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Article

The Role of Benthic TA and DIC Fluxes on Carbon Sequestration in Seagrass Meadows of Dongsha Island

by
Lan-Feng Fan
1,2,
En-Cheng Kang
1,2,
Mariche B. Natividad
1,3,4,
Chin-Chang Hung
5,6,
Yung-Yen Shih
5,7,
Wei-Jen Huang
5 and
Wen-Chen Chou
1,8,*
1
Institute of Marine Environment and Ecology, National Taiwan Ocean University, Keelung 202301, Taiwan
2
Precious Instrument Center of New Ocean Researcher 2, National Taiwan Ocean University, Keelung 202301, Taiwan
3
Ecosystems Research and Development Bureau, Laguna 4031, Philippines
4
Doctoral Degree Program on Ocean Resources and Environmental Changes, College of Ocean Science and Resources, National Taiwan Ocean University, Keelung 202301, Taiwan
5
Department of Oceanography, National Sun Yat-sen University, Kaohsiung 804201, Taiwan
6
Research Center for Environmental Changes, Academia Sinica, Taipei 115201, Taiwan
7
Department of Applied Science, R.O.C. Naval Academy, Kaohsiung 81345, Taiwan
8
Center of Excellence for the Oceans, National Taiwan Ocean University, Keelung 202301, Taiwan
*
Author to whom correspondence should be addressed.
J. Mar. Sci. Eng. 2024, 12(11), 2061; https://doi.org/10.3390/jmse12112061
Submission received: 21 October 2024 / Revised: 10 November 2024 / Accepted: 11 November 2024 / Published: 13 November 2024
(This article belongs to the Section Chemical Oceanography)
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Abstract

:
Coastal blue carbon ecosystems sequester carbon, storing it as plant biomass and particulate organic matter in sediments. Recent studies emphasize the importance of incorporating dissolved inorganic and organic forms into carbon assessments. As sediment-stored organic matter decomposes, it releases dissolved inorganic carbon (DIC) and total alkalinity (TA), both of which are critical for regulating the partial pressure of CO2 (pCO2) and thus carbon sequestration. This study investigated the role of benthic DIC and TA fluxes in carbon sequestration within seagrass meadows in Dongsha Island’s inner lagoon (IL) during the winter and summer seasons. Chamber incubation experiments revealed elevated benthic DIC and TA fluxes compared to global averages (107 ± 75.9 to 119 ± 144 vs. 1.3 ± 1.06 mmol m−2 d−1 for DIC, and 69.7 ± 40.7 to 75.8 ± 81.5 vs. 0.52 ± 0.43 mmol m−2 d−1 for TA). Despite DIC fluxes being approximately 1.5 times higher than TA fluxes, water pCO2 levels remained low (149 ± 26 to 156 ± 18 µatm). Mass balance calculations further indicated that benthic DIC was predominantly reabsorbed into plant biomass through photosynthesis (−135 to −128 mmol m−2 d−1). Conversely, TA accumulated in the water and was largely exported (−60.3 to −53.7 mmol m−2 d−1), demonstrating natural ocean alkalinity enhancement (OAE). This study highlights the crucial role of IL seagrass meadows in coastal carbon sequestration through net autotrophy and carbonate dissolution. Future research should explore the global implications of these processes and assess the potential of natural OAE in other coastal blue carbon ecosystems.

1. Introduction

Coastal blue carbon ecosystems, such as mangroves, seagrass beds, and salt marshes, play a critical role in mitigating climate change by sequestering atmospheric CO2 through photosynthesis [1,2,3,4]. These systems store carbon in both plant biomass and sediments [5,6]. Mangroves sequester the most carbon due to their large canopies, while seagrass contributes significantly to sediment carbon through rapid growth, decomposition, and accumulation [2,3,7]. The current blue carbon assessments have predominantly focused on particulate forms of carbon, including aboveground live and dead biomass, belowground biomass, and sediment organic carbon [8]. More recent studies, however, emphasize the importance of integrating dissolved forms of carbon—such as dissolved organic carbon (DOC) and dissolved inorganic carbon (DIC)—into these assessments [3,9,10].
Carbon storage within coastal systems is dynamic [11]. Organic matter buried in sediments decomposes over time, releasing inorganic forms such as DIC and total alkalinity (TA), both of which directly influence the overall carbon balance [10,12,13,14,15,16,17]. DIC consists of aqueous carbon dioxide (CO2*), bicarbonate (HCO3−), and carbonate (CO32−), while TA represents the sum of HCO3−, twice the concentration of CO32−, and other weak bases, such as borate and organic alkalinity [18]. In coastal sediments, aerobic respiration primarily produces DIC, while the anaerobic degradation of organic matter and carbonate dissolution generate both DIC and TA [19].
The relative proportions of DIC and TA fluxes play a crucial factor in determining whether a coastal system acts as a carbon sink or source [18,20]. Theoretically, DIC release can increase pCO2 and thus contribute to atmospheric CO2, while TA generation can decrease and promote CO2 sequestration [9,14,21,22,23]. A balance between the DIC and TA effects on pCO2 is achieved when their ratio approaches 0.87 [20,24]. The DIC-to-TA ratio is critical in regulating the buffering capacity of seawater and its ability to absorb CO2 [25]. Recently, studies have highlighted ocean alkalinity enhancement (OAE) as a strategy to alter this ratio and improve marine CO2 removal [26,27]. Monitoring both DIC and TA fluxes in benthic environments, surface waters, and during outwelling events provides insights into how these fluxes regulate coastal blue carbon storage.
Previous studies have indicated that intertidal systems, such as mangroves, seagrass beds, and salt marshes, often exhibit higher levels of DIC than TA in both porewater and benthic fluxes [15,16,28]. Elevated DIC concentrations, relative to TA, have also been observed in overlying waters, driving the outwelling of DIC into the sea [15,16,29]. This outwelling, particularly in the form of bicarbonate, plays a critical role in long-term carbon storage in oceans [16,29,30]. In fact, over 70% of intertidal wetlands export more DIC than TA, which can potentially increase pCO2 level in adjacent waters [17].
Recent studies have also consistently shown that DIC levels in intertidal systems are higher than TA levels in porewater, benthic fluxes, and overlying water, which may elevate the pCO2 of coastal waters [15,16,17,29]. In contrast, semi-enclosed systems like the inner lagoon of Dongsha Island display unique dynamics. The seagrass beds within this inner lagoon hold substantial carbon stocks with both living biomass and the underlying sediment [30]. While DIC and TA levels remain high in the porewater, the overlying water column exhibits elevated TA but relatively lower DIC [31]. This leads to lower pCO2 and higher pH in the water column [14,32], differing from the conditions observed in more open coastal systems [17]. Hung et al. [33] suggested that Taiwan should explore novel carbon dioxide removal methods such as seaweed cultivation, ocean alkalinity enhancement, etc., to achieve carbon neutrality by 2050. These distinct patterns suggest that biogeochemical processes in lagoons differ from those in intertidal systems, highlighting the need for further investigation through benthic incubation studies.
This study aims to explore the relationship between sediment-derived DIC and TA fluxes and its effect on carbon sequestration in seagrass ecosystems, with a focus on the carbonate chemistry dynamics in the system of coral-structured seagrass meadows of Dongsha Island. Specifically, the objective is to evaluate how benthic DIC and TA fluxes influence CO2 absorption and the overall carbon balance in these seagrass ecosystems. The findings will provide valuable insights for coastal carbon management and conservation efforts.

