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    Scientific Research Paper:
    Nature Sustainability | Volume 7 | December 2024 | 1592–1595
1592
nature sustainability
Brief Communication
https://doi.org/10.1038/s41893-024-01471-y
Potential increase of neurotoxic mercury risk
in global blue carbon nature-based solutions
Zhijia Ci
  1
, Wenjie Shen2,3, Baowei Chen1, Yanbin Li
  4, Yongguang Yin
  5,
Xiaoshan Zhang5 & Yong Cai
  6
Blue carbon strategies are now being proposed as a promising, nature-based
solution for achieving multiple benefits. Here we provide field evidence
on the co-accumulation of organic carbon and neurotoxic mercury (Hg)
in coastal environments. We estimate the global Hg stock in the top
metre of sediment of blue carbon ecosystems to be 21,306 to 125,018 Mg
(mean = 73,162 Mg), highlighting that Blue Hg stock is an important,
dynamic, reactive, but overlooked Hg pool in global Hg cycle and health risk.
Anthropogenic emissions of both carbon dioxide (CO2) and mercury
(Hg) are long-term stressors for the global environment and human
society1. The global community is seeking common and coordinated
actions, such as the Paris Agreement on Climate Change (https://unfccc.
int/process-and-meetings/the-paris-agreement) and the Minamata
Convention on Mercury (https://minamataconvention.org/en), to
reduce their emissions and associated adverse effects, although both
treaties still are in the early stages of implementation.
The term ‘Blue C’ entered the lexicon in 2009 and refers to organic
C (OC) that is captured and stored by vegetated coastal ecosystems
(VCEs, including seagrass meadows, tidal marshes and mangrove for-
ests)2,3. Recently, tidal mudflats have also entered the Blue C discourse4.
The importance of implementation of Blue C strategies—expanding
and restoring Blue C ecosystems—to cope with climate crisis, biodi-
versity loss and natural hazards3 has now reached a consensus in the
international community. However, there are few empirical studies on
the negative effects of Blue C strategies5.
As a persistent and ubiquitous global pollutant, Hg deposition
of any species and concentration poses potential risks to humans
and ecosystems1. Compared with terrestrial environments, coastal
environments have many physical, ecological and biogeochemical
characteristics that increase the accumulation of Hg and the produc-
tion, release, bioaccumulation and risk of neurotoxic methylmercury
(MMHg, the methylated form of Hg) (refs. 6–10). Therefore, Hg stored
in coastal ecosystems poses more serious risks to human health and
ecosystem safety10. Field measurements show that OC-rich coastal
sediments are generally accompanied by elevated Hg concentrations
since sedimentary OC provides a large surface area and strong binding
sites for divalent Hg (Hg(II)) (ref. 9). However, the sample size, spatial
distribution and ecosystem type of coastal sediments in existing stud-
ies are extremely limited9,11–13. More critically, no attempt has been
made to estimate the Hg stock in global coastal sediments, which is
one of the fundamental constraints on determining Hg risks in global
coastal ecosystems.
In this Brief Communication, we carry out a targeted survey along
Chinese coastlines to document the relationship between coastal habi-
tat types and sediment Hg and OC contents (Fig. 1a and Methods). We
find that the sediment of VCEs not only is OC-rich, but also is a Hg-rich
system (Fig. 1b), and that there is a strong and positive correlation
between OC contents and Hg concentrations in coastal sediments
(Fig. 1c). Notably, the Hg/OC ratio reported here (3.95 µg g–1) is ~2.5
times that of permafrost soils (1.6 µg g–1 (ref. 14)), indicating that coastal
sediment is a more favourable environment for storing Hg under the
same OC sequestration potential as permafrost soils. The Hg release
from anthropogenic activities around coastal regions could addition-
ally increase the Hg/soil OC ratio of coastal sediments9.
We combine the linear correlation between OC and Hg in all coastal
sediments (N = 819) generated from our regional observational data
(Fig. 1c) and the Blue C stock in the top metre of sediment in different
Blue C ecosystems compiled by others to roughly estimate the Hg
stock in global Blue C ecosystems (Methods). We find that over 95%
of the Hg stock in global Blue C ecosystems is stored in VCEs (Fig. 1d).
