Global estimates of seagrass blue carbon stocks in biomass and net primary production

Domain: earth_science
Published: 2025-10-29
Editorial outcome: minor_revision

Abstract
Seagrass meadows play a key role in the global carbon cycle through storing carbon in their biomass and soils. However, the lack of global assessments of carbon stocks in seagrass biomass and net primary production (NPP) limits our understanding of their role in the carbon cycle. Here, we provide global estimates of biomass carbon stocks and NPP encompassing seagrasses with different life-history strategies, across bioregions and 66 countries. Seagrass meadows show up to 800-fold differences in biomass across different life-history strategies and bioregions (mean ± SE; 1551 ± 40 kg C ha−1), whereas seagrass NPP (5833 ± 557 kg C ha−1 yr−1) ranks among the highest within photosynthetic ecosystems. We estimate seagrass biomass carbon stocks at 24–40 Tg C and NPP at 83–137 Tg C yr−1 within 160,387 to 266,500 km2 of global seagrass extent. This study showcases the role of seagrasses in the global carbon cycle and provides the basis for their inclusion in carbon crediting schemes towards implementing conservation actions for climate change mitigation.

Introduction
Seagrass meadows are key coastal ecosystems that maintain the well-being of people and the planet, providing a range of ecological, social, and economic services1. These ecosystems provide critical habitat for marine biodiversity, stabilize sediments, cycle nutrients, and contribute substantially to climate regulation through their ability to sequester and store organic carbon2. The capacity of seagrass meadows to act as carbon sinks has gathered increasing attention, particularly within the framework of blue carbon, the carbon removed and stored as organic carbon by coastal and marine ecosystems3. In recent years, much of the focus around seagrass blue carbon has been placed on soil organic carbon stocks and accumulation rates, whereas carbon stores in seagrass above- and below-ground biomass, and seagrass net primary production (NPP) have often not been included, potentially leading to underestimates4. This oversight persists despite the critical importance of seagrass biomass and production in maintaining carbon fluxes and stabilizing sedimentary carbon stocks5.

The capacity of seagrass to act as carbon sinks is partially explained by their high NPP rates, which rank amongst the highest within marine ecosystems and are comparable to those of terrestrial forests and other highly efficient photosynthetic systems like seaweed forests, mangroves, and salt marshes6. Environmental factors such as temperature, rainfall patterns, irradiance, water depth, and sediment characteristics play key roles in shaping the distribution of seagrass meadows globally together with their biomass and NPP rates7. These drivers vary across seagrass bioregions8 and interact with seagrass life-history strategies to shape the carbon storage potential and ecological functioning of seagrass meadows9. Understanding how these factors influence biomass carbon stocks and seagrass NPP is essential for predicting the responses of seagrass ecosystems to global environmental change.

Life-history strategies in seagrasses—defined as the set of traits that species exhibit to thrive in their environment—can be categorized into persistent, opportunistic, and colonizing10. Persistent meadows, often dominated by genera such as Posidonia and Thalassia, are characterized by large, slow-growing plants with extensive rhizome systems that promote meadow stability and longevity, making them highly resilient carbon sinks11,12. In contrast, opportunistic and colonizing genera like Zostera and Halophila, respectively, are smaller in size, exhibit rapid growth, and have higher turnover rates, allowing them to quickly establish in bare or disturbed areas. However, colonizing meadows are even more ephemeral than opportunistic meadows, with faster turnover, lower persistence, and reduced resistance to disturbances, but high recovery capacity10.

Bioregions and specific climate conditions further modulate the nature and dynamics of seagrass meadows, influencing their productivity and temporal patterns. In temperate regions, species often experience marked seasonal variability in biomass and NPP, with slower metabolism and growth during autumn and winter due to lower temperatures and light availability7,13. Conversely, tropical seagrass meadows can maintain relatively high biomass and NPP rates year-round but can exhibit periods of complete dieback in response to extreme climatic events, including prolonged heatwaves14,15. The interaction between life-history strategies and environmental dynamics underscores potential differences in the relative contribution of different seagrass ecosystems to the carbon cycle.

Seagrass biomass plays a dual role in carbon cycling: it serves as a transient but dynamic carbon reservoir, but also contributes organic matter to sediments, bolstering the long-term carbon storage in the soil compartment16. The turnover time of seagrass biomass ranges from weeks to years, depending on species and environmental conditions. Seagrass biomass constitutes a dynamic carbon sink that can persist in a near-steady state over annual, decadal, or millennial time scales17. However, the loss of seagrass canopy due to either natural or anthropogenic disturbances can disrupt this cycle as well as the role of seagrass ecosystems as carbon sinks and reservoirs by reducing organic matter inputs to the soil carbon pool and exposing the soil carbon reservoir to erosion, likely resulting in carbon emissions5. These disruptions are particularly concerning given the global decline of seagrass meadows over the past century, driven by threats, such as coastal development, pollution, eutrophication, and climate change18.

The rapid advancement of blue carbon science highlights the urgent need for equitable and effective approaches to implement blue carbon strategies that can yield multiple benefits for people and the planet. Although debate around elements of the blue carbon framework remain19–22, most peer-reviewed studies endorse the role of these dynamic carbon pools in climate mitigation16,23,24. Indeed, the Intergovernmental Panel on Climate Change (IPCC) provided conversion factors that include seagrass biomass, and current carbon crediting methodologies, such as VERRA VM0033 and the Australian carbon methodology for Wetlands, include seagrass biomass in carbon accounting25–27. Their inclusion is contingent upon conservation and restoration actions that enhance carbon removals or avoided emissions by increasing or maintaining these pools in a steady state condition. However, these initiatives lack global datasets of biomass carbon stocks and NPP that could contribute baseline data to aid and enhance current estimates and methodologies, ultimately promoting the large-scale uptake of seagrass conservation and restoration actions under crediting mechanisms for climate change mitigation and adaptation.