2. Materials and Methods

2.1. Site Description

Dongsha Island is a coral-constructed island on the western margin of Dongsha Atoll in the northern South China Sea (Figure 1). This island is approximately 2.80 km long and 0.87 km wide, with a total area of ~1.74 km2. Surrounding the island are extensive seagrass meadows, covering ~1.85 km2, with seagrass coverage ranging from 20% to 95% [30]. These meadows support about 14% of global seagrass diversity, with seven species identified across six genera and two families [34]. The dominant species are Thalassia hemprichii and Cymodocea rotundata.

2.2. Seawater and Porewater Sampling

Two field investigations were conducted in distinct seasons (11–17 January 2021 for winter; 9–16 September 2021 for summer) at two seagrass meadows on the island to compare diurnal cycles across season variations (Figure 1). The southern shore (SS) meadow has open ocean exchange, featuring a sandy habitat, while the inner lagoon (IL) meadow has restricted water exchange due to a structural barrier, resulting in a close nutrient cycle with dense seagrass detritus accumulation on the seabed.
Water depth, temperature, and salinity were recorded at both sites during winter using a multiparameter probe (Ocean Seven 316 Plus CTD, IDRONAUT, Brugherio, Italy) and during summer with an HI98194 waterproof meter (Hanna Instruments, Leighton Buzzard, UK). Dissolved oxygen (DO) was monitored with a HOBO U26 sensor, recording at 10 min intervals. A HOBO Pendant Temperature/Light Logger was placed 0.1 m above the seafloor to record light intensity. Wind speed data were also obtained from a local meteorological station on Dongsha Island (the Naval Meteorological and Oceanographic Office, Kaohsiung, Taiwan).
Seawater samples for DIC, TA, and pH were collected daily at 06:00, 12:00, 18:00, and 24:00 (summer only) using a 9 L Nalgene™ HDPE carboy. Samples were stored in 350 mL borosilicate bottles, preserved with 200 μL of saturated HgCl2, and transported to the National Taiwan Ocean University for analysis.
Porewater samples were collected at 2, 4, 6, 8, 12, 16, and 20 cm sediment depths using modified porewater wells [35]. Samples were extracted using Luer-Lok syringes; preserved according to Kindeberg et al. [36]; and analyzed for DIC, TA, pH, and calcium ion concentrations.

2.3. Sample Analyses

DIC, TA, and pH measurements followed the standard methods of Dickson and Millero [37], as in previous studies [14,20,32,38]. DIC was measured using a nondispersive infrared method with a DIC analyzer (AS-C3, Apollo SciTech Inc., Lincoln, NB, USA), and TA was quantified by Gran titration (AS-ALK2, Apollo SciTech Inc., Lincoln, NB, USA). Calibration was performed using certified reference materials (CRMs) from Dr. A. Dickson’s laboratory at Scripps Institution of Oceanography. Measurement precision was within ±0.15%.
pH was measured spectrophotometrically at 25 °C, following Clayton and Byrne [39], with a precision of 0.005. The aragonite saturation state (Ωa) was calculated using CO2SYS [40], using DIC, TA, temperature, and salinity. Calcium ion concentration was measured using an inductively coupled plasma mass spectrometry (ICP-MS) system (Agilent 7700×, Agilent Technologies, Santa Clara, CA, USA), following the method outlined by Su and Ho [41].

