc is Schmidt number and calculated based on the formulations given by Pilson (1998).

    2.3.3   Diffusion from sediments

    The diffusive flux of 222Rn from sediments can be determined based on the following equation (Peng et al., 1974; Martens et al., 1980):

    $$ {{F}}_{\text{sed}}\text{= (}{\text{λ}}_{\text{Rn-222}}\times \mathrm{\varphi }\times {{D}}_{{m}}{\text{)}}^{\text{0.5}}\text{(}{{C}}_{\text{eq}}-{{C}}_{\text{o}}\text{)}\text{,} $$ (7)

    where λRn-222 is the decay constant of 222Rn (0.181 d−1); φ is porosity of sediment; Dm is a function of temperature described by Peng et al. (1974); Ceq is the 222Rn activity (Bq/m3) in porewater that is equilibrium with that in sediment; Co is 222Rn activity (Bq/m3) in overlying water.

    2.3.4   Decay loss of 222Rn and production from 226Ra

    The 222Rn flux from the 226Ra decay and the 222Rn decay loss can be calculated by the following equations (Chen et al., 2018a):

    $$\ {{F}}_{\text{Ra-226}}\text={\text{λ}}_{\text{Rn-222}}\times {{C}}_{\text{Ra-226}}\times {h}\text{,} $$ (8)
    $$ {{F}}_{\text{dec}}\text={\text{λ}}_{\text{Rn-222}}\times {{C}}_{{{\rm{w}}}}\times (1-{\text{e}}^{-{\text{λ}}_{\text{Rn-222}}{\text{∆}}t})\times {h}\text{,} $$ (9)

    where CRa-226 is the dissolved 226Ra activity in the surface waters (Bq/m3). Other items are described as above.

    2.3.5   Mixing loss and porewater exchange

    Net 222Rn flux (Fnet) can be evaluated by taking the change in inventories over time after correcting for atmospheric evasion, tidal effect, 226Ra support, 222Rn decay, and sediment diffusion (Zhang et al., 2016):

    $$ {{F}}_{\text{net}}{\text{= ∆}}F-{{F}}_{\text{in}}-{{F}}_{\text{Ra-226}}-{{F}}_{\text{sed}}\text+{{F}}_{\text{out}}\text+{{F}}_{\text{atm}}\text+{{F}}_{\text{dec}}\text{.} $$ (10)

    Based on the Eqs (8) and (9), Fnet should be a balance between the FPWE and Fmix. To evaluate the conservative estimates of the Fmix, the maximum negative Fnet are selected as Fmix in adjacent periods because the conservative estimates of the mixing loss provide estimations of the minimum porewater exchange flux (Burnett and Dulaiova, 2003). Then the 222Rn fluxes via porewater exchange can be determined based on Eqs (1) and (2). And the porewater exchange rates (PW, cm/d) can be obtained by dividing the 222Rn fluxes via porewater exchange by the 222Rn activity of porewater end-member (CPW, Bq/m3):

    $$\ {{\rm{P}}}{{\rm{W}}}\text=\frac{{{F}}_{\text{PWE}}}{{{C}}_{{{\rm{PW}}}}}\text{.} $$ (11)

    3.1 Time series observation

    All measured parameters such as salinity, DO, 222Rn, nutrients and DIC fluctuated during time series observation without an obvious tidal pattern except the temperature (Figs 2 and 3). Salinity, temperature and DO ranged from 2.3 to 10.7, 10.5°C to 11.5°C and 10.8 mg/L to 11.4 mg/L, with averages of 6.3±2.4, (10.9±0.3)°C and (11.1±0.2) mg/L, respectively. 222Rn activity ranged from 10.6 Bq/m3 to 42.7 Bq/m3 with an average of (26.2±9.5) Bq/m3. Concentrations of DIN, DIP, DSi, and DIC ranged from 91.5 μmol/L to 112 μmol/L, 1.24 μmol/L to 1.76 μmol/L, 87.8 μmol/L to 115 μmol/L, and 1.68×103 μmol/L to 2.03×103 μmol/L, with averages of (102±6) μmol/L, (1.68±0.16) μmol/L, (104±7) μmol/L, and (1.85±0.09)×103 μmol/L, respectively.