This study aims to address these knowledge gaps by providing a global-scale assessment of carbon stocks in seagrass biomass, NPP, and emissions from seagrass biomass linked to habitat loss according to different life-history strategies and countries within the world’s seagrass bioregions. This research offers insights into the ecological contributions of seagrass to the global carbon cycle and lays the groundwork for their inclusion in carbon crediting schemes linked to conservation and restoration actions. The results compiled not only enhance the recognition of seagrass meadows as climate mitigation tools but also highlight their broader ecological significance as foundational coastal ecosystems.

Results
Carbon stocks in seagrass biomass and NPP across genera and life-history strategies
Seagrass biomass carbon stocks per unit area and NPP varied by up to six orders of magnitude, ranging from 0.03 to 25,340 kg C ha−1 (mean ± SE; 1551 ± 40 kg C ha−1), and from 2.82 to 51,456 kg C ha−1 yr−1 (6696 ± 557 kg C ha−1 yr−1), respectively. Biomass and NPP refer to total biomass and total NPP (i.e., above plus below-ground) unless stated otherwise. Clear patterns in seagrass biomass carbon stocks and NPP were observed among seagrass genera, life-history strategies, and bioregions. Relatively large, long-living seagrass genera that typify persistent meadows (Posidonia, Enhalus, Amphibolis, Thalassia, Thalassodendron) held up to 4–fold higher carbon stocks in biomass (3699 ± 179 kg C ha−1) than opportunistic genera (Syringodium, Zostera, Cymodocea, and Phyllospadix), and up to 11–fold higher carbon stocks than colonizing (Halodule, Ruppia, and Halophila) genera (944 ± 26 and 327 ± 17 kg C ha−1; p-value < 0.001; Fig. 1 and Supplementary Table 1). Carbon stocks in above- and below-ground biomass followed a similar pattern among life-history strategies, with higher stores in persistent meadows than in opportunistic and colonizing meadows (p-value < 0.001; Supplementary Fig. 1 and Supplementary Table 1). Notably, Amphibolis, Phyllospadix, and Posidonia had the highest above-ground biomass, whereas Posidonia had the largest below-ground biomass. Both above- and below-ground biomass were lowest in Halophila, Halodule, Ruppia, and Syringodium (p-value < 0.001; Supplementary Fig. 2 and Supplementary Table 2). Among the 12 seagrass genera explored, Posidonia held the highest total biomass carbon stocks (9780 ± 877 kg C ha−1), being 50–fold higher than the genus Halophila, supporting the lowest carbon stocks in biomass (190 ± 10 kg C ha−1) (p-value < 0.001; Supplementary Fig. 2 and Supplementary Table 2).

On the contrary, seagrass NPP per unit area in persistent genera was 2–fold lower (3126 ± 205 kg C ha−1 yr−1) than in meadows of opportunistic and colonizing life-history strategies (10,276 ± 1096 and 7271 ± 716 kg C ha−1 yr−1, respectively; p-value < 0.001; Fig. 1 and Supplementary Table 1). Remarkably, Phyllospadix showed an exceptionally high NPP, reaching 32,986 ± 3248 kg C ha−1 yr−1, far exceeding the NPP observed in other genera, which ranged from 1380 ± 250 in Enhalus to 11,877 ± 701 kg C ha−1 yr−1 in Syringodium (p-value < 0.001; Fig. 1). Above-ground NPP varied up to three orders of magnitude, from 12 kg C ha−1 yr−1 in Thalassodendron meadows to 19,129 ± 2122 kg C ha−1 yr−1 in Phyllospadix. Below-ground NPP was lowest in Posidonia and Enhalus (49 ± 16 and 324 kg C ha−1 yr−1, respectively), and highest in Phyllospadix (13,612 ± 1290 kg C ha−1 yr−1), whereas other genera exhibited relatively similar below-ground NPP (ranging from 1047 to 3871 kg C ha−1 yr−1; p-value < 0.001, Supplementary Fig. 2).

Carbon stocks in seagrass biomass and NPP among bioregions
The Mediterranean bioregion hold the highest seagrass biomass carbon stock per unit area (2328 ± 189 kg C ha−1; p-value < 0.001), followed by the Tropical Indo Pacific and Atlantic bioregions (1420 ± 88 and 1346 ± 85 kg C ha−1, respectively). The Temperate Southern (1102 ± 97 kg C ha−1), North Pacific (736 ± 36 kg C ha−1), North Atlantic East (660 ± 33 kg C ha−1), and the North Atlantic West (594 ± 45 kg C ha−1) bioregions had lower carbon stocks in seagrass biomass (Fig. 1 and Supplementary Table 2). Carbon stocks in below-ground biomass followed the same pattern as in total biomass, whereas carbon stocks in above-ground biomass were lowest in the Tropical Indo Pacific and Atlantic and Temperate North Atlantic East bioregions (299 ± 17, 404 ± 26, and 372 ± 22 kg C ha−1, respectively) compared to Temperate North Atlantic West (418 ± 27 kg C ha−1 yr−1), Temperate Southern (551 ± 28 kg C ha−1 yr−1), Mediterranean (582 ± 36 kg C ha−1 yr−1), and the Temperate North Pacific (729 ± 38 kg C ha−1 yr−1) bioregions (p-value < 0.001; Supplementary Fig. 3 and Supplementary Table 2).

Conversely, the opposite pattern was observed for NPP, with the Mediterranean (2554 ± 323 kg C ha−1 yr−1), Tropical Atlantic (2843 ± 289 kg C ha−1 yr−1), Temperate North Atlantic West (4413 ± 1280 kg C ha−1 yr−1), and Tropical Indo Pacific (5050 ± 328 kg C ha−1 yr−1) bioregions showing from 2– to 3–fold lower NPP than Temperate Southern (11,917 ± 1403 kg C ha−1 yr−1), North Pacific (14,765 ± 26,769 kg C ha−1 yr−1) and North Atlantic East (15,858 ± 1490 kg C ha−1 yr−1) bioregions (p-value < 0.001; Fig. 1 and Supplementary Table 2). Above- and below-ground NPP exhibited similar patterns to total NPP across bioregions (Supplementary Fig. 3).