The Hg stock in global Blue C ecosystems (mean = 73,162 Mg, mini-
mum–maximum: 21,306–125,018 Mg) is ~15 times the Hg pool in the
Received: 15 May 2024
Accepted: 28 October 2024
Published online: 29 November 2024
 Check for updates
1Guangdong Provincial Key Laboratory of Marine Resources and Coastal Engineering, School of Marine Sciences, Sun Yat-sen University, Zhuhai,
China. 2School of Earth Sciences and Engineering, Sun Yat-sen University, Zhuhai, China. 3Guangdong Key Laboratory of Geological Process and
Mineral Resources Exploration, Zhuhai, China. 4Frontiers Science Center for Deep Ocean Multispheres and Earth System, and Key Laboratory of Marine
Chemistry Theory and Technology, Ministry of Education, and College of Chemistry and Chemical Engineering, Ocean University of China, Qingdao,
China. 5Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing, China. 6Department of Chemistry and Biochemistry and
Institute of Environment, Florida International University, Miami, FL, USA.
 e-mail: cizhj@mail.sysu.edu.cn

Nature Sustainability | Volume 7 | December 2024 | 1592–1595
1593
Brief Communication
https://doi.org/10.1038/s41893-024-01471-y
TM
4,369 Mg
3,405–5,532 Mg
(5.79%)
MF
20,342 Mg
7,505–33,180 Mg
(27.80%)
TF
3,555 Mg
(4.86%)
SM
44,895 Mg
6,841–82,950 Mg
(61.36%)
TF
(19)
SM
(10)
TM
(14)
MF
(15)
0
10
20
30
Sediment LOI
Sediment Hg
Sediment LOI (%)
(181)
(239)
(137)
(262)
0
200
400
600
Sediment Hg (ng g–1)
TF
TM
SM
MF
a
b
c
d
800
600
400
200
0
0
5
10
15
Sediment LOI (%)
Sediment Hg (ng g–1)
20
25
Sediment Hg (ng g–1) = 22.896 × LOI (%) + 21.96
r2 = 0.62, P = 1.42 × 10
–9
TF
SM
TM
MF
Fig. 1 | Estimate of Hg stock in global blue C ecosystems. a, Sampling sites
(N = 58) in diverse coastal ecosystem locations of China. A jitter is used to offset
closely placed sites. b, Organic carbon contents (loss on ignition (LOI), a proxy for
organic carbon) and Hg concentrations in the sediments of four coastal habitat
types. Central box lines and squares show the median and mean, respectively;
box boundaries show the 25th and 75th percentiles; whiskers show 1.5 times the
interquartile range; triangles show the maximum and minimum. Text in brackets
above and below the panel show the number of sediment samples and sampling
locations in each coastal habitat type, respectively. c, Strong and positive linear
correlation between OC and Hg in the sediments (N = 819) across four coastal
habitat types, P = 1.42 × 10–9 (two-sided Student’s t test). d, Hg stock in the top
metre of sediment in different Blue C ecosystems in the world. Data in d are
reported as average and minimum–maximum (except tidal flat), and percentage
(%) represents the relative contribution of each coastal habitat type to the
Hg stock in global blue carbon ecosystems. TF, tidal flat; TM, tidal marsh; SM,
seagrass meadow; MF, mangrove forest. Data used to generate a–c are available
in Supplementary Table 1. Basemap in a from Earthdata (https://www.earthdata.
nasa.gov).
–
–
–
CO2 + Hg(0) + Hg(II)
Input
CO2, Hg(0), Hg(II)
CO2, Hg(0), Hg(II)
CO2, CH4, Hg(0)
CO2, CH4, Hg(0)
CO2, CH4, Hg(0)
CO2, CH4, Hg(0)
MMHg
DOC, DIC, Hg, MMHg
CO2
Sediment OC
mineralization
Hg
Hg
DOC
DOC
MMHg
POC
CH4
Tidal marsh
Mangrove forest
Seagrass meadow
Output
CO2 + CH4 + Hg(0)
Fig. 2 | Close linkage between C and Hg biogeochemical cycles in coastal
ecosystems. As a high primary production ecosystem, coastal vegetation
effectively assimilates atmospheric CO2 into OC via photosynthesis,
accompanied by the uptake of atmospheric Hg (Hg(0) and Hg(II)) into biomass,
mainly in foliage20. Submerged canopies and fine root systems of VCEs greatly
increase sediment deposition and reduce sediment resuspension2,3. The
litterfall deposition and particles trapped from surrounding ecosystems lead to
synchronous and effective accumulation of OC and Hg in surface sediments2,13.