    Figure  2.  Variations of tidal height, temperature, salinity, DO concentration, and 222Rn activity during the time series observation.
    DownLoad: Full-Size Img PowerPoint
    Figure  3.  Variations between tidal height and nutrients, and DIC during the time series observation.
    DownLoad: Full-Size Img PowerPoint

    3.2 Porewater observation

    Salinity, temperature and DO of porewater in Chongming Dongtan wetland changed from 1.4 to 20.5, 9.8°C to 12.3°C and 0.35 mg/L to 3.32 mg/L, with averages of 12.6±5.1, (10.9±0.7)°C and (1.06±0.88) mg/L, respectively. 222Rn activity of porewater varied from 1.59×103 Bq/m3 to 4.35×103 Bq/m3 with an average of (2.87±0.97)×103 Bq/m3 (Table 1), which was about 110 fold higher than that in surface water.

    Table  1.  Summary of sampling stations, 222Rn activities, temperature, salinity, DO, nutrient (DIN, DIP and DSi) and DIC concentrations of porewater samples in the Chongming Dongtan
    Station222Rn
    activity/
    (Bq·m−3)    		Temperature/
    °C    		SalinityDO
    concentration/
    (mg·L−1)    		DIN
    concentration/
    (μmol·L−1)    		DIP
    concentration/
    (μmol·L−1)    		DSi
    concentration/
    (μmol·L−1)    		DIC
    concentration/
    (μmol·L−1)
    P11.69×10311.412.80.703390.422167.71×103
    P23.09×10310.712.30.653860.212339.20×103
    P32.63×10310.713.20.553391.182039.06×103
    P41.59×10310.812.20.508401.251818.12×103
    P53.67×10310.319.00.352080.0924912.0×103
    P64.20×10310.420.51.05––––
    P74.35×10311.4 9.91.66 120.0718411.5×103
    P82.24×1039.812.13.321300.031857.09×103
    P92.35×10312.3 1.40.73 130.081948.27×103
    Average(2.87±0.97)×10310.9±0.712.6±5.11.06±0.88283±2510.42±0.48206±23(9.12±1.65)×103
     | Show Table
    DownLoad: CSV

    Nutrients and DIC displayed considerable variability in intertidal porewater (Table 1). DIN (range: 12–840 μmol/L; mean: (283±251) μmol/L), DSi (range: 181–249 μmol/L; mean: (206±23) μmol/L) and DIC (range: (7.09×103–12.0×103) μmol/L; mean: (9.12±1.65)×103 μmol/L) concentrations were significantly higher than those in surface water. However, DIP concentration ranged from 0.03 μmol/L to 1.25 μmol/L with an average of (0.42±0.48) μmol/L, which was significantly lower than that in surface water.

    3.3 222Rn mass balance and porewater exchange

    According to Eqs (1)–(5), all 222Rn sources and sinks were estimated (Table 2). Based on 222Rn mass balance, porewater-derived 222Rn fluxes were calculated to be 0–138 Bq/(m2·h), mean: (43.8±42.4) Bq/(m2·h). In terms of the 222Rn sources in Chongming Dongtan wetland, porewater exchange flux, incoming surface water during flood tide, 222Rn diffusion from sediments and 226Ra decay accounted for 88.0%, 11.2%, 0.6% and 0.2% of the 222Rn sources, respectively. In terms of the 222Rn sinks, mixing loss with offshore waters, outgoing surface waters during ebb tide, atmospheric evasion and 222Rn decay accounted for 83.0%, 14.4%, 2.6% and <0.1% of the 222Rn sinks, respectively. 222Rn diffusion from sediments, 226Ra decay and 222Rn decay were minor components of the 222Rn mass balance and can be negligible as these items were <1% of the 222Rn sources or sinks. Similar results were found in other coastal areas (Burnett and Dulaiova, 2006; Chen et al., 2018a, 2018b; Sadat-Noori et al., 2018).

