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

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The manuscript “The Role of Benthic TA and DIC Fluxes on Carbon Sequestration in Seagrass Meadows of Dongsha Island” is well written and very interesting and, in my opinion, could be published after some revisions. I would suggest to lighten the text in Results by increasing the Tables at the expense of the data reported directly in the text. There are some aspects of physical chemistry for which I asked for further explanations. In addition, I report some suggestions below.

 

Lines 168-172 – How do you explain that the shallow, internal IL area has a lower temperature than the open SS area, even in summer?

 

Lines 174-177 – How can it be explained that in IL an evaporation effect does not increase salinity more than in SS?

 

Lines 179-182 – How do you explain that DO values ​​are lower in IL than in SS, given that theoretically photosynthesis should over-saturated the modest water column?

 

There are values ​​that I do not understand how it is possible:

the summer temperature is higher in SS (contact with the ocean) than in IL (shallow water area and in the absence of water renewal).

DO has relatively low values ​​both in open areas and in closed lagoon areas, in meadows that should saturate the water with oxygen, especially in IL.

195 micromoles per liter correspond to 3.12 mg/L which is very little for a meadow!

 

Lines 196-198 – This is a sentence to be moved to Discussion. 

Lines 221-222 – the same

 

Lines 212-216 – If the ranges were included in Table 1, the text would be lighter for the reader.

 

Lines 225-234 – It would be appropriate to prepare a table with all these ranges, reading in the text is tiring

 

Lines 247-248 – Transfer to Discussion. Drawing conclusions from results is not a result but a speculative element that should be transferred to the discussion.

 

Lines 250-258 – I would suggest increasing the tables with all the data that are inserted in the text, reducing the text of the Results, then moving on to resume the deductions in Discussion.

 

In an overall balance, the mitigation of ocean acidity determined by the meadows in the lagoon area seems to me to be little. Isn't that so?

So this phenomenon could have an effect, but limited to narrow coastal areas.

 

Lines 338-340 – transfer to M&M

 

I would move the conceptual model of Figure 7 to Discussion

Author Response

Lines 168-172 – How do you explain that the shallow, internal IL area has a lower temperature than the open SS area, even in summer?

Response: We have clarified in the Discussion section that the IL meadow's lower temperature in summer may result from rapid heat dissipation at night in the smaller water volume and increased shading from the dense seagrass cover, which limits direct sunlight absorption. (L310-313)

Lines 174-177 – How can it be explained that in IL an evaporation effect does not increase salinity more than in SS?

Response:  We clarified in the Discussion that salinities in IL were generally higher than in SS, though the difference was modest. This may be due to frequent tidal flushing, especially during higher tidal flows, which helps moderate evaporation effects. (L313-316)

Lines 179-182 – How do you explain that DO values ​​are lower in IL than in SS, given that theoretically photosynthesis should over-saturated the modest water column?

Response: We addressed this in the Discussion, noting that lower DO levels in IL could result from limited water mixing and the decomposition of organic material within the sediment, which consumes oxygen. (L317-319)

There are values ​​that I do not understand how it is possible:

the summer temperature is higher in SS (contact with the ocean) than in IL (shallow water area and in the absence of water renewal).

Response: There are two possible reasons for the lower summer temperature in IL: rapid heat dissipation at night due to the smaller water volume and increased shading from the dense seagrass cover, which limits direct sunlight absorption. (L310-313)

DO has relatively low values ​​both in open areas and in closed lagoon areas, in meadows that should saturate the water with oxygen, especially in IL.

195 micromoles per liter correspond to 3.12 mg/L which is very little for a meadow!

Response: The relatively low average DO values in both SS and IL are likely due to high rates of both gross production and respiration, which balance each other. Consequently, the net DO production is relatively low. Similar findings have been reported by Chou et al. [31]. (L320-324)

Lines 196-198 – This is a sentence to be moved to Discussion. 

Response: This section has been moved to the Discussion to streamline the Results section. (L327-329)

Lines 221-222 – the same

Response: This part was also moved to the Discussion for better flow. (L329-331)

Lines 212-216 – If the ranges were included in Table 1, the text would be lighter for the reader.