Global and national carbon inventories in seagrass biomass and NPP
On a global scale, we estimated the biomass carbon stocks in seagrass meadows at 24–40 Tg C, and NPP at 83–137 Tg C yr−1, distributed across an estimated extent of 160,387–266,500 km2. Total carbon stocks in biomass widely varied across countries, ranging from 0.014 Gg C in Slovenia to 12,718 Gg C in Australia (Fig. 2 and Supplementary Table 3). Similarly, total carbon stocks in biomass per unit area varied widely among countries, ranging from 20 ± 2 kg C ha−1 in Vietnam to 12,483 ± 1770 kg C ha−1 in Malta. We observed substantial variation in total seagrass NPP summed across the total spatial extent of seagrass meadows of countries, ranging from 0.013 Gg C yr−1 in Slovenia to 58,805 Gg C yr−1 in Australia (Fig. 2 and Supplementary Table 3). NPP per unit area varied up to two orders of magnitude among countries, ranging from 144 ± 35 kg C ha−1 yr−1 in Italy to 32,985 ± 3248 kg C ha−1 yr−1 in Mexico. Clear patterns in seagrass biomass carbon stocks and NPP per unit area among life-history strategies within countries were observed, with up to one order of magnitude higher carbon stocks and NPP in persistent compared to colonizing genera within the same country (Fig. 3 and Supplementary Table 3).

Methods
Data collection and quality control
Data on seagrass above-, below-ground, and total biomass and NPP was sourced from “Global dataset on seagrass meadow structure, biomass and production”49,50. This published dataset was complemented with a new literature review search conducted in Web of Science (Clarivate™; webofknowledge.com) and Google Scholar (Google™; scholar.google.com) to encompass posterior and previously unidentified peer-reviewed literature. We conducted a search across titles, keywords, and abstracts, for the 1980-2024 time period, using the terms: “seagrass*” AND (“biomass” OR “carbon” OR “nutrient*” OR “product*” OR “NPP” OR “growth”). We identified 128 new papers and extracted relevant data. We also extracted data from 33 peer-reviewed manuscripts on organic carbon content (%) in above- and below-ground biomass for the 12 seagrass genera.

To evaluate and select relevant seagrass studies, we established specific criteria to ensure robust and reliable data extraction. The studies selected were required to report above-ground, below-ground, or total biomass, or NPP per unit area explicitly on a per-area basis in the text, figures, or tables, excluding those presenting only relative measures. Selected studies had to provide geolocation (latitude and longitude) and specify seagrass genera; those lacking these details or involving mixed meadows without genus-level resolution were excluded. Experiments conducted in mesocosms or natural environments with substantial modifications (e.g., nutrient enrichment or acidification) were not considered, focusing instead on in situ measurements. Additionally, studies needed to include original data not previously published, ensuring the use of novel information. These criteria facilitated the selection of studies offering accurate, spatially explicit estimates of seagrass biomass and/or NPP.

After applying the selection criteria, we compiled and reviewed 351 peer-reviewed scientific papers, encompassing 8576 data points on seagrass biomass and NPP from 791 coordinates distributed worldwide (Fig. 5). Of these, 342 papers contributed 8077 data points on biomass, including above-ground (N = 3353), below-ground (N = 1981), and total biomass (N = 2743). In contrast, only 38 papers provided the 499 data points compiled on NPP (N = 196 for above-ground, N = 93 for below-ground, and N = 210 for total NPP), with some of these manuscripts also containing biomass data. Additionally, 33 papers reported 761 data points on carbon content (%OC) in above-ground (N = 404) and below-ground (N = 357) biomass.

Global, bioregion- and country-specific estimates of seagrass extent were sourced from McKenzie et al.7, which provides estimates categorized by confidence levels: high confidence (supported by both qualitative and quantitative data) and low confidence (based only on qualitative data). For each country, the high-confidence extent estimate was prioritized when available; otherwise, the low-confidence estimate was used (Supplementary Table 4).

Numerical analyses
Standardized units of living biomass (g DW m−2) and NPP (g DW m−2 day−1 or g DW m−2 yr−1) were extracted from the reviewed literature. The biomass and NPP of leaves and sheaths were considered above-ground, whereas roots and rhizomes were considered below-ground. The sum of above- and below-ground biomasses and NPP constituted total biomass and total NPP (when total biomass and NPP were not explicitly reported). The % organic carbon in above- and below-ground biomass was weight-averaged for each genus based on leaf and root/rhizome percentages sourced from published literature, respectively (Supplementary Table 5). Biomass and NPP expressed as dry weight (DW) data were transformed into organic carbon by multiplying each value by the seagrass genus-specific mean % organic carbon content (Supplementary Table 5). For seagrass genera where there were no data available for % organic carbon content in below-ground biomass (i.e., Amphibolis and Phyllospadix), the mean % organic carbon value in below-ground across all genera with available data (38%) was used. For the calculation of total carbon in total biomass and total NPP in each seagrass genera, a weighted calculation was performed based on the proportion of above- and below-ground biomass. This was run as the sum of the weighted values of above- and below-ground biomass multiplied by the % organic carbon content in above- or below-ground biomass, respectively, and then multiplying by the total biomass. Data on seagrass carbon biomass and NPP are reported as kg C ha−1 and kg C ha−1 yr−1, respectively.

We assigned each data point to a specific seagrass bioregion defined as Temperate North Atlantic East and West, Tropical Atlantic, Mediterranean, Tropical Indo Pacific, Temperate Southern, Temperate North Pacific8 (Fig. 5); and classified seagrass genera as colonizing (i.e, Halodule, Halophila, and Ruppia), opportunistic (i.e., Syringodium, Zostera, Cymodocea and Phyllospadix,) or persistent (i.e., Posidonia, Enhalus, Amphibolis, Thalassia, Thalassodendron) according to its life-history strategy10. Global, bioregional, and national estimates of biomass and NPP were obtained by applying weight averages based on life-history strategies and seagrass extents. Global biomass and NPP values were calculated by multiplying the mean global biomass and NPP value by the high and low confidence global extent estimate7.

