Preprint Review Version 1 Preserved in Portico This version is not peer-reviewed

Carbon Balance in Salt Marsh and Mangrove Ecosystems: A Global Synthesis

Version 1 : Received: 9 September 2020 / Approved: 10 September 2020 / Online: 10 September 2020 (11:22:38 CEST)

A peer-reviewed article of this Preprint also exists.

Alongi, D.M. Carbon Balance in Salt Marsh and Mangrove Ecosystems: A Global Synthesis. J. Mar. Sci. Eng. 2020, 8, 767. Alongi, D.M. Carbon Balance in Salt Marsh and Mangrove Ecosystems: A Global Synthesis. J. Mar. Sci. Eng. 2020, 8, 767.


Mangroves and salt marshes are among the most productive ecosystems in the global coastal ocean. Mangroves store more carbon (739 Mg CORG ha-1) than salt marshes (334 Mg CORG ha-1), but the latter sequester proportionally more (24%) net primary production (NPP) than mangroves (12%). Mangroves exhibit greater rates of gross primary production (GPP), above-ground net primary production (NPP) and plant respiration (RC) with higher PGPP/RC ratios, but salt marshes exhibit greater rates of below-ground NPP. Mangroves have greater rates of subsurface DIC production and, unlike salt marshes, exhibit significant microbial decomposition to a soil depth of 1 m. Salt marshes release more soil CH4 and export more dissolved CH4 , but mangroves release more CO2 from tidal waters and export greater amounts of POC, DOC and DIC to adjacent waters. Both ecosystems contribute only a small proportion of GPP, RE (ecosystem respiration) and NEP (net ecosystem production) to the global coastal ocean due to their small global area, but contribute 72% of air-sea CO2 exchange from the world’s wetlands and estuaries and contribute 34% of DIC export and 17% of DOC + POC export to the world’s coastal ocean. Thus, both wetland ecosystems contribute disproportionately to carbon flow of the global coastal ocean.


Biogeochemistry; Carbon; Carbon balance; Ecosystem; Ecosystem processes; Mangrove; Salt marsh; Wetland


Environmental and Earth Sciences, Environmental Science

Comments (0)

Comment 1
Received: 21 September 2020
Commenter: Judith Rosentreter
The commenter has declared there is no conflict of interests.
Comment: Dear Dan Alongi,

Thank you for this interesting review. I enjoyed reading your synthesis. However, I have several questions regarding your global estimates of water-air fluxes from mangrove and salt-marsh waters.

1. You cite your recently published review "Carbon Cycling in the World ’ s Mangrove Ecosystems Revisited : Significance of Non-Steady State Diagenesis and Subsurface Linkages between the Forest Floor and the Coastal Ocean" (Alongi, 2020). When I convert your values from Table 1 in Alongi (2020) they don't match the values you present in this table 3. This may be due to a conversion error that I would like to bring to your attention.

2. In your published synthesis (Alongi, 2020) that you refer to in this study, you report that water-air fluxes are based on 134 measurements for CH4 and 143 for CO2. Looking at your references (66-93) this seems odd. Only very few of these studies measured the CH4 flux for example. Several are from open water estuary studies and as such do not reflect gas exchange in mangrove or marsh creeks (water-air flux). Many published studies are missing ! What are the 134 and 143 measurements? Are these individual flux measurements or sites? For global assumptions, it would be expected to have a flux measurement per site to not have any sampling bias towards more frequently measured sites. It would be helpful if you could make your database open access and available, so everyone can understand where these global numbers come from. This has been done in a previous study by Al-Haj and Fulweiler (2020) (Global Change Biology). Regarding concerns and uncertainties in global estimates of coastal ecosystems I would also like to bring your attention to the paper by Rosentreter and Williamson (2020) (Global Change Biology).

3. In your table 3 the references for saltmarsh studies also include several open water estuary studies. I would suggest to at least note this somewhere in your text or table caption, so the reader understands these are not studies from actual salt marsh habitats but estuaries that have some salt marsh in the catchment area. The study by Trifunovic (2020) is the only one that is in fact from a tidal marsh creek. Other studies (that were not included in your synthesis) include Ferron et al. (2007) and Tong et al. (2013).

4. Several of your cited studies include the sediment-air flux. Were these separated from water-air fluxes?

5. The global mangrove area has been updated in 2018 by Bunting et al. Can you explain why you chose the study by Hamilton 2016 instead?

6. As mentioned above, it would be desired to have an open access database. This would help to trace data and to build on in the future.

My comments are intended to help to improve your global synthesis. Happy to chat about everything.

Best regards
Judith Rosentreter
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