The water-logged anoxic environment favours the long-term storage of OC
and Hg in the sedimentary system21. Both the sediments of VCEs and the
coastal water column are hotspots of MMHg production6–8,22, and the coastal
seafood webs are hotspots of MMHg bioaccumulation, biomagnification and
human exposure1,10. The release of C (including CO2, CH4, dissolved OC (DOC),
dissolved inorganic C (DIC) and particulate OC (POC)) and Hg (including Hg(0),
dissolved and particulate Hg and MMHg) to coastal waters via bioturbation and
multiple hydrological processes such as tidal pumping, submarine groundwater
discharge and diffusion and to the atmosphere via ebullition and diffusion
greatly affects the OC and Hg stock and influences the biogeochemistry and
toxicology of marine environments3,6,9,22. DOC is the dominant mobile carrier of
dissolved Hg and MMHg and greatly regulates Hg speciation and bioavailability
in coastal environments22. The sediment OC mineralization during early
diagenesis highlights that the upper part of coastal sediment (approximately
Blue C sediment) is not a stable sink for C and Hg.

Nature Sustainability | Volume 7 | December 2024 | 1592–1595
1594
Brief Communication
https://doi.org/10.1038/s41893-024-01471-y
present-day atmosphere (4,400 Mg (ref. 15)), ~25% of the Hg stored in
present-day ocean water (260,000 to 350,000 Mg) and ~80% of the Hg
stored in the upper 1,000 m of ocean water (62,800 to 120,000 Mg)
(refs. 16,17). Our methodology underestimates the Hg stock since we
calculate only the Hg stored in the top metre of sediment and do not
account for Hg stored in biomass, particularly in mangrove forests,
which have large biomass.
Global VCEs and tidal flats are estimated to annually bury
80–220 Tg OC (ref. 18) and 6.8 Tg OC (ref. 4), respectively. Correspond-
ingly, 316 to 869 Mg (mean = 592 Mg) and 26.9 Mg Hg could be seques-
tered by global VCEs and tidal flats annually, although the degradation
of VCEs2 may lead to Hg release. This Hg burial flux is comparable to
~25% of global anthropogenic Hg emissions of 2,500 Mg yr–1 (ref. 15),
close to half of the global river Hg discharge of 893–1,224 Mg yr–1 to
coastal seas19 and roughly equivalent to 70% of the annual oceanic Hg
sequestration of 800 Mg (ref. 15).
Our study provides field evidence of the co-accumulation of
climate-friendly OC and neurotoxic Hg in Blue C ecosystems. On the
basis of the strong commonality and close linkage between C and Hg
storage and cycling in coastal ecosystems (Fig. 2), by analogy to the
term ‘Blue C’, we propose the term ‘Blue Hg’ to refer to Hg captured
and stored in coastal ecosystems, particularly in VCEs.
Our study reveals that extremely limited extents of VCEs (~0.2%
of the ocean surface18) and their substantial stock and burial flux of Hg
highlight the disproportionate importance of the VCEs to global Hg
storage and cycling. Blue Hg stock is therefore an important, dynamic,
reactive but overlooked Hg pool in the global Hg cycle and health risk.
Blue C ecosystems can provide a range of ecosystem services such as
protecting biodiversity and fisheries and increasing water quality.