Global, bioregional and national estimates of seagrass biomass carbon emissions from seagrass biomass loss were calculated using the data on seagrass loss extent, expressed as the percentage of net change per bioregion over a century18. Country-specific annual seagrass extent loss was calculated by proportionally allocating the net percentage change of each bioregion to individual countries based on their proportional coastline extent within each bioregion (Supplementary Table 4). This allocation was then multiplied by the total seagrass extent for each country7, and divided by the temporal period over which seagrass loss was assessed to calculate annual rates. Similarly, bioregion-specific annual seagrass extent loss was estimated by multiplying the net percentage change per bioregion by the total seagrass extent within that bioregion7 and dividing by the same temporal period of estimates.

To estimate carbon emissions from biomass, the annual extent of losses was multiplied by the mean seagrass total biomass for each country or bioregion. It was assumed that 90% of the biomass would ultimately be mineralized following seagrass loss, whereas the remaining 10% would be preserved elsewhere28. These biomass values were subsequently converted to carbon dioxide equivalents (CO2-eq) using a conversion factor of 3.67, reflecting the stoichiometric relationship between carbon and CO2. This methodological framework provides a robust, spatially explicit assessment of carbon emissions from seagrass biomass across global, bioregional, and national scales.

Statistical analyses
To evaluate the differences in above, below and total biomass and NPP for different life-history strategies, genera and bioregions (considered as fixed factors) we used generalized linear models (GLM). The biomass and NPP data were log- and square root-transformed, respectively, to meet model assumptions. These models were evaluated in R Studio software with the glmmTMB function from the glmmTMB package51,52. The significance of the fixed factors analyzed was evaluated with the Anova function of the car package53 that performs Wald chi-square tests. Post-hoc pairwise comparisons were conducted using Estimated Marginal Means (EMM) and were performed with the emmeans function of the emmeans package with Tukey adjustment54. The normality and homoscedasticity assumptions were assessed by visually inspecting the residuals in a Q-Q plot. These analyses were implemented using R studio software51 and the results were visualized using ggplot2 package55 with a combination of box-and-whisker plots and bar plots. The boxplots y-axes were transformed using the 4th-root to enhance visualization while maintaining the display of real values. Global heatmaps were generated in QGIS56 to show the distribution of the data compiled, and the geographical patterns of total and per unit area biomass carbon and NPP across life-history strategies and countries.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "Authors have estimated seagrass blue carbon stock in biomass and net primary productivity at global scale.\nCurrently, blue carbon habitats have been recognized to be part of nature based solutions and mitigating climate change. This study provides biomass carbon and NPP which is quite important to know the current state of seagrass in climate\nchange mitigation process.\nGlobal estimates seagrass carbon stocks in biomass and soil have been published in the past (Fourqurean et al. 2012).\nSince than seagrass carbon related research are increasing, so the data points too. This study added new biomass carbon\nnumber to increase the uncertainties related to biomass carbon estimates.\nNPP is something not easy to measure and monitor and such a dataset is quite important to understand carbon removal\npotential in seagrasses. Therefore, this study will certainly help in future carbon or nature-based solutions project.\nMajor Comment\n1- I think it would be good to see how much uncertanity have been reduced in estimating biomass carbon as compare to\nFourqurean et al. 2012. Because if tsomeone wants to do same kind of research after 10 or 15 yrs adding more datasets, i do\nnot know if its necessary to conduct research but its always important to reduce uncertainities. Which could help in\ndeveloping more robust models.\n2- It would be good to see if there is any relationship with biomass carbon and NPP? This would be something important to\nunderstand seagrass ecology at global scale.\nBelow are some minor comments\nAbstract: L 40-42: lack of global assessments of carbon stocks in seagrass biomass and net primary production (NPP) limits\nour understanding of their role in the carbon cycle.\nAccording to literature carbon stocks in biomass and soil have been already reported- If this study has utilised other data\nsets after publishing a particular literature ( ).\nBetter to mention how much biomass uncertainties has been reduced since last published manuscript.\nI think net primary productivity is something we can take in to account for this manuscript.\nLine 45: Typo mistake Kg C ha-1 repeated – Delete one\nLine 47: Typo – Its kg C ha-1 yr-1\nLine 66-69- Why seagrass have high NPP as compared to mangroves and saltmarshes?\nMethodology\nLine 419- 426 – This paragraph is quite confusing and need to rewritten. Total manuscript identified 351 with total 8576 data\npoints.\n342 article – 8077 data points\n38 article – 502 data points\nTotal 380 articles (some may be overlapped but its confusing), which is higher than 342 articles mentioned earlier. And total\ndata point is 8579, which is 3 data points higher than earlier mentioned.\nLine 424: What is the difference between NPP and total NPP. This need to clarify, its confusing.", "reviewer_2": "This paper is, in many ways, a straightforward study that provides a clear and structured overview\nof seagrass biomass, NPP, and carbon stocks. The results are highly interesting and highlight the\nsignificant global role of seagrasses. The study is based on a comprehensive review of existing\ndata from around the world, which is synthesized in an engaging and relevant manner. It\ndemonstrates the crucial importance of seagrasses for NPP in marine areas and the global role of\nplant biomass compared to other marine and terrestrial ecosystems. The study emphasizes the\nimportance of managing seagrasses, preventing their loss, and restoring them, as this has a\nsubstantial impact on the global CO2 cycle.