Response: Data ranges have been added to Table 1 to facilitate reader access, and the text was revised for clarity. (L214-220)

Lines 225-234 – It would be appropriate to prepare a table with all these ranges, reading in the text is tiring

Response: We have rephrased this section, removing the range values from the text since the full information is provided in Figure 4. An additional table was not added due to space constraints. (L222-238)

Lines 247-248 – Transfer to Discussion. Drawing conclusions from results is not a result but a speculative element that should be transferred to the discussion.

Response: Conclusions have been transferred to the Discussion. (L334, 335)

Lines 250-258 – I would suggest increasing the tables with all the data that are inserted in the text, reducing the text of the Results, then moving on to resume the deductions in Discussion.

Response: We condensed this section by removing average values and moving inferences to the Discussion, with the full data available in Figure 5. An additional table was not included due to space limitations. (L240-245, 345, 346)

In an overall balance, the mitigation of ocean acidity determined by the meadows in the lagoon area seems to me to be little. Isn't that so?

So this phenomenon could have an effect, but limited to narrow coastal areas.

Response: Thank you for this insight. We agree that while this phenomenon is indeed limited to coastal areas, it could serve as a natural method to help mitigate global climate change. By simulating the carbon chemistry characteristics of IL and applying them in artificial facilities, like wastewater treatment plants, we could potentially expand these benefits.

Lines 338-340 – transfer to M&M

Response: This section has been relocated to the M&M. (L119-120).

I would move the conceptual model of Figure 7 to Discussion

Response: We have moved Figure 7 to the Discussion section. (L355, 385, 386, 407)

Reviewer 2 Report

Comments and Suggestions for Authors

Review for the paper “The Role of Benthic TA and DIC Fluxes on Carbon Sequestration in Seagrass Meadows of Dongsha Island” by Lan-Feng Fan, En-Cheng Kong, Mariche B. Natividad, Chin-Chang Hung, Yung-Yen Shih, Wei-Jen Huang, Wen-Chen Chou submitted to “JMSE”.

The authors of this research paper conducted an analysis to evaluate the role of benthic dissolved inorganic carbon and total alkalinity fluxes in carbon sequestration within seagrass meadows located in Dongsha Island's inner lagoon. The dissolved inorganic carbon flux was found to be roughly 1.5 times higher than the total alkalinity flux, yet the partial pressure of carbon dioxide in the water remained low. They found that the mass balance calculations indicated a predominant reabsorption of benthic dissolved inorganic carbon into plant biomass via photosynthesis, while the total alkalinity accumulated in the water and was mostly exported. The results of this study may have important implications for understanding the mechanisms of carbon sequestration in coastal ecosystems, particularly the potential role of seagrass meadows in enhancing ocean alkalinity.

 

Some revisions are needed to improve the clarity of the conclusions.

 

Introduction. L 39. The authors should mention how mangroves, seagrass beds, and salt marshes differ in their mechanisms of carbon sequestration.

 

Introduction. L 51. The authors should indicate how aerobic and anaerobic diagenetic processes contribute differently to the production of DIC in coastal sediments.

 

Methods. L 101-102. The authors should explain the rationale behind the timing of sample collection.

 

Methods. L 103-104. The authors should expand the description of the study area by explaining how these structural characteristics affect water exchange, nutrient cycling, and habitat quality.

 

Results. Section 3.1. The authors should compare the environmental data statistically using an appropriate method to confirm differences between sites and seasons.

 

Discussion. The authors did not discuss the results presented in Section 3.1.

They should explain what factors contribute to the more stable water depths observed in the IL meadow compared to the SS meadow, how the seasonal and diurnal temperature variations in both meadows impact the biological processes of the seagrass ecosystems, what might be causing the site-specific salinity variations observed between the IL and SS meadows.

 

Discussion. Section 4.1. The authors should explain more clearly what specific biogeochemical processes within the IL meadow contribute to the exceptionally high TA production observed, exceeding global averages by orders of magnitude.