However, the co-accumulation and contrary ecological effects of OC
and Hg in coastal ecosystems indicate that expanding Blue C ecosystems
are very likely to add to the Blue Hg stock, consequently resulting in an
increase of Hg risk in global coastal seas (Fig. 2). Delay and failure to
implement the Blue C strategies may well accelerate the impending cli-
mate catastrophe and other coastal and ecological disasters, inevitably
resulting in substantial economic, ecological and human losses3. We
refer to increased Blue Hg stocks and the risk caused by expanding Blue
C ecosystems to mitigate climate change and achieve co-benefits as the
‘Blue Hg dilemma’. The close interactions between C and Hg in coastal
ecosystems point to the importance of a holistic understanding of the
benefits, risks and trade-offs of Blue C sequestration strategies. We high-
light that the Blue Hg dilemma should be accounted for in future Blue
C strategy assessments to inform policymakers and help them design
better management and restoration policies across sectors and actors
that can balance economic, ecological, climatic and health interests for
future generations, particularly in the context of the Paris Agreement
on Climate Change and the Minamata Convention on Mercury.
Methods
We collected 819 sediment samples from 58 coastal wetlands, includ-
ing unvegetated (tidal flats, N = 19) and vegetated (seagrass meadows,
N = 10; tidal marshes, N = 14; mangrove forests, N = 15) coastal ecosys-
tems along Chinese coastlines from the cold temperate zone (~40° N)
to the tropical zone (~19° N) (Fig. 1a) during 2010 to 2022. We measured
sediment Hg concentrations using a direct Hg analyser (DMA–80,
Milestone), following the US Environmental Protection Agency Method
7473 (ref. 9). The limit of detection of the Hg analyser was 0.1 ng g−1. The
performance of the Hg analyser was evaluated by multiple measure-
ments of certified reference materials GBW07314 (marine sediment)
and GBW07405 (soil). Results were not statistically different from the
certified values, with Hg concentrations of 47.1 ± 3.9 ng g−1 (1σ, N = 57,
certified value of 48 ± 12 ng g−1) for GBW07314 and 282.9 ± 12.5 ng g−1 (1σ,
N = 16, certified value of 290 ± 40 ng g−1) for GBW07405. We measured
LOI as the loss of weight of a sediment sample ignited at 550 °C for 8 h
by a Muffle furnace9.
We multiplied a single general linear trend (Hg/OC ratio:
3.95 µg g–1) between OC and Hg in all coastal sediments generated
from our regional observational data (Fig. 1c) by the Blue C stock in
different Blue C ecosystems compiled by others to roughly estimate the
Hg stock in the top metre of sediment in global coastal environments
(Blue Hg stock; Fig. 1d). The van Bemmelen factor (LOI/OC = 1.724) is
used to convert sediment LOI to OC content. The Blue C stock in global
mangrove forests (mean = 5,150 Tg OC, range = 1,900–8,400 Tg OC),
seagrass meadows (mean = 11,366 Tg OC, range = 1,732–21,000 Tg OC)
and tidal marshes (mean = 1,106 Tg OC, range = 862–1,350 Tg OC) was
obtained from ref. 3 and tidal flats (900 Tg OC) from ref. 4. We applied
the identical method to estimate the Blue Hg fluxes. Since the huge
uncertainties of current Blue C stocks and fluxes dominate the uncer-
tainties of Blue Hg stock and flux estimates in the present study, we did
not consider the error propagation of sediment LOI and Hg determina-
tion and the regression coefficient of OC and Hg in the calculation.
Reporting summary
Further information on research design is available in the Nature
Portfolio Reporting Summary linked to this article.
Data availability
The data supporting the findings of this study are available within the
paper and its Supplementary Table 1.
References
1.
Schartup, A. T. et al. Climate change and overfishing increase
neurotoxicant in marine predators. Nature 572, 648–650
(2019).
2.
Duarte, C. M., Losada, I. J., Hendriks, I. E., Mazarrasa, I. &
Marbà, N. The role of coastal plant communities for climate
change mitigation and adaptation. Nat. Clim. Change 3, 961–968
(2013).
3.
Macreadie, P. I., Costa, M. D. P., Atwood, T. B. & Friess, D. A. Blue
carbon as a natural climate solution. Nat. Rev. Earth Environ. 2,
826–839 (2021).
4.
Chen, Z. L. & Lee, S. Y. Tidal flats as a significant carbon reservoir
in global coastal ecosystems. Front. Mar. Sci. 9, 900896 (2022).
5.
Boyd, P. W. et al. Potential negative effects of ocean afforestation
on offshore ecosystems. Nat. Ecol. Evol. 6, 675–683 (2022).