\nThe study contributes new insights by focusing on the significance of life history (e.g., persistent,\nopportunistic, colonizing) and bioregion. The conclusions are well-supported by the results, which\nwere obtained through straightforward analyses of a very large dataset. The methods used are\nappropriate, and the study addresses the dataset's limitations, primarily due to the lack of mapping\nin several seagrass-harboring countries and the absence of NPP and biomass-C data from many\ncountries.\nI have a few overall comments to the text and the figures below, as well as a few editorial\ncomments.\nOverall comments\nLine 82 In contrast, opportunistic and colonizing genera like Zostera and Halophila\nComment: As the classification of genera has not been presented at this stage of the paper, I\nsuggest including “respectively” at the end of the sentence\nLine 216-218\nFor instance, while Australia holds the largest total biomass carbon stock (12.7 Tg C and 1,667 kg\nC ha ¹), countries like Malta exhibit the highest biomass carbon stock per unit area (12,483 kg C\nha ¹) but lower total carbon stocks (88 Gg C), showcasing the heterogeneity in seagrass carbon\n⁻\nstorage potential.\n⁻\nComment: Is it relevant to indicate total carbon stocks per country? It seems obvious that the size\nof carbon stocks per country will depend on the country's size and be independent of for instance\nseagrass genera or bioregion. This could send misleading signals, suggesting that seagrasses\nhave little significance for the country but still be highly significant for the bioregion. For example, it\nis clear that seagrasses are very important for Australia due to its large size, while they seem less\nsignificant for Malta. Conversely, Posidonia is extremely important for the entire Mediterranean,\nand all countries should have a common strategy to ensure the protection of seagrasses there.\nMy suggestion is to only discuss total stocks on the level of bioregion, but still keep the detailed\ndata pr. Country in the supplementary materials.\nLine 285-286\nBased on the seagrass loss trend calculated for each country over the past century, we estimated\nannual global emissions from seagrass loss and the decomposition of biomass at 154–256 Gg\nCO2-eq yr ¹\nComment⁻: it is a little difficult to understand this sentence without having read the methods. Could\nthe paragraph be expanded to provide a short definition?\nLine 307-308\nIn contrast, emissions from seagrass biomass in the Temperate North Atlantic East bioregion were\nrelatively low (2.6 Gg CO2-eq, Fig. 4), whereas emissions within the Temperate North Pacific and\nNorth Atlantic West bioregions were negligible due to the absence of net seagrass meadow loss in\nthese bioregions.\nComment: an explanation of the relatively low emissions from the Temperate North Atlantic East\nbioregion is missing. The net loss rates are very high in these countries (Supp Table 4).\nLine 336-338\nFor example, avoiding losses of seagrass meadows worldwide, which have been estimated at\n615,600 ha over a century, could result in the avoidance of 154–256 Gg CO2-eq yr ¹ of emissions\nfrom remineralization of lost seagrass meadow biomass alone, which equals the annual emissions\n⁻\nof 60,000 cars (4.6 metric tonnes CO2-eq car ¹ yr ¹)\nCo∼mment: Is it correct to do the comparison on⁻ an ⁻annual basis as the seagrasses are lost once?\nIt may be correct. I am just uncertain here and other readers may be as well.\nLine 340-343\nYet, similar to biomass in terrestrial forests and other carbon accounting methodologies, it seems\ndifficult to maintain this steady state condition for >100 years, but other approaches allow shorter\npermanence periods (often at a discounted crediting rate), which may be more appropriate for\nopportunistic and colonizing species.\nComment: Is the problem with defining a stable biomass similar to the definition for terrestrial\nforests or is seagrass biomass less stable? And only less stable for opportunistic and colonizing\nspecies? This is critical for understanding how to handle seagrass ecosystems in carbon\naccounting methods.\nFigures:\nFig. 5 It is very difficult to distinguish the different colors of the dots and match them up with the\ninformation given in Supp. Table 4. The figure can be moved to Supplementary to show the\ndefinition of the bioregions and with further explanation of the datapoints.\nEditorial\nLine 113-114\nIndeed, the Intergovernmental Panel on Climate Change (IPCC) provided conversion factors that\ninclude seagrass biomass, whereas current carbon crediting methodologies such as VERRA\nVM0033 and the Australian carbon methodology for Wetlands include seagrass biomass in carbon\naccounting\nComment: Change whereas with “and”\nLine 269\nGlobally, seagrass soil carbon stocks per unit area in the top 20 cm have been estimated at\n22,000±800 (mean±SE) kg C ha ¹\nComment: move parenthesis in ⁻front of the numbers (see line 133)\nLine 272-273\nHowever, this percentage would differ when considering the range of soil thicknesses (from 0 m\nwhen seagrasses grow on top of rock to more than 10 m) reported for seagrass ecosystems.\nComment: include “of below-ground material in Posidonia oceanica”\nDespite the uncertainties, these 291 estimates underscore the potential contribution of seagrass\nbiomass losses to carbon 292 emissions globally, whereas all focus on emissions with seagrass\nloss had been on the carbon 293 socks in the denuded sediments.\nComment: check spelling\nOn the other hand, emissions from the Mediterranean and Temperate Southern (10 302 and 7.4\nGg CO2-eq, respectively) bioregions were up to 10-fold lower\nComment: Is it “on the other hand” or rather “similarly”\nLine 358-360\nThe estimates of annual NPP provided here contribute to further understanding the key role of\nseagrasses in the carbon cycle, consolidating seagrass as some of the most productive\necosystems in the biosphere\nComment: should it be “seagrasses”\nLine 369\nGlobal estimates of carbon stocks in seagrass biomass and NPP were obtained throughtout\nmultiple years and seasons and therefore\nLine 393-394\nCollaboration among governments, researchers, and conservation organizations will be critical to\nunlocking the full potential of 394 seagrass ecosystems for climate adaptation and mitigation.\nComment: “to unlock”"}, "author_response": "RESPONSE TO REFEREES\nWe would like to express our sincere gratitude for the time and effort the Reviewers dedicated\nto reviewing our manuscript. Your insightful comments and constructive suggestions have\nbeen invaluable in enhancing the clarity, rigor, and overall quality of our work.\nREVIEWER #1 (REMARKS TO THE AUTHOR):\nAuthors have estimated seagrass blue carbon stock in biomass and net primary productivity\nat global scale.Currently, blue carbon habitats have been recognized to be part of nature\nbased solutions and mitigating climate change. This study provides biomass carbon and NPP\nwhich is quite important to know the current state of seagrass in climate change mitigation\nprocess.