    Table  2.  222Rn budget during December 2017 in the Chongming Dongtan
    Sources and sinks222Rn fluxes
    /(Bq·m−2·h−1)    		Percentage to
    total 222Rn
    sources and sinks/%
    Sources
    226Ra contribution0.13±0.020.2
    Sediment diffusion0.30±0.020.6
    Influx during flood tide5.56±3.7411.2
    Porewater exchange flux43.8±42.488.0
    Sinks
    222Rn decay0.007±0.0030.01
    Atmospheric evasion1.25±0.672.6
    Outflux during ebb tide6.90±2.8914.4
    Mixing loss39.9±36.983.0
     | Show Table
    DownLoad: CSV

    Defining the porewater end-member is a crucial step in the estimates of porewater exchange and associated nutrient or carbon flux calculations, which has been recognized as a major source of uncertainty (Santos et al., 2012a; Cook et al., 2018; Chen et al., 2020b). In this study, a spatial porewater survey were conducted in Chongming Dongtan wetland (Fig. 1). The mean 222Rn value ((2.87±0.97)×103 Bq/m3) in the spatial porewater survey was selected as the most representative groundwater end-member. Hence, the porewater exchange rate was calculated to be (37±35) cm/d by using porewater exchange flux divided by this groundwater end-member value.

    3.4 Porewater-derived nutrient and DIC fluxes

    Previous studies have confirmed that porewater exchange plays an important even decisive role in coastal ecosystems (Cai et al., 2014, 2015; Tait et al., 2016; Chen et al., 2019). However, there are limited studies on nutrient and carbon fluxes through porewater exchange in saltmarsh ecosystem (Charette, 2007; Santos et al., 2019) due to the heterogenic geological matrix and highly dynamic aquifer system. Therefore, based on the porewater exchange rates calculated by 222Rn mass balance, the porewater-derived nutrient and DIC fluxes in Chongming Dongtan saltmarshes were estimated.

    3.4.1   Porewater-derived nutrient fluxes

    DIN and DSi had high porewater/surface water ratios while porewater DIP was significantly lower than that in surface water (Fig. 4). The low porewater/surface water ratio of DIP is due to mineralization (Slomp and Van Cappellen, 2004) and utilization of phosphate by saltmarshes (Patrick and Delaune, 1976), indicating that porewater exchange in Chongming Dongtan was not DIP source to coastal waters. Therefore, multiplying the porewater exchange rate by the corresponding concentration differences of DIN and DSi in porewater and overlying seawater yields porewater-derived DIN and DSi fluxes of 67 mmol/(m2·d) and 38 mmol/(m2·d), respectively, which are within the wide ranges (DIN: 0.1–680 mmol/(m2·d); DSi: 0.04×103–1.40×103 mmol/(m2·d)) of other coastal ecosystems (summarized in Chen et al. (2018a, 2020a) and Wang et al. (2018b)). Similarly, by multiplying the radon-derived porewater exchange by the concentration differences of ${\rm {NO}}_2^- $, ${\rm {NO}}_3^- $ and ${\rm {NH}}_4^+ $ in porewater and overlying seawater, the ${\rm {NO}}_2^- $, ${\rm {NO}}_3^- $ and ${\rm {NH}}_4^+ $ fluxes via porewater exchange were estimated to be −0.05 mmol/(m2·d), −37 mmol/(m2·d) and 104 mmol/(m2·d), respectively.

    Figure  4.  Ratios of measured 222Rn, nutrients and DIC for porewater to surface water.
    DownLoad: Full-Size Img PowerPoint

    Based on the analysis of different inorganic nitrogen species, the concentrations of ${\rm {NO}}_2^- $, ${\rm {NO}}_3^- $ and ${\rm {NH}}_4^+ $ in the porewater of Chongming Dongtan ranged from 0.08 μmol/L to 0.41 μmol/L, 0.28 μmol/L to 3.87 μmol/L, and 9 μmol/L to 838 μmol/L, respectively. Almost all of the DIN in porewater of Chongming Dongtan was ${\rm {NH}}_4^+ $ (>99% of total inorganic nitrogen species) (Table 3, Fig. 5). The concentrations of ${\rm {NO}}_2^- $, ${\rm {NO}}_3^- $ and ${\rm {NH}}_4^+ $ in groundwater/porewater of other ecosystems were summarized in Table 3. Whether compared with small-scale bays, lagoons and seagrass beds, or large-scale estuaries, continental shelves and marginal seas, the ${\rm {NO}}_3^- $ concentration in porewater of Chongming Dongtan ((1.48±0.98) μmol/L) was significantly lower than that in other ecosystems (range: 19.6–1 460 μmol/L; mean: (342±431) μmol/L), while ${\rm {NH}}_4^+ $ in porewater of Chongming Dongtan ((282±251) μmol/L) was much higher than that in others (range: 0.38–38.1 μmol/L). Compare with other ecosystems (Fig. 5), the high ${\rm {NH}}_4^+ $ concentration and ${\rm {NH}}_4^+ $/DIN ratio of porewater are mainly due to the degradation of organic matter (White and Howes, 1994; Tobias and Neubauer, 2019).