6.
Bergamaschi, B. A. et al. Tidally driven export of dissolved
organic carbon, total mercury and methylmercury from a
mangrove-dominated estuary. Environ. Sci. Technol. 46, 1371–1378
(2012).
7.
Mitchell, C. P. J. & Gilmour, C. C. Methylmercury production in
a Chesapeake Bay salt marsh. J. Geophys. Res. Biogeosci. 113,
G00C04 (2008).
8.
Lei, P., Zhong, H., Duan, D. & Pan, K. A review on mercury
biogeochemistry in mangrove sediments: hotspots of
methylmercury production? Sci. Total Environ. 680, 140–150
(2019).
9.
Ci, Z., Yin, Y., Shen, W. & Chen, B. Non-conservative mixing
behaviors of mercury in subterranean estuary: coupling effect of
hydrological and biogeochemical processes and implications for
rapidly changing world. Water Res. 244, 120455 (2023).
10. Médieu, A. et al. Evidence that Pacific tuna mercury levels are
driven by marine methylmercury production and anthropogenic
inputs. Proc. Natl Acad. Sci. USA 119, e2113032119 (2022).
11.
Fostier, A. H., Do, N., Costa, F. & Korn, M. D. G. A. Assessment
of mercury contamination based on mercury distribution in
sediment, macroalgae and seagrass in the Todos os Santos Bay,
Bahia, Brazil. Environ. Sci. Pollut. Res. 23, 19686–19695 (2016).
12. Wolswijk, G. et al. Distribution of mercury in sediments, plant and
animal tissues in Matang Mangrove Forest Reserve, Malaysia.
J. Hazard. Mater. 387, 121665 (2020).

Nature Sustainability | Volume 7 | December 2024 | 1592–1595
1595
Brief Communication
https://doi.org/10.1038/s41893-024-01471-y
13. Wang, Y., Wang, Z., Zheng, X. & Zhou, L. Influence of Spartina
alterniflora invasion on mercury storage and methylation in the
sediments of Yangtze River estuarine wetlands. Estuar. Coast.
Shelf Sci. 265, 107717 (2022).
14. Schuster, P. F. et al. Permafrost stores a globally significant
amount of mercury. Geophys. Res. Lett. 45, 1463–1471 (2018).
15. Outridge, P. M., Mason, R. P., Wang, F., Guerrero, S. &
Heimbürger-Boavida, L. E. Updated global and oceanic mercury
budgets for the United Nations Global Mercury Assessment 2018.
Environ. Sci. Technol. 52, 11466–11477 (2018).
16. Sunderland, E. M. & Mason, R. P. Human impacts on open ocean
mercury concentrations. Glob. Biogeochem. Cycles 21, GB4022
(2007).
17. Lamborg, C. H. et al. A global ocean inventory of anthropogenic
mercury based on water column measurements. Nature 512,
65–68 (2014).
18. Spivak, A. C., Sanderman, J., Bowen, J. L., Canuel, E. A. &
Hopkinson, C. S. Global-change controls on soil-carbon
accumulation and loss in coastal vegetated ecosystems.
Nat. Geosci. 12, 685–692 (2019).
19. Liu, M. et al. Rivers as the largest source of mercury to coastal
oceans worldwide. Nat. Geosci. 14, 672–677 (2021).
20. Zhou, J., Obrist, D., Dastoor, A., Jiskra, M. & Ryjkov, A. Vegetation
uptake of mercury and impacts on global cycling. Nat. Rev. Earth
Environ. 2, 269–284 (2021).
21. Biswas, A., Blum, J. D., Bergquist, B. A., Keeler, G. J. & Xie, Z.
Natural mercury isotope variation in coal deposits and organic
soils. Environ. Sci. Technol. 42, 8303–8309 (2008).
22. Soerensen, A. L., Schartup, A. T., Skrobonja, A. & Björn, E. Organic
matter drives high interannual variability in methylmercury
concentrations in a subarctic coastal sea. Environ. Pollut. 229,
531–538 (2017).