2.4. Benthic Incubations and Calculations

Benthic flux measurements were conducted using cylindrical polymethyl methacrylate chambers (35 cm diameter, 40 cm height), inserted 10–15 cm into the sediment. Chambers were equipped with HOBO U26 DO loggers and HOBO Pendant Temperature/Light Loggers (HOBO Data Loggers, Bourne, MA, USA), measuring at 10 min intervals. Water circulation pumps were installed to minimize chemical gradients [42].
Chamber incubations at the IL site spanned two days, covering both diurnal and seasonal variations. Seawater samples were collected from the chambers at 06:00, 12:00, and 18:00, with an additional sample at 24:00 (summer only). The benthic flux (FSEDI) of DIC and TA was calculated as follows:
FSEDI = ΔDIC or TA/Δt × hρ,
where ΔDIC or ΔTA is the concentration change during incubation, h is chamber height (volume/surface), and ρ is seawater density.

2.5. Statistics

Wilcox’s robust ANOVA (WR-ANOVA) was employed to account for heteroscedasticity and skewed carbonate chemistry data [43]. WR-ANOVA, which tests differences in medians, was used to compare pH, pCO2, DIC, and TA across seasons and sites. The “med1way” function from the R package “WRS2” [44] was used for analysis. All statistical analyses were performed in R (version 4.1.1) [45] with a significance level of 95%.

3. Results

3.1. Water Properties in Two Seagrass Meadows Across Two Seasons

The water properties in the IL and SS seagrass meadows, including water depth, temperature, salinity, and DO, are shown in Table 1 and Figure 2. Measurements of water depth and DO were taken only during winter. The IL meadow exhibited shallower and more stable water depths than the SS meadow (0.4 ± 0.1 vs. 1.0 ± 0.4 m, p < 0.01, Figure 2A). Water depth at the SS meadow followed trends predicted from the 2021 Tide Tables, consistent with previous observations [14,32].
Water temperature and salinity levels varied seasonally across the meadows. Temperatures were generally higher in summer than in winter at both sites (Figure 2B). During winter, temperatures in the IL meadow were higher than in the SS meadow (21.1 ± 1.1 vs. 19.0 ± 0.4 °C, p < 0.01). In summer, however, the IL meadow recorded slightly lower temperatures than the SS meadow (31.5 ± 0.8 vs. 32.1 ± 1.1 °C, p < 0.01).
Salinity was generally lower in winter than in summer (Figure 2C). In both seasons, salinity was higher in the IL meadow compared to the SS meadow. During winter, the IL meadow had a mean salinity of 34.7 ± 0.3, while the SS meadow averaged 33.9 ± 0.1 (p < 0.01). In summer, the salinity of the IL meadow averaged 33.0 ± 0.2, compared to 31.5 ± 0.4 in the SS meadow (p < 0.01).
DO levels also differed between sites (Figure 2D). The IL meadow had consistently lower DO concentrations than the SS meadow (mean ± SD: 183 ± 15 vs. 195 ± 27 μmol L−1, p < 0.01).

3.2. Water Carbonate Chemistry in Two Seagrass Meadows Across Two Seasons

The water carbonate chemistry of both seagrass meadows is presented in Table 1 and Figure 3. Diurnal variations in pH and pCO2 were observed in both seasons. In winter, pH ranged from 8.23 to 8.40 in IL and from 7.86 to 8.13 in SS, with corresponding pCO2 levels ranging from 138 to 187 μatm in IL and from 241 to 504 μatm in SS (Figure 3A,B). In summer, pH ranged from 8.34 to 8.53 in IL and from 7.93 to 8.42 in SS, while pCO2 ranged from 112 to 202 μatm in IL and 146 to 618 μatm in SS. The IL meadow consistently exhibited higher pH values than the SS meadow in winter (8.33 ± 0.07 vs. 7.99 ± 0.11, p < 0.01) and summer (8.44 ± 0.06 vs. 8.20 ± 0.16, p < 0.01). The pH of the IL meadow also exceeded the South China Sea average (~8.1, Chou et al. [46]). Conversely, pCO2 was significantly lower at the IL meadow than at the SS meadow in both winter (156 ± 18 vs. 374 ± 102 μatm, p < 0.01) and summer (149 ± 26 vs. 323 ± 150 μatm, p < 0.01). Both meadows exhibited pCO2 levels below the atmospheric average (~406 μatm, NOAA-ESRL MLO) [47].
Diurnal variations in DIC and TA were observed in both seasons (Figure 3C,D). There were no significant differences in DIC between the two meadows in both winter (1970 ± 39 μmol kg−1 at IL vs. 2033 ± 77 μmol kg−1 at SS, p = 0.08) and summer (1938 ± 64 μmol kg−1 at IL vs. 1874 ± 113 μmol kg−1 at SS, p = 0.17). However, TA was significantly higher in the IL meadow compared to the SS meadow during both winter (2490 ± 17 μmol kg−1 vs. 2300 ± 32 μmol kg−1, p < 0.01) and summer (2592 ± 30 μmol kg−1 vs. 2276 ± 36 μmol kg−1, p < 0.01).

3.3. Porewater Carbonate Chemistry in Two Seagrass Meadows

Porewater carbonate chemistry was analyzed in winter and summer (Figure 4). In winter, porewater pH generally decreased with depth, with a steeper trend observed in the IL meadow compared to the SS meadow (Figure 4A). Similar patterns were noted in summer, with an even steeper decline in the IL meadow.
DIC and TA concentrations both increased with depth. In winter, DIC showed a steeper increase in the IL meadow than in the SS meadow, while in summer, the increase in the IL meadow was notably steeper (Figure 4B). Similar trends were observed for TA, with a steeper trend in the IL meadow, especially during summer (Figure 4C).
The saturation state of aragonite (ΩAr) decreased with depth. In winter, the IL meadow exhibited a steeper decline compared to the SS meadow, with an even sharper decline observed in summer (Figure 4D). Calcium ion (Ca2+) concentrations also differed between the two sites, with IL meadow displaying higher and more variable levels than the SS meadow (Figure 4E).