    Table  3.  The concentrations of different inorganic nitrogen species in groundwater of different marine ecosystems
    Study
    area    		Ecosystem
    type    		${\rm {NO} }_2^- $
    concentration
    /(μmol·L−1)    		${\rm {NH} }_4^+ $
    concentration
    /(μmol·L−1)    		${\rm {NO} }_3^- $
    concentration
    /(μmol·L−1)    		DIN
    concentration
    /(μmol·L−1)    		Reference
    Yucatan Coast, Mexicoseagrass bed0.4210.419.630.4Kantún-Manzano et al. (2018)
    Laoye Lagoon, Chinalagoon0.845.36212218Ji et al. (2013)
    Sanggou Bay, Chinabay0.731.171.64×1031.64×103Wen (2013)
    Shipu Bay, Chinabay0.131.66395397Ji (2013)
    La Paz, Mexicobay0.474.80122127Urquidi-Gaume et al. (2016)
    Sansha Bay, Chinabay1.7313.8889905Wang et al. (2018a)
    Maowei Sea, Chinabay/mangrove0.7627.5196224Chen et al. (2018a)
    Shengsi, Chinasandy beach0.9938.189.9129Chen et al. (2019)
    Yucatan Peninsula, Mexicobeach5.538.22132146Null et al. (2014)
    Kinvara Bay, Irelandbay/karst aquifer0.240.3879.480.0Rocha et al. (2015)
    Zaton Bay, Croatiabay/anchialine cave0.090.89164165Chen et al. (2020a)
    Yellow Sea, Chinacontinental shelf/marginal sea0.427.72182190Ye et al. (2016)
    Xiangshan, Chinacoastal aquifer0.242.18325327Wen (2013)
    Chongming Dongtan, Chinasaltmarsh0.312821.48283this study
     | Show Table
    DownLoad: CSV
    Figure  5.  Ternary diagram illustrating the percentage of different inorganic nitrogen species in groundwater of different ecosystems. The concentrations of different inorganic nitrogen species in groundwater of other marine ecosystems are summarized in Table 3.
    DownLoad: Full-Size Img PowerPoint
    3.4.2   Porewater-derived DIC flux

    The DIC concentration in coastal groundwater is usually higher than that in surface waters (Maher et al., 2013; Tan et al., 2018; Reithmaier et al., 2021). The porewater DIC concentration in Chongming Dongtan ((9.12±1.65)×103 μmol/L) was also significantly higher than that in surface waters ((1.85±0.09)×103 μmol/L). In addition, the porewater DIC concentration in Chongming Dongtan was higher than that in other ecosystems such as bays, coral reefs, estuaries, continental shelves and marginal seas (range: 0−5.38 mmol/L; mean: 2.63 mmol/L), except for the Zhujiang River Estuary (1.2−19.5 mmol/L) (Table 4). Multiplying the porewater exchange rate by the corresponding concentration differences of DIC in porewater and overlying seawater yields porewater-derived DIC flux of 2.69×103 mmol/(m2·d), which is higher than that in other coastal ecosystems (1.6×103–1.96×103 mmol/(m2·d), summarized in Sadat-Noori et al. (2016) and Chen et al. (2018b)). Therefore, the DIC flux into coastal waters via porewater exchange in saltmarsh wetlands may be an important DIC source, which may have an important contribution to the coastal carbon budget.