\nGlobal estimates seagrass carbon stocks in biomass and soil have been published in the past\n(Fourqurean et al. 2012). Since than seagrass carbon related research are increasing, so the\ndata points too. This study added new biomass carbon number to increase the uncertainties\nrelated to biomass carbon estimates. NPP is something not easy to measure and monitor and\nsuch a dataset is quite important to understand carbon removal potential in seagrasses.\nTherefore, this study will certainly help in future carbon or nature-based solutions project.\nMajor Comment\n1- I think it would be good to see how much uncertanity have been reduced in estimating\nbiomass carbon as compare to Fourqurean et al. 2012. Because if tsomeone wants to do\nsame kind of research after 10 or 15 yrs adding more datasets, i do not know if its necessary\nto conduct research but its always important to reduce uncertainities. Which could help in\ndeveloping more robust models.\nOur manuscript increased the dataset reported in Fourqurean et al. (2012) from 251 to 2,743\ndata points, equivalent to a 91% increase. We included a statement in the Discussion section\nto reflect the novelty of our manuscript compared to Fourqurean et al. (2012):\nNew text reads (Lines 362-369): “This study increased the number of biomass data reported\nin Fourqurean et al. (2012) by 91% (from 251 to 2,743 data points). Our estimates of total\nseagrass biomass were up to 1.5–fold lower than those reported by Fourqurean et al. (2012),\nwhereas median estimates are comparable (1.1 and 1.0 Mg OC ha⁻¹, respectively). This study\nreduced the uncertainties associated with previous estimates by increasing the dataset\nincluding data from previously underrepresented regions, and by obtaining total seagrass\nbiomass estimates by applying weigthed-averages based on life-history strategies, accounting\nfor the disproportionate number of observations associated with opportunistic and colonizing\ngenera.”\n2- It would be good to see if there is any relationship with biomass carbon and NPP? This\nwould be something important to understand seagrass ecology at global scale\nWe fully agree. However, the ecological relationships between NPP and biomass have been\nexplored in detail in a separate manuscript, which is currently under revision in the Journal of\nBiogeography. Therefore, it is not possible to include this manuscript; also to keep the focus\non carbon storage linked to biomass and NPP and their influencing factors.\nPlease find below the key Figure that has been included in the other manuscript, showing that\nseagrass genera are adapted to specific environmental conditions which result in a trade-off\nbetween NPP and biomass balancing the allocation of energy to thrive in specific habitats.\nBelow are some minor comments\nAbstract: L 40-42: lack of global assessments of carbon stocks in seagrass biomass and net\nprimary production (NPP) limits our understanding of their role in the carbon cycle.\nAccording to literature carbon stocks in biomass and soil have been already reported- If this\nstudy has utilised other data sets after publishing a particular literature ( ).\nBetter to mention how much biomass uncertainties has been reduced since last published\nmanuscript.\nI think net primary productivity is something we can take in to account for this manuscript.\nWe modified the abstract to clarify the novelty of our study. This text aims at highlighting the\nresearch gap before entering into the aims and results obtained.\nNew text reads (Lines 40-42): “However, the limited information on carbon stocks in seagrass\nbiomass and the lack of global assessments of net primary production (NPP) limits our\nunderstanding of their role in the carbon cycle.”\nLine 45: Typo mistake Kg C ha-1 repeated – Delete one Corrected as suggested.\nLine 47: Typo – Its kg C ha-1 yr-1 Corrected as suggested.\nLine 66-69- Why seagrass have high NPP as compared to mangroves and saltmarshes?\nSome opportunistic and colonising seagrass genera have higher NPP rates compared to\nsaltmarshes and mangroves, mainly due to faster turnover rates. Nonetheless, we agree that\nthe previous text could lead to wrong interpretation as not all seagrass species exceed NPP\nin mangroves, seaweed forests and saltmarshes. Consequently, we deleted “or exceed” from\nthe sentence, and highlight that NPP are comparable only.\nNew text reads (Lines 66-69): “The capacity of seagrass to act as carbon sinks is partially\nexplained by their high NPP rates, which rank amongst the highest within marine ecosystems\nand are comparable to those of terrestrial forests and other highly efficient photosynthetic\nsystems like seaweed forests, mangroves and saltmarshes.”\nMethodology\nLine 419- 426 – This paragraph is quite confusing and need to rewritten. Total manuscripts\nidentified 351 with total 8576 data points. 342 article – 8077 data points\n38 article – 502 data points. Total 380 articles (some may be overlapped but its confusing),\nwhich is higher than 342 articles mentioned earlier. And total data point is 8579, which is 3\ndata points higher than earlier mentioned.\nWe rewrite the paragraph because it was confusing, as stated. We also corrected the error\nyou spotted: NPP number of data was 499, not 502 as initially stated.\nNew text reads (Lines 422-430): “After applying the selection criteria, we compiled and\nreviewed 351 peer-reviewed scientific papers, encompassing 8,576 data points on seagrass\nbiomass and net primary production (NPP) from 791 coordinates distributed worldwide (Fig.\n5). Of these, 342 papers contributed 8,077 data points on biomass, including above-ground\n(N = 3,353), below-ground (N = 1,981), and total biomass (N = 2,743). In contrast, only 38\npapers provided the 499 data points compiled on NPP (N =196 for above-ground, N = 93 for\nbelow-ground, and N = 210 for total NPP), with some of these manuscripts also containing\nbiomass data. Additionally, 33 papers reported 761 data points on carbon content (%OC) in\nabove-ground (N = 404) and below-ground (N = 357) biomass.”\nLine 424: What is the difference between NPP and total NPP. This need to clarify, its confusing.\nThanks for pointing this out. Essentially it’s the same. Now, across the manuscript we refer to\nNPP as total NPP, and to above-ground or below-ground NPP when referring to NPP in\nspecific plant compartments. We have included a sentence to clarify this issue.\nNew text reads (Lines 133-134): “Biomass and NPP refer to total biomass and total NPP\nunless stated otherwise.”