3.4. Benthic Chamber Incubations

Benthic chamber incubations in the IL meadow during winter showed a pH decrease from 8.21 to 7.61 (Figure 5A). Concurrently, DIC and TA concentrations increased, ranging from 2100 to 3000 μmol kg−1 and 2523 to 3081 μmol kg−1, respectively (Figure 5B,C). In summer, the IL meadow exhibited an even sharper pH decrease, from 8.63 to 7.79, while DIC and TA levels increased, ranging from 1712 to 2961 μmol kg−1 and 2535 to 3202 μmol kg−1, respectively.

3.5. Benthic Flux Calculations

Benthic fluxes in the IL meadow displayed diurnal variations. In winter, DIC fluxes ranged from 0.75 ± 5.37 to 8.17 ± 3.34 mmol m−2 h−1 during the day and night, respectively. In summer, DIC fluxes varied from −6.27 ± 1.36 to 16.2 ± 11.9 mmol m−2 h−1. Daily DIC fluxes were at 107 ± 75.9 mmol m−2 d−1 in winter and 119 ± 144 mmol m−2 d−1 in summer. TA followed a similar pattern, with fluxes ranging from 1.66 ± 2.95 to 4.15 ± 1.68 mmol m−2 h−1 during winter and from −0.60 ± 0.14 to 6.92 ± 6.79 mmol m−2 h−1 during summer. Daily TA fluxes were 69.7 ± 40.7 mmol m−2 d−1 in winter and 75.8 ± 81.5 mmol m−2 d−1 in summer. These benthic flux results indicate a substantial release of DIC and TA from the sediment into the overlying water in both winter and summer, contributing to the carbon cycle within the seagrass meadow ecosystem.

3.6. Carbonate Chemistry Budgets

The budget accounts for various factors, including air–sea CO2 exchange (FGAS), sediment emissions (FSEDI), lateral advection (FADVH), and net biological community (NBC) effects (Table 2, Figure 6). The mass balance equation for TA and DIC in the IL system is as follows:
d(HL Con)/dt = FSEDI + FGAS + FADVH + NBC,
where d(HL Con)/dt represents the rate of change in the carbonate chemistry budget, including DIC and TA. In winter, DIC and TA changed at the rate of −2.91 ± 4.86 and −1.83 ±0.73 mmol m−2 d−1, respectively. Meanwhile, in summer, the rates of DIC and TA were −2.55 ± 1.59 and −0.34 ± 2.17 mmol m−2 d−1, respectively. The average water depth (HL) during the study period was 0.25 m.
Benthic flux (FSEDI) was determined using chamber incubations, which allowed for the assessment of metabolic variations within both the water column and sediment. The water column contribution was minimal, indicated by near-zero water NCP values [31].
Air–sea exchange (FGAS) was calculated using the following:
FGAS = kz × S × (ΔpCO2),
where kz represents the CO2 gas transfer velocity [48], S is the CO2 solubility, and ΔpCO2 is the difference in the partial pressure of CO2 between air and seawater. During both winter and summer, the system functioned as a carbon sink, with CO2 absorption averaging 25.1 mmol m−2 d−1 in winter and 2.99 mmol m−2 d−1 in summer.
Lateral advection (FADVH) also plays an essential role in the semi-enclosed IL system where tidal forces transport mixed water over a single day, with the advection of outer source water into the lagoon during flood tides, followed by the mixing of this water with lagoon water, which is then flushed out during ebb tides. FADVH is calculated using the following equation:
FADVH = HS × N × (CM − CS),
where HS denotes tidal variation (0.52 in winter and 0.17 m in summer, respectively), N represents the number of tidal cycles per day (diurnal in winter and semidiurnal in summer), and CS and CM denote the concentrations of outer source and mixed water, respectively. The value of CS is derived from data by Fan et al. [20], while CM is calculated using the following:
CM = (CL × HL + CS × HS)/(HL + HS),
where CL denotes the concentration of inner lagoon water. HL represents the average depth of the inner lagoon water, approximately 0.25 m. Winter saw the higher advection of both DIC (−7.42 mmol m−2 d−1) and TA (−44.0 mmol m−2 d−1) compared to summer (−6.16 and −77.3 mmol m−2 d−1).
Substituting these values into Equation (2), the net biological community effect (NBC) of DIC, representing primary production, was calculated as −128 mmol m−2 d−1 in winter and −119 mmol m−2 d−1 in summer. For TA, NBC values were lower in absolute values, with −27.6 mmol m−2 d−1 in winter and 1.08 mmol m−2 d−1 in summer.

4. Discussion

4.1. DIC and TA Production Hotspot in the Enclosed Lagoon

During this study, we found that the environmental properties of the IL meadow differed significantly from those of the open shore SS meadow (Table 1). Factors such as the enclosed system characteristics, small water volume, limited water mixing, shading by seagrass, and tidal influence contribute to the unique properties of the IL meadow [30,32]. The IL meadow exhibited more stable water depths compared to the SS meadow, reflecting the characteristic of enclosed system.
Temperatures in the IL meadow were higher in winter but lower in summer compared to the SS meadow, likely due to limited water mixing, smaller water volume, and shading from seagrass. The elevated winter temperatures were probably due to restricted water mixing, which retains heat from sunlight effectively. In summer, the reduction in temperature may have resulted from rapid heat dissipation at night in the smaller water volume and increased shading from the dense seagrass cover, which limited direct sunlight absorption. Salinity in the IL meadow remained consistently higher throughout both seasons, likely due to rapid evaporation in its restricted area. However, the difference in salinity between the IL and SS meadows was modest, possibly because frequent tidal flushing, particularly during higher tidal flows, helps moderate evaporation effects.
DO concentrations in the IL meadow were generally lower than in the SS meadow, which may have been due to limited water mixing and the decomposition of organic material in the sediment, both of which consume oxygen. Additionally, various heterotrophic processes in the IL meadow may have also contributed to these reduced DO levels. The relatively low average DO values observed in both the SS and IL meadows likely reflect high rates of both gross production and respiration, which balance each other out and result in the lower net production of DO. These findings align with similar results reported by Chou et al. [31].