    Table  4.  A summary of the DIC concentrations in different subterranean estuaries
    Study areaEcosystem typeDIC concentration/(mmol·L–1)Reference
    Western Port Bay, Australiamangrove11.4Lee et al. (2016)
    Watson Inlet, Australiamangrove2.6Faber et al. (2014)
    Chinaman Inlet, Australiamangrove2.7Faber et al. (2014)
    Blackwater River estuary, USAmangrove4.62Atekwana et al. (2003)
    Mangrove tidal creek, Australiamangrove5.15Maher et al. (2013)
    Haughton River Estuary, Australiamangrove13.8Kristensen and Alongi (2006)
    Ras Dege, Tanzaniamangrove2.82Bouillon et al. (2007)
    Can Gio Mangrove Biosphere Reserve, Vietnammangrove4.86Taillardat et al. (2018)
    Twin Cays, Belizemangroveabout 12.5Lee et al. (2008)
    Celestun Lagoon, Mexicomangrove0.74−1.53Young et al. (2005)
    Indian Sundarbans, Indiamangrove2.18Ray et al. (2018)
    Maowei Sea, Chinamangrove1.6Chen et al. (2018b)
    Rarotonga, Australiacoral reef0−3.06Cyronak et al. (2013)
    Sanya Bay, Chinacoral reef1.44−2.54Wang et al. (2014)
    Northern South China Seamarginal sea4Liu et al. (2012)
    Northern South China Seamarginal sea4.71Tan et al. (2018)
    East China Seamarginal sea4.65Tan et al. (2018)
    Waquoit Bay, USAbay0.7−1.9Liu et al. (2017)
    Sanggou Bay, Chinabay0.07−0.12Jiang et al. (2015)
    Bay of Puck, PolandBay5.38Szymczycha et al. (2014)
    Had Head, Australiaestuary1.46Sadat-Noori et al. (2016)
    Jiulong River Estuary, Chinaestuary0.5−1.4Wang et al. (2015)
    Zhujiang River Estuaryestuary1.2−19.5Cai et al. (2015)
    Weitou Bay, Chinasandy beach2.2−3.1Cai et al. (2020)
    Okatee, USAsaltmarsh3.51−22.7Moore et al. (2006)
    North Inlet, USAsaltmarsh2−12Cai et al. (2003)
    Chongming Dongtan, Chinasaltmarsh8.65this study
     | Show Table
    DownLoad: CSV

    3.5 Implications of porewater exchange in saltmarshes on nutrient and DIC fluxes into the Changjiang River Estuary

    The concentrations of ${\rm {NO}}_2^- $, ${\rm {NO}}_3^- $ and DIP in the porewater of Chongming Dongtan were significantly lower than those in surface waters, while the ${\rm {NH}}_4^+ $ concentration in the porewater was much higher than that in surface waters. Because the ${\rm {NO}}_3^- $ concentration in porewater of Chongming Dongtan was much lower than that in other ecosystems while the ${\rm {NH}}_4^+ $ concentration was significantly higher than that in other ecosystems (Table 3), the DIN concentration in porewater of Chongming Dongtan (range: 12–840 μmol/L; mean: (283±251) μmol/L) has the same order of magnitude as other coastal ecosystems (range: 30–1640 μmol/L; mean: (352±429) μmol/L) when compared with other ecosystems. However, due to the low DIP concentration in porewater, the N/P ratios (range: 160–4.0×103; mean: 681) in porewater of Chongming Dongtan were much higher than those in surface waters (61) and Redfield ratio (N: P=16: 1). Based on the porewater-derived DIN and DSi flux per unit area and the area of this study site (145 km2), the DIN and DSi fluxes through porewater exchange in the dry season of Chongming Dongtan were estimated to be 9.72×106 mol/d and 5.46×106 mol/d, respectively, which accounted for 12% and 5% of DIN and DSi fluxes in the Changjiang River (Zhang et al., 2007) (Fig. 6). These ratios of porewater exchange to river are within the broad range (DIN: 0.14–16.3; DSi: 0.07–2.8) observed in other estuarine systems worldwide (Table 5). Therefore, the nutrient fluxes with high N/P ratio via porewater exchange in Chongming Dongtan may change the nutrient components and the phytoplankton community structure in surface waters, thereby affect the nutrient cycles and the marine ecological environment of coastal waters.