 

Discussion. L 276. The authors should explain how the elevated porewater Ca2+ concentrations in the IL meadow relate to the enhanced TA production. Is there a direct causal link, and what are the underlying mechanisms?

 

Discussion. L 296.  The authors mention that the DIC flux is approximately 1.5 times higher than the TA flux. They should discuss the potential implications of this imbalance for the overall carbon budget of the IL meadow and the adjacent coastal waters.

 

Discussion. L 305. The authors suggested that detritus transport to deep waters plays a role. It would be useful to clarify how significant this transport mechanism in mitigating the high DIC fluxes observed at the IL site and maintaining the overall carbon sink function.

 

Discussion. L 307-358. This text presents results and should be laced in the corresponding section.

 

Discussion. L 365-368. The NBC calculation suggests seagrass primary production balances the DIC budget.  However, there is a discrepancy between the calculated DIC NBC (-128 to -119 mmol m⁻² d⁻¹) and the independently estimated net production (338 ± 48.9 mmol m⁻² d⁻¹). How can this difference be reconciled?

In addition, the authors stated that the TA NBC values are considerably lower than the DIC NBC values. They should explain what are the key processes responsible for this imbalance in the net biological community effect on TA and DIC?

Author Response

Introduction. L 39. The authors should mention how mangroves, seagrass beds, and salt marshes differ in their mechanisms of carbon sequestration.

Response: We have expanded the introduction to clarify these differences as mangroves sequester the most carbon due to their large canopies, while seagrasses contribute substantially to sediment carbon through rapid growth, decomposition, and accumulation. This addition highlights the distinct carbon sequestration processes among these habitats. (L40-41)

Introduction. L 51. The authors should indicate how aerobic and anaerobic diagenetic processes contribute differently to the production of DIC in coastal sediments.

Response: We have revised this section, specifying that aerobic respiration primarily generates DIC, while anaerobic degradation of organic matter and carbonate dissolution produce both DIC and TA, highlighting their distinct contributions to DIC production. (L53-55)

Methods. L 101-102. The authors should explain the rationale behind the timing of sample collection.

Response: We elaborated on the timing rationale, explaining its alignment with diurnal cycles to capture representative seasonal variations. (L104-105)

Methods. L 103-104. The authors should expand the description of the study area by explaining how these structural characteristics affect water exchange, nutrient cycling, and habitat quality.

Response: We added details on site-specific characteristics, discussing their effects on water exchange, nutrient cycling, and habitat quality. (L106-108)

Results. Section 3.1.

The authors should compare the environmental data statistically using an appropriate method to confirm differences between sites and seasons.

Response: We included statistical analyses using robust ANOVA methods to confirm significant differences in environmental parameters between the IL and SS sites across seasons, as presented in Table 1 and elaborated in the Results section. (L164, 165, 174-177, 179-185, 207, 208)

Discussion. The authors did not discuss the results presented in Section 3.1.

Response: We added a discussion consistent with the findings in Section 3.1. (L300-324)

They should explain what factors contribute to the more stable water depths observed in the IL meadow compared to the SS meadow, how the seasonal and diurnal temperature variations in both meadows impact the biological processes of the seagrass ecosystems, what might be causing the site-specific salinity variations observed between the IL and SS meadows.

Response: We discussed factors such as the enclosed system characteristics, limited water volume, restricted mixing, seagrass shading, and tidal influences as contributors to the observed stability in water depth, temperature, salinity, and DO in the IL. (L302-304)

Discussion. Section 4.1. The authors should explain more clearly what specific biogeochemical processes within the IL meadow contribute to the exceptionally high TA production observed, exceeding global averages by orders of magnitude.

Response: We elaborated on carbonate dissolution as a key biogeochemical process contributing to the high TA production, noting the elevated porewater Ca²⁺ concentrations associated with it. (L334-336)

Discussion. L 276. The authors should explain how the elevated porewater Ca2+ concentrations in the IL meadow relate to the enhanced TA production. Is there a direct causal link, and what are the underlying mechanisms?