4.2. DIC and TA Production Hotspot in the Enclosed Lagoon

This study highlights the fact that the seagrass meadow in the inner lagoon (IL) of Dongsha Island acts as a significant hotspot for TA production. The higher water pH and lower water pCO2 levels observed in the IL meadow suggest that it has a greater capacity to mitigate ocean acidification and absorb atmospheric CO2 [14,32]. Higher water TA levels also underscore the critical role of elevated TA in the IL meadow in facilitating carbon sink formation and mitigating ocean acidification [14,21]. Elevated porewater TA and Ca2+ concentrations were observed at the IL site, far exceeding those of the southern shore (SS) site. Porewater Ca2+ concentrations are primarily generated from carbonate dissolution which is a key biogeochemical process directly linked to the porewater TA contribution [14,32]. Porewater Ca2+ concentrations generally increased with depth, suggesting much more intense metabolic carbonate dissolution which contributes to the exceptionally high TA production. The benthic TA fluxes at IL, ranging from 69.7 ± 40.7 to 75.8 ± 81.5 mmol m−2 d−1, were orders of magnitude higher than the global average for seagrass meadows (0.52 ± 0.43 mmol m−2 d−1) [23]. This alkalinity production contributes to increasing TA concentrations in the water column, underscoring the IL meadow’s role in mitigating ocean acidification. In contrast, the SS site exhibited moderate porewater TA and Ca2+ levels, which were comparable to those found in the overlying water. Similar processes of enhanced alkalinity production have been documented in other seagrass meadows, such as those in the Bahamas [49,50] and Florida Bay [51,52], suggesting a broader ecological phenomenon of seagrass ecosystems acting as alkalinity sources.
The incubation observations indicate a continued release of DIC and TA from the sediment in both winter and summer (Figure 6). The DIC fluxes at the IL site, ranging from 107 ± 75.9 to 119 ± 144 mmol m−2 d−1, also far exceeded the global average for seagrass meadow (1.3 ± 1.06 mmol m−2 d−1) [23]. Additionally, the DIC flux at the IL site was much higher than the average carbon dioxide removal (42 mg-C m−2 d−1,= 144 mg-CO2 m−2 d−1 = 3.5 mmol-CO2 m−2 d−1) by phytoplankton in the South China Sea [33,53], though slightly lower than the average CO2 removal rate by red seaweed sarcodia suae (2.45 g-C m−2 d−1 = 204 mmol-CO2 m−2 d−1) [54]. These findings reveal the importance of natural DIC supply, which may positively impact the growth rate of seaweeds [55], particularly in mixed coastal zones containing both seaweeds and seagrass meadows. Interestingly, these DIC fluxes were approximately 1.5 times higher than TA fluxes (processes 3 plus 4 in Figure 7), suggesting that IL sediments may act as a CO2 source to the overlying water [17]. This could lead to CO2 release from the IL sediments and potential outwelling to adjacent coastal waters [7,15,16,17,28,29]. Intertidal ecosystems, such as mangroves, seagrass beds, and salt marshes, typically exhibit higher levels of DIC than TA in porewater and benthic fluxes, leading to significant DIC accumulation in the water column and potentially increasing pCO2 in nearby waters [15,16,17,21,56]. However, despite higher DIC fluxes, the decrease in water pCO2 indicates that the IL system functions as a carbon sink (Figure 2). This suggests that processes such as air–sea CO2 exchange, lateral advection, and biological community effects are balancing the carbon budget, preventing significant DIC accumulation in the water column [14,29,38,53]. More recently, Chang et al. [57] reported that some detritus from coastal blue carbon (including seagrass leaves and macroalgae) can be transported to deep waters in the South China Sea, which supports our postulation. Briefly, the DIC fluxes from the IL sediment can be stored in plant biomass as blue carbon, which eventually decomposes into detritus and sinks into deeper waters.