    Figure  6.  A comparison of DIN, DSi and DIC to the Changjiang River Estuary via porewater exchange of saltmarshes and surface runoff of the Changjiang River in the dry season.
    DownLoad: Full-Size Img PowerPoint
    Table  5.  Nutrient and DIC fluxes via porewater exchange or SGD versus local riverine input for estuarine systems worldwide
    Study areaSGD/(106 mol·d–1)River/(106 mol·d–1)Ratio of SGD or porewater
    exchange to river    		Reference
    DINDSiDICDINDSiDICDINDSiDIC
    Chao Phraya River
    Estuary, Thailand    		0.19−0.480.11−0.65−0.49−2.30.26−4.3−0.21−0.400.15−0.44−Burnett et al. (2007)
    Yarra River Estuary,
    Australia    		0.01−0.230.04−0.410.25−0.55Santos et al. (2012a)
    Caloosahatchee River
    Estuary, USA    		0.017−−0.05−−0.35−−Charette et al. (2013)
    Tuckean Swamp,
    Australia    		0.005−−0.013−−0.41−−Santos et al. (2013)
    Jiulong River Estuary,
    China    		0.88−4.81.5−7.913−7211−2010−2235−580.14−0.320.07−0.190.24−1.1Wang et al. (2015)
    Krka River Estuary,
    Croatia    		0.05−0.330.05−0.273.7−220.020.1565.92.3−16.30.31−2.80.06−0.33Liu et al. (2019)
    Urbanized Estuary,
    China    		0.180.069−0.610.093−0.30.7−Xiao et al. (2019)
    Zhujiang River
    Estuary, China    		8.2−11533.4−6490.25−0.18Cai et al. (2015)
    Jiulong River Estuary,
    Chinaa    		−1.4−1.534.6−85.4−6.2−12.628.1−39.5−0.11−0.231.2−2.4Hong et al. (2017)
    Jiulong River Estuary,
    Chinab    		−2.7−14.420.7−118−6.2−12.628.1−39.5−0.44−1.10.74−2.4Hong et al. (2017)
    Chongming Dongtan,
    China    		9.725.56390831182 1900.120.050.18this study
    Note: aporewater exchange; bSGD.
     | Show Table
    DownLoad: CSV

    The DIC flux via porewater exchange in Chongming Dongtan was significantly higher than that in other ecosystems as described above, which indicates that porewater exchange has an important contribution to coastal DIC budget. Similar with those of nutrients, the DIC flux via porewater exchange in the dry season of Chongming Dongtan was estimated to be 3.90×108 mol/d, which accounted for 18% of DIC flux in the Changjiang River (Zhang et al., 2014) (Fig. 6). The ratio of porewater exchange to river for DIC are also within the range (0.06–2.4) observed in other estuarine systems worldwide (Table 5). Therefore, the high porewater-derived DIC flux may have a buffering effect on the decreased DIC concentration/flux in the Changjiang River Estuary due to upstream dam construction (Zhang et al., 2014).

    Chongming Dongtan, the largest and youngest saltmarsh in the Changjiang River Estuary, is a typical subtropical saltmarsh ecosystem in China. In the present study, porewater and surface water (time series observation) data sets were collected from the Chongming Dongtan during December 2017. Based on the distribution patterns of the 222Rn, nutrients and DIC in porewater and surface water, the following conclusions have been drawn:

    (1) Porewater exchange rate in Chongming Dongtan was estimated to be (37±35) cm/d during December 2017 using a 222Rn mass balance model. The estimated DIN, DSi and DIC fluxes via porewater exchange were 67 mmol/(m2·d), 38 mmol/(m2·d) and 2.69×103 mmol/(m2·d), respectively. Porewater-derived ${\rm {NO}}_2^- $, ${\rm {NO}}_3^- $ and ${\rm {NH}}_4^+ $ fluxes were −0.05 mmol/(m2·d), −37 mmol/(m2·d) and 104 mmol/(m2·d), respectively.

    (2) Compared with the nutrient and carbon fluxes through the Changjiang River, porewater-derived DIN, DSi and DIC fluxes in Chongming Dongtan was equivalent to 12%, 5% and 18% of the riverine inputs during December 2017.

    (3) Even though there is large riverine inputs, saltmarsh porewater exchange seems to be important sources of biogenic elements such as DIN and DIC into estuary or coastal area. This suggests that saltmarsh porewater exchange should be included in carbon and nutrient assessments in the Changjiang River Estuary.

    We would like to thank Yali Wang and Fule Zhang from SKLEC/ECNU for their kind assistance in the field and laboratory. We thank two reviewers for their constructive comments and suggestions that helped improve the manuscript.

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