\nREVIEWER #2 (REMARKS TO THE AUTHOR):\nThis paper is, in many ways, a straightforward study that provides a clear and structured\noverview of seagrass biomass, NPP, and carbon stocks. The results are highly interesting and\nhighlight the significant global role of seagrasses. The study is based on a comprehensive\nreview of existing data from around the world, which is synthesized in an engaging and\nrelevant manner. It demonstrates the crucial importance of seagrasses for NPP in marine\nareas and the global role of plant biomass compared to other marine and terrestrial\necosystems. The study emphasizes the importance of managing seagrasses, preventing their\nloss, and restoring them, as this has a substantial impact on the global CO2 cycle.\nThe study contributes new insights by focusing on the significance of life history (e.g.,\npersistent, opportunistic, colonizing) and bioregion. The conclusions are well-supported by the\nresults, which were obtained through straightforward analyses of a very large dataset. The\nmethods used are appropriate, and the study addresses the dataset's limitations, primarily due\nto the lack of mapping in several seagrass-harboring countries and the absence of NPP and\nbiomass-C data from many countries.\nI have a few overall comments to the text and the figures below, as well as a few editorial\ncomments.\nOverall comments\nLine 82 In contrast, opportunistic and colonizing genera like Zostera and Halophila\nComment: As the classification of genera has not been presented at this stage of the paper,\nI suggest including “respectively” at the end of the sentence\nIncluded “respectively” as suggested.\nNew text reads (Lines 80-83): “In contrast, opportunistic and colonizing genera like Zostera\nand Halophila, respectively, are smaller in size, exhibit rapid growth, and have higher turnover\nrates, allowing them to quickly establish in bare or disturbed areas.”\nLine 216-218\nFor instance, while Australia holds the largest total biomass carbon stock (12.7 Tg C and 1,667\nkg C ha−1), countries like Malta exhibit the highest biomass carbon stock per unit area (12,483\nkg C ha−1) but lower total carbon stocks (88 Gg C), showcasing the heterogeneity in seagrass\ncarbon storage potential.\nComment: Is it relevant to indicate total carbon stocks per country? It seems obvious that the\nsize of carbon stocks per country will depend on the country's size and be independent of for\ninstance seagrass genera or bioregion. This could send misleading signals, suggesting that\nseagrasses have little significance for the country but still be highly significant for the bioregion.\nFor example, it is clear that seagrasses are very important for Australia due to its large size,\nwhile they seem less significant for Malta. Conversely, Posidonia is extremely important for\nthe entire Mediterranean, and all countries should have a common strategy to ensure the\nprotection of seagrasses there.\nMy suggestion is to only discuss total stocks on the level of bioregion, but still keep the detailed\ndata pr. Country in the supplementary materials.\nThanks for your insights. Althought it is obvious that total carbon stocks in each country are\nlargely driven by national seagrass extent, highlighting these data and facts is revelant for\npolicy (e.g., reporting national seagrass carbon inventories). Nevertheless and following your\nsuggestion, we deleted text to focus the Discussion on biomass carbon stocks and NPP at the\nbioregional level, and retained detailed national data in the Supplementary Information\n(Supplementary Table 3).\nDeleted text read (Line 213): “For instance, while Australia holds the largest total biomass\ncarbon stock (12.7 Tg C and 1,667 kg C ha-1), countries like Malta exhibit the highest biomass\ncarbon stock per unit area (12,483 kg C ha-1) but lower total carbon stocks (88 Gg C),\nshowcasing the heterogeneity in seagrass carbon storage potential.”\nPrevious subheading read (Lines 200-201): “Patterns in seagrass biomass carbon stocks and\nNPP among genera, life-history strategies, bioregions and countries”\nNew subheading reads (Lines 200-201): “Patterns in seagrass biomass carbon stocks and\nNPP among genera, life-history strategies and bioregions”\nLine 285-286\nBased on the seagrass loss trend calculated for each country over the past century, we\nestimated annual global emissions from seagrass loss and the decomposition of biomass at\n154–256 Gg CO2-eq yr−1\nComment: it is a little difficult to understand this sentence without having read the methods.\nCould the paragraph be expanded to provide a short definition?\nWe acknowledge that having the Methods section at the end of the document could difficult\nthe understanding of some aspects of the Discussion. Therefore, we provided details on how\nthese values were obtained in the Discussion.\nNew text reads (Lines 281-289): “Based on previous estimates of seagrass loss trends over\nthe past century calculated for each country18, and the mean country-specific seagrass total\nbiomass obtained in this study, we estimated annual global emissions from seagrass loss at\n154–256 Gg CO yr⁻¹ (based on high (160,387 km2) and low (266,500 km2) confidence world\n2-eq\nextent estimates, respectively7). These estimates assumed that 90% of the biomass would\nultimately be mineralized following seagrass loss.”\nLine 307-308\nIn contrast, emissions from seagrass biomass in the Temperate North Atlantic East bioregion\nwere relatively low (2.6 Gg CO2-eq, Fig. 4), whereas emissions within the Temperate North\nPacific and North Atlantic West bioregions were negligible due to the absence of net seagrass\nmeadow loss in these bioregions.\nComment: an explanation of the relatively low emissions from the Temperate North Atlantic\nEast bioregion is missing. The net loss rates are very high in these countries (Supp Table 4).\nThanks for pointing this out. We provided a plausible explanation as suggested.\nNew text reads (Lines 300-305): “In contrast, although net seagrass loss rates reported for the\nTemperate North Atlantic East bioregion are high (~69%)Error! Reference source not found., the\nemissions from seagrass biomass in this bioregion were relatively low (2.6 Gg CO , Fig. 4)\n2-eq\nowing to the limited biomass carbon storage capacity of the dominant seagrass genera in this\nregion (i.e., opportunistic Zostera spp.) and the limited extent compared to other bioregions.”