4.3. Fate of Benthic DIC and TA Fluxes

A carbonate chemistry budget was constructed to evaluated the fate of benthic DIC and TA fluxes for the IL seagrass meadows (Table 2, Figure 6 and Figure 7). As depicted in Figure 7, the carbonate chemistry budget analysis suggests that lateral advection and NBC likely determine the fate of benthic fluxes by either exporting out or utilizing benthic fluxes accumulated in the water column. Positive air–ea exchange values indicate a role as a carbon sink under conditions of active benthic fluxes.
In the IL meadow, elevated TA advection was exported out of the lagoon during tidal mixing, which is notable as other seagrass ecosystems often show lower absolute levels of TA advection out of the system (−44.0 to −77.3 vs. −5 ± 6 mmol m−2 d−1) [29]. The high TA advection observed in Dongsha’s IL suggests that natural alkalinity production enhances carbon sequestration by exporting TA-rich waters to adjacent areas (process 5 in Figure 7), potentially mitigating ocean acidification regionally. By contrast, absolute levels of DIC flow were much lower compared to other seagrass ecosystems (−6.16 to −7.42 vs. −114 ± 61 m−2 d−1) [29]. These comparative low levels of DIC advection may result from limited DIC flux, potentially moderated by biological processes within the system.
The NBC effect further indicates that seagrass production in the IL meadow balanced the DIC budget. NBC values for DIC, representing primary production, were consistent in magnitude with the net production estimated for the seagrass meadow in the IL (−128 to −119 vs. −338 ± 48.9 mmol m−2 d−1) [30]. These NBC DIC values, representing the portion of seagrass primary production involved in DIC reabsorption, are expected to fall within the estimated net production range. Despite high benthic DIC flux, primary production within the seagrass meadow was sufficient to counteract DIC influx from both sedimentary and atmospheric sources, maintaining a nearly net-zero carbon balance in the system (processes 1 plus 3 in Figure 7). For TA, associated with calcification, NBC values were lower than for DIC, likely due to differences in the underlying processes. While photosynthesis, which primarily affects DIC levels, is light-dependent, calcification, which influences TA, requires energy.
OAE is an emerging carbon dioxide removal (CDR) strategy with significant potential to mitigate climate change by enhancing the ocean’s ability to absorb atmospheric CO2 [25,26,58,59]. While field experiments, laboratory studies, mesocosm experiments, and models have provided insights into OAE [21,60,61,62,63,64,65,66], this study is the first to provide evidence of natural OAE processes in the high-alkalinity waters of the inner lagoon in Dongsha Island. The TA vented from IL sediments acts as a natural alkalinity enhancer, facilitating long-term carbon sequestration and CO2 absorption. Over time, this natural OAE could serve as a significant process for combating ocean acidification and increasing the resilience of coastal ecosystems to climate change. The elevated TA fluxes and efficient carbon sink observed in the IL meadow highlight the potential for seagrass meadows to be leveraged as natural systems for OAE, providing a cost-effective and ecologically sustainable CDR strategy.

5. Conclusions

This study underscores the critical role of benthic-derived TA and DIC fluxes from seagrass meadows in regulating coastal carbon budgets. As depicted in the conceptual diagram (Figure 7), the seagrass meadows in the inner lagoon of Dongsha Island exhibit elevated benthic fluxes, with DIC fluxes approximately 1.5 times higher than TA fluxes (reflected in processes 3 and 4). Both DIC and TA are released from the sediments into the water column, where their released DIC is primarily reutilized through photosynthesis into plant biomass, contributing to a nearly net-zero carbon budge (processes 1 and 3). In contrast, TA released to the water column can be accumulated (processes 1 and 4) and is largely advected out of the lagoon (process 5), illustrating natural OAE. These seagrass meadows act as effective carbon sinks through enhanced alkalinity production and autotrophic activities. By regulating DIC fluxes and exporting elevated TA, they play a crucial role in coastal carbon dynamics. Moreover, the natural OAE observed in this study offers a promising mechanism for mitigating ocean acidification and enhancing carbon sequestration. Future research should investigate the broader implications of these processes on global carbon cycles and assess the potential of natural OAE in other coastal ecosystems.

Author Contributions

Conceptualization, L.-F.F.; Validation, M.B.N., C.-C.H., Y.-Y.S. and W.-J.H.; Investigation, E.-C.K., Y.-Y.S., W.-J.H. and W.-C.C.; Data curation, E.-C.K.; Writing—original draft, L.-F.F.; Writing—review & editing, M.B.N., C.-C.H. and W.-C.C.; Funding acquisition, W.-C.C. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the National Science and Technology Council of Taiwan (grant numbers NSTC112-2611-M-019-005 and NSTC112-2119-M-019-008), which were awarded to W-CC.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The raw data supporting the conclusions of this article will be made available by the authors on request.