Response: We clarified that porewater Ca²⁺ concentrations, generated primarily from carbonate dissolution, directly contribute to TA production in the IL meadow. (L332-334)

Discussion. L 296. The authors mention that the DIC flux is approximately 1.5 times higher than the TA flux. They should discuss the potential implications of this imbalance for the overall carbon budget of the IL meadow and the adjacent coastal waters.

Response: We noted that this imbalance in the carbon budget could lead to CO₂ release from IL sediments and potential outwelling to adjacent coastal waters. This point has been included in the discussion section. (L356-358)

Discussion. L 305. The authors suggested that detritus transport to deep waters plays a role. It would be useful to clarify how significant this transport mechanism in mitigating the high DIC fluxes observed at the IL site and maintaining the overall carbon sink function.

Response: We clarified that DIC fluxes from IL sediments may be sequestered as blue carbon within plant biomass, which decomposes into detritus that eventually sinks into deeper waters. (L367-369)

Discussion. L 307-358. This text presents results and should be laced in the corresponding section.

Response: We reorganized by relocating portions of the text that present results to the Results section to better align findings with the discussion. (L261-298, 383-410)

Discussion. L 365-368. The NBC calculation suggests seagrass primary production balances the DIC budget.  However, there is a discrepancy between the calculated DIC NBC (-128 to -119 mmol m⁻² d⁻¹) and the independently estimated net production (338 ± 48.9 mmol m⁻² d⁻¹). How can this difference be reconciled?

Response: The calculated DIC NBC represents the portion of seagrass primary production involved in DIC reabsorption. These values are anticipated to fall within the estimated net production. (L402-404)

In addition, the authors stated that the TA NBC values are considerably lower than the DIC NBC values. They should explain what are the key processes responsible for this imbalance in the net biological community effect on TA and DIC?

Response: The imbalance arises from differing processes. Photosynthesis, primarily affecting DIC levels, is light-dependent, while calcification, which influences TA, requires energy. This distinction contributes to the disparity in DIC and TA effects. (L407-410)

Reviewer 3 Report

Comments and Suggestions for Authors

please, see the attached report.

Comments for author File: Comments.pdf

Author Response

Figure 2, caption: Please specify that solid black lines in A1 to A4 represent trends predicted from the 2021 Tide Tables.

Response: We have clarified this notation in the figure cation. (L171-172)

Table 1, caption: p<0.01 instead of p<0.05

Please note that the p-value at the bottom of table 1 refers to Summer-Winter.

Response: We have corrected the p value to < 0.01. Additionally, we clarified in the table caption that “the p value in the right column refers to the IL-SS comparison, while the p value at the bottom corresponds to the summer–winter comparison.” (L212, 213)

Table 2, caption: ?^????

Response: We have corrected this to ?^????. (L382)

Line 358: “other seagrass ecosystems often show lower levels of TA advection.” Please, clarify: In this case the TA flux is imported whereas the TA flux is exported in Dongsha’s IL. This inversion also occurs for the DIC flow.

Response: Apologies for the previous misinterpretation. Both the literature and our results indicate advection out of the system (negative values). We have revised the phrase to read: “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]),” and “DIC flux levels were significantly lower compared to those in other seagrass ecosystems (-6.16 to -7.42 vs. -114 ± 61 mmol m^-2 d^-1; [29]).” (L390-392, 395-398)

Lines 364-367 the reasoning concerning the DIC flux in seagrass meadow seems not clear.

Response: In our discussion, we now state: “Comparatively low levels of DIC advection may result from limited DIC flux, potentially moderated by other processes, such as the influence of biological communities.” (L397-398)

Lines 367-371 require more explanation.

Response: We added the following clarification: “The values of DIC NBC are anticipated to fall within the estimated net production, explaining why primary production in the seagrass meadow was sufficient to counteract DIC influx. Additionally, differences in contributions from DIC NBC and TA NBC arise from distinct processes. While photosynthesis, which primarily affects DIC levels, is light-dependent, calcification, influencing TA, requires energy.” (L404-404, 408-410)

Figure 7: what is the meaning of DIC +2, TA -1…

Response: "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. We have added this note to the figure caption. (L447-449)

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

No further comments.