\nLine 336-338\nFor example, avoiding losses of seagrass meadows worldwide, which have been estimated\nat 615,600 ha over a century, could result in the avoidance of 154–256 Gg CO2-eq yr−1 of\nemissions from remineralization of lost seagrass meadow biomass alone, which equals the\nannual emissions of ∼60,000 cars (4.6 metric tonnes CO2-eq car−1 yr−1)\nComment: Is it correct to do the comparison on an annual basis as the seagrasses are lost\nonce? It may be correct. I am just uncertain here and other readers may be as well.\nWe deleted this paragraph to avoid confussion.\nDeleted text read (Line 334): “For example, avoiding losses of seagrass meadows worldwide,\nwhich have been estimated at 615,600 ha over the next century, could result in the avoidance\nof 154–256 Gg CO yr⁻¹ of emissions from remineralization of lost seagrass meadow\n2-eq\nbiomass alone, which equals the annual emissions of ∼60,000 cars (4.6 metric tonnes CO\n2-eq\ncar-1 yr-1). \nLine 340-343\nYet, similar to biomass in terrestrial forests and other carbon accounting methodologies, it\nseems difficult to maintain this steady state condition for >100 years, but other approaches\nallow shorter permanence periods (often at a discounted crediting rate), which may be more\nappropriate for opportunistic and colonizing species.\nComment: Is the problem with defining a stable biomass similar to the definition for terrestrial\nforests or is seagrass biomass less stable? And only less stable for opportunistic and\ncolonizing species? This is critical for understanding how to handle seagrass ecosystems in\ncarbon accounting methods.\nThe inclusion of seagrass biomass in carbon crediting methodologies is at its onset. From a\nscientific point of view, deciphering whether seagrass biomass of either persistent or\ncolonizing/opportunistic genera meets the permanence requirements for being considered a\nrelevant carbon pool for climate change mitigiation remains controversial. In our Discussion,\nwe provided some insights around this topic, but developing a comprehensive discussion\naround it is outside the scope of our manuscript.\nFigures:\nFig. 5 It is very difficult to distinguish the different colors of the dots and match them up with\nthe information given in Supp. Table 4. The figure can be moved to Supplementary to show\nthe definition of the bioregions and with further explanation of the datapoints.\nWe consider that this figure is comprehensive and self-explanatory, and no changes were\nmade. We agree that the large number of data difficults distinguishing the different dots and\ncolours (i.e. overlapping) but provides a clear overview of data distribution within each\nbioregion. The Supp. Tables together with the raw dataset that will be published in open\naccess provide the original data to check the data, and to advance science in the future.\nEditorial\nLine 113-114\nIndeed, the Intergovernmental Panel on Climate Change (IPCC) provided conversion factors\nthat include seagrass biomass, whereas current carbon crediting methodologies such as\nVERRA VM0033 and the Australian carbon methodology for Wetlands include seagrass\nbiomass in carbon accounting\nComment: Change whereas with “and”\nChanged as suggested (Line 112).\nLine 269\nGlobally, seagrass soil carbon stocks per unit area in the top 20 cm have been estimated at\n22,000±800 (mean±SE) kg C ha−1\nComment: move parenthesis in front of the numbers (see line 133)\nChanged as suggested (Line 265).\nLine 272-273\nHowever, this percentage would differ when considering the range of soil thicknesses (from 0\nm when seagrasses grow on top of rock to more than 10 m) reported for seagrass ecosystems.\nComment: include “of below-ground material in Posidonia oceanica”\nIncluded as suggested (Line 288).\nLine 297-300\nDespite the uncertainties, these estimates underscore the potential contribution of seagrass\nbiomass losses to carbon emissions globally, whereas all focus on emissions with seagrass\nloss had been on the carbon socks in the denuded sediments.\nComment: check spelling\nText modified as suggested.\nNew text reads (Lines 267-270): “Although the focus on emissions associated with seagrass\nloss had mostly been on the carbon stocks in the denuded sediments, the results obtained in\nthis study underscore the potential contribution of seagrass biomass losses to carbon\nemissions globally.”\nLine 308-311\nOn the other hand, emissions from the Mediterranean and Temperate Southern (10 302 and\n7.4 Gg CO2-eq, respectively) bioregions were up to 10-fold lower\nComment: Is it “on the other hand” or rather “similarly”\nWe removed the term “on the other hand” to avoid confusion.\nLine 358-360\nThe estimates of annual NPP provided here contribute to further understanding the key role\nof seagrasses in the carbon cycle, consolidating seagrass as some of the most productive\necosystems in the biosphere\nComment: should it be “seagrasses” Line 369\nCorrected as suggested.\nLine 376\nGlobal estimates of carbon stocks in seagrass biomass and NPP were obtained throughtout\nmultiple years and seasons and therefore\nCorrected as suggested.\nLine 393-394\nCollaboration among governments, researchers, and conservation organizations will be\ncritical to unlocking the full potential of 394 seagrass ecosystems for climate adaptation and\nmitigation.\nComment: “to unlock”\nCorrected as suggested."}, {"round": 2, "reviewer_comments": {"reviewer_1": "Authors have improved the manuscript following reviewers comments. I apprciate authours effort to clarify things very clearly.\nJust minor comments to authors:\nAs per our knowledge (based on literature) still seagrass area covers at global, regional and local scale is not very well\nquantified well due to several factorr. Do you feel seagrass area is much more higher than what has been reported till now\nacross the globe?\nBecause mapping seagrass area cover using satellite based remote sensing, currently using drones, acoustics ect, would\nincrease the carbon capture capacity of seagrass which is still missing.\nfor example turbid water, shallower water and diffcult to access the site to visit seagrsses.\nAlso, it would be also good to mention in the manuscript to come up with a standrad proptocol to measure or estimate\nseagrass NPP like segrass carbon stock quantification (Howard et al. 2014). Comparable data sets across the site collected\nusing same method can help in improving models with lower errors.\nI would like to add few sentences in the discussion, especially gaps and how it can chnage the seagrass biomass and NPP\nvalues in the future as well as species ocuurence. After that this manuscript would be very valuable not only in terms of\ncurrent status but how it can be improved in the feature.", "reviewer_2": "The authors have addressed all comments and the manuscript reads very well. I have no further comments to the\nmanuscript."}, "author_response": ""}]

Decision letter