Acknowledgments

We are grateful to the Dongsha Atoll Research Station, Dongsha Atoll National Park, and Coast Guard Administration for their assistance in field sampling. We thank Hsin-Yu Chou for field sampling and laboratory assistance and Shin-E Wang for the basemap plate design. We also express our gratitude to Assistant Professor Cheng-Kuan Su from the Department of Chemistry at National Chung Hsing University for providing us with the ICP-MS system.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Geographical map showing the locations of Dongsha Atoll, Dongsha Island, and the designated sampling sites around Dongsha Island (upper panels). The lower panels include photographs of the seagrass meadow on the southern shore (SS) and the chamber incubation setup in the seagrass meadow of the inner lagoon (IL).
Figure 1. Geographical map showing the locations of Dongsha Atoll, Dongsha Island, and the designated sampling sites around Dongsha Island (upper panels). The lower panels include photographs of the seagrass meadow on the southern shore (SS) and the chamber incubation setup in the seagrass meadow of the inner lagoon (IL).
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Figure 2. Time-series observations of water depth (A1–A4), temperature (B1–B4), salinity (C1–C4), and DO (D1,D2) at the southern shore (SS, red squares) and inner lagoon (IL, blue circles) of Dongsha Island during summer (solid symbols) and winter (open symbols). Solid black lines in A1 to A4 represent trends predicted from the 2021 Tide Tables.
Figure 2. Time-series observations of water depth (A1–A4), temperature (B1–B4), salinity (C1–C4), and DO (D1,D2) at the southern shore (SS, red squares) and inner lagoon (IL, blue circles) of Dongsha Island during summer (solid symbols) and winter (open symbols). Solid black lines in A1 to A4 represent trends predicted from the 2021 Tide Tables.
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Figure 3. Daily variations in pH (A1–A4) at 25 °C, pCO2 in situ (B1–B4), DIC (C1–C4) and TA (D1,D2) at the southern shore (SS, red squares) and inner lagoon (IL, blue circles) of Dongsha Island during summer (solid symbols) and winter (open symbols). The horizontal dashed lines in panels (A1–A4,B1–B4) represent the average pH in the open northern South China Sea (8.1, Chou et al. [46]) and the monthly average atmospheric pCO2 (406 µatm, NOAA-ESRL MLO [47]), respectively.
Figure 3. Daily variations in pH (A1–A4) at 25 °C, pCO2 in situ (B1–B4), DIC (C1–C4) and TA (D1,D2) at the southern shore (SS, red squares) and inner lagoon (IL, blue circles) of Dongsha Island during summer (solid symbols) and winter (open symbols). The horizontal dashed lines in panels (A1–A4,B1–B4) represent the average pH in the open northern South China Sea (8.1, Chou et al. [46]) and the monthly average atmospheric pCO2 (406 µatm, NOAA-ESRL MLO [47]), respectively.
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Figure 4. Vertical porewater profiles of pH (A), DIC (B), TA (C), ΩAr (D), and Ca2+ (E) at the southern shore (SS, red square) and inner lagoon (IL, blue circles) of Dongsha Island during summer (solid symbols) and winter (open symbols).
Figure 4. Vertical porewater profiles of pH (A), DIC (B), TA (C), ΩAr (D), and Ca2+ (E) at the southern shore (SS, red square) and inner lagoon (IL, blue circles) of Dongsha Island during summer (solid symbols) and winter (open symbols).
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Figure 5. Chamber incubation results showing pH at 25 °C (A1,A2), DIC (B1,B2), and TA (C1,C2) in the inner lagoon of Dongsha Island during winter (left, open symbols) and summer (right, solid symbols).
Figure 5. Chamber incubation results showing pH at 25 °C (A1,A2), DIC (B1,B2), and TA (C1,C2) in the inner lagoon of Dongsha Island during winter (left, open symbols) and summer (right, solid symbols).
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Figure 6. Conceptual diagrams summarizing the mass balance (mmol m−2 d−1) of dissolved inorganic carbon (DIC, 1) and total alkalinity (TA, 2) in the inner lagoon of Dongsha Island during winter (A1,A2) and summer (B1,B2). The water system dynamics (d(H DIC)/dt and d(H TA)/dt) are influenced by sediment emissions (FSEDI), air–sea CO2 exchange (FGAS), lateral advection (FADVH), and net biological community (NBC) effects. Positive values represent carbon fluxes into the lagoon, while negative values indicate carbon fluxes out of the lagoon.
Figure 6. Conceptual diagrams summarizing the mass balance (mmol m−2 d−1) of dissolved inorganic carbon (DIC, 1) and total alkalinity (TA, 2) in the inner lagoon of Dongsha Island during winter (A1,A2) and summer (B1,B2). The water system dynamics (d(H DIC)/dt and d(H TA)/dt) are influenced by sediment emissions (FSEDI), air–sea CO2 exchange (FGAS), lateral advection (FADVH), and net biological community (NBC) effects. Positive values represent carbon fluxes into the lagoon, while negative values indicate carbon fluxes out of the lagoon.
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Figure 7. A conceptual diagram illustrating two different pathways for organic carbon cycles: one solely focused on the metabolic cycle (green title, enclosed by a green dashed line), where both DIC and TA budgets show a net-zero impact; the other coupled with carbonate dissolution and DIC reabsorption by the autotrophic activity of seagrasses (blue title), leading to elevated TA accumulation in the water column, which is largely advected out of the lagoon. This represents natural ocean alkalinity enhancement and contributes to more effective carbon sequestration. Under the blue title, for example, “DIC +1 & TA +2” indicates that DIC increases by 1 unit and TA increases by 2 units when processes 1 and 4 are combined. We have noted this in the figure caption. “DIC +1 & TA +2” under the blue title, for instance, indicates that DIC increases by 1 unit and TA by 2 units when processes 1 and 4 are combined. Changes in DIC and TA during other processes are expressed similarly.
Figure 7. A conceptual diagram illustrating two different pathways for organic carbon cycles: one solely focused on the metabolic cycle (green title, enclosed by a green dashed line), where both DIC and TA budgets show a net-zero impact; the other coupled with carbonate dissolution and DIC reabsorption by the autotrophic activity of seagrasses (blue title), leading to elevated TA accumulation in the water column, which is largely advected out of the lagoon. This represents natural ocean alkalinity enhancement and contributes to more effective carbon sequestration. Under the blue title, for example, “DIC +1 & TA +2” indicates that DIC increases by 1 unit and TA increases by 2 units when processes 1 and 4 are combined. We have noted this in the figure caption. “DIC +1 & TA +2” under the blue title, for instance, indicates that DIC increases by 1 unit and TA by 2 units when processes 1 and 4 are combined. Changes in DIC and TA during other processes are expressed similarly.
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Table 1. Comparison of range and mean (mean ± SD) values for water depth (m), surface seawater temperature (°C), salinity, DO (μmol L−1), pH, pCO2 (μatm), DIC (μmol kg−1), and TA (μmol kg−1) between the inner lagoon (IL) and the southern shore (SS) sites (right columns) and between summer and winter seasons (bottom rows). Statistically significant differences across seasons and sites were determined using Wilcoxon’s robust Analysis of Variance, with post hoc test results indicating significance at p < 0.01. The p value in the right column refers to the IL-SS comparison, while the p value at the bottom of table corresponds to the summer–winter comparison.
Table 1. Comparison of range and mean (mean ± SD) values for water depth (m), surface seawater temperature (°C), salinity, DO (μmol L−1), pH, pCO2 (μatm), DIC (μmol kg−1), and TA (μmol kg−1) between the inner lagoon (IL) and the southern shore (SS) sites (right columns) and between summer and winter seasons (bottom rows). Statistically significant differences across seasons and sites were determined using Wilcoxon’s robust Analysis of Variance, with post hoc test results indicating significance at p < 0.01. The p value in the right column refers to the IL-SS comparison, while the p value at the bottom of table corresponds to the summer–winter comparison.
SeasonParameterInner LagoonSouth ShoreIL—SSp Value
WinterWater depth0.22–0.74
(0.36 ± 0.14)		0.32–1.78
(0.96 ± 0.44)		−0.60<0.01
Temperature19.1–23.1
(21.1 ± 1.1)		18.2–19.8
(19.0 ± 0.4)		2.1<0.01
Salinity34.4–35.4
(34.7 ± 0.3)		33.7–34.0
(33.9 ± 0.1)		0.8<0.01
DO136–216
(183 ± 15)		160–256
(195 ± 27)		−11<0.01
pH8.23–8.40
(8.33 ± 0.07)		7.86–8.13
(7.99 ± 0.11)		0.34<0.01
pCO2138–187
(156 ± 18)		241–504
(374 ± 102)		−218<0.01
DIC1923–2035
(1970 ± 39)		1919–2123
(2033 ± 77)		−64=0.08
TA2461–2490
(2490 ± 17)		2257–2351
(2300 ± 32)		190<0.01
SummerWater depth----
Temperature28.8–33.2
(30.8 ± 1.1)		30.1–33.8
(32.2 ± 1.1)		−1.4<0.01
Salinity32.8–33.3
(33.0 ± 0.2)		30.5–33.1
(31.5 ± 0.4)		1.5<0.01
DO----
pH8.34–8.53
(8.44 ± 0.06)		7.93–8.42
(8.20 ± 0.16)		0.24<0.01
pCO2112–202
(149 ± 26)		146–618
(323 ± 150)		−174<0.01
DIC1852–2034
(1938 ± 64)		1657–2039
(1874 ± 113)		64=0.17
TA2544–2623
(2592 ± 30)		2204–2329
(2276 ± 36)		316<0.01
S—WpH0.110.21
pCO2−7−51
DIC−32−159
TA102−23
p valuepH<0.01<0.01
pCO2=0.53=0.43
DIC=0.25<0.01
TA<0.01=0.20
Table 2. Summary of mass balance results for dissolved inorganic carbon (DIC) and total alkalinity (TA) in the inner lagoon of Dongsha Island during winter (January 2019) and summer (September 2019). The water system dynamics (d(H con.)/dt) are influenced by sediment emissions (FSEDI), air–sea CO2 exchange (FGAS), lateral advection (FADVH), and net biological community (NBC) effects. Positive values represent carbon fluxes into the lagoon, while negative values indicate carbon fluxes out of the lagoon.
Table 2. Summary of mass balance results for dissolved inorganic carbon (DIC) and total alkalinity (TA) in the inner lagoon of Dongsha Island during winter (January 2019) and summer (September 2019). The water system dynamics (d(H con.)/dt) are influenced by sediment emissions (FSEDI), air–sea CO2 exchange (FGAS), lateral advection (FADVH), and net biological community (NBC) effects. Positive values represent carbon fluxes into the lagoon, while negative values indicate carbon fluxes out of the lagoon.
(mmol m−2 d−1)DICTA
SeasonWinterSummerWinterSummer
d(H con.)/dt−2.91 ± 4.86−2.55 ± 1.59−1.83 ± 0.73−0.34 ± 2.17
FGAS25.12.99--
FADVH−7.42−6.16−44.0−77.3
FSED107 ± 75.9119 ± 14469.7 ± 40.775.8 ± 81.5
FBIOA−128−119−27.61.08
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MDPI and ACS Style

Fan, L.-F.; Kang, E.-C.; Natividad, M.B.; Hung, C.-C.; Shih, Y.-Y.; Huang, W.-J.; Chou, W.-C. The Role of Benthic TA and DIC Fluxes on Carbon Sequestration in Seagrass Meadows of Dongsha Island. J. Mar. Sci. Eng. 2024, 12, 2061. https://doi.org/10.3390/jmse12112061

AMA Style

Fan L-F, Kang E-C, Natividad MB, Hung C-C, Shih Y-Y, Huang W-J, Chou W-C. The Role of Benthic TA and DIC Fluxes on Carbon Sequestration in Seagrass Meadows of Dongsha Island. Journal of Marine Science and Engineering. 2024; 12(11):2061. https://doi.org/10.3390/jmse12112061

Chicago/Turabian Style

Fan, Lan-Feng, En-Cheng Kang, Mariche B. Natividad, Chin-Chang Hung, Yung-Yen Shih, Wei-Jen Huang, and Wen-Chen Chou. 2024. "The Role of Benthic TA and DIC Fluxes on Carbon Sequestration in Seagrass Meadows of Dongsha Island" Journal of Marine Science and Engineering 12, no. 11: 2061. https://doi.org/10.3390/jmse12112061

APA Style

Fan, L. -F., Kang, E. -C., Natividad, M. B., Hung, C. -C., Shih, Y. -Y., Huang, W. -J., & Chou, W. -C. (2024). The Role of Benthic TA and DIC Fluxes on Carbon Sequestration in Seagrass Meadows of Dongsha Island. Journal of Marine Science and Engineering, 12(11), 2061. https://doi.org/10.3390/jmse12112061

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