Acknowledgements
We greatly appreciate Joel D. Blum of University of Michigan for
providing valuable comments on drafts of the paper. We thank
Yingying Liu, Zhilong Shi and Dongping Chen for sample collection
and analysis, and Peng Wang and the volunteers who helped in the
field sampling. This study was supported by the National Natural
Science Foundation of China (nos. 42477224 and 41773123 to
Z.C. and 21777198 to B.C.) and the Innovation Group Project of
Southern Marine Science and Engineering Guangdong Laboratory
(Zhuhai) (no. 311022010 to W.S.).
Author contributions
Z.C. designed and performed research. W.S. and X.Z. contributed
analytic tools. Z.C., W.S. and B.C. acquired funding. Z.C. analysed
data, generated figures and wrote the original draft of the paper.
All co-authors contributed to the discussion, editing and approval of
the paper.
Competing interests
The authors declare no competing interests.
Additional information
Supplementary information The online version contains
supplementary material available at
https://doi.org/10.1038/s41893-024-01471-y.
Correspondence and requests for materials should be addressed to
Zhijia Ci.
Peer review information Nature Sustainability thanks Jean-Francois
Lapierre, Maodian Liu and Ke Pan for their contribution to the peer
review of this work.
Reprints and permissions information is available at
www.nature.com/reprints.
Publisher’s note Springer Nature remains neutral with regard to
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Springer Nature or its licensor (e.g. a society or other partner) holds
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the author(s) or other rightsholder(s); author self-archiving of the
accepted manuscript version of this article is solely governed by the
terms of such publishing agreement and applicable law.
© The Author(s), under exclusive licence to Springer Nature Limited
2024

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Corresponding author(s):
Zhijia Ci
Last updated by author(s): Oct 18, 2024
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Study description
In this study, we carried out a targeted survey along Chinese coastlines to document the relationship between coastal habit types
and sediment Hg and OC contents. We then based on the correlation between OC and Hg in coastal sediments generated from our
regional observational data and global blue carbon stock and flux data from literatures to estimate the Hg stock in the top meter
sediment of global blue C ecosystems and annual burial flux of Hg to global blue C ecosystems. We also propose two terms, "Blue Hg"
and "Blue Hg dilemma", to advance our understanding of the carbon-climate-Hg nexus.
Research sample
Sediment samples (n=819) collected from diverse coastal environments along Chinese coastlines were used in this study.
Sampling strategy
Sampling sites designed to encompass typical coastal habitats, including tidal flats, seagrass meadows, tidal marshes and mangrove
forest.
Data collection
Data collection was mainly done by Zhijia Ci with the assistance of the co-workers and volunteers.
Timing and spatial scale
Sediment samples were collected along Chinese coastlines from the cold temperate zone (~40 oN) to the tropical zone (~19 oN)
during 2010 to 2022.
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Field conditions
Sediment samples were collected from diverse coastal habitats, including unvegetated (tidal flats, n=19) and vegetated (seagrass
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Sediment samples were collected along Chinese coastlines from the cold temperate zone (~40 o) to the tropical zone (~19 oN).

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Novel plant genotypes
Describe the methods by which all novel plant genotypes were produced. This includes those generated by transgenic approaches,
gene editing, chemical/radiation-based mutagenesis and hybridization. For transgenic lines, describe the transformation method, the
number of independent lines analyzed and the generation upon which experiments were performed. For gene-edited lines, describe
the editor used, the endogenous sequence targeted for editing, the targeting guide RNA sequence (if applicable) and how the editor
was applied.
Seed stocks
Report on the source of all seed stocks or other plant material used. If applicable, state the seed stock centre and catalogue number. If
plant specimens were collected from the field, describe the collection location, date and sampling procedures.
Authentication
Describe any authentication procedures for each seed stock used or novel genotype generated. Describe any experiments used to
assess the effect of a mutation and, where applicable, how potential secondary effects (e.g. second site T-DNA insertions, mosiacism,
off-target gene editing) were examined.
Plants

    Scientific Research Study Methods:

Answer: Our study consisted of three parts. First, we analysed 819 sediment samples collected from 58 coastal wetlands along Chinese coastlines to document the relationship between coastal habitat types and sediment OC and Hg contents. Second, we combined the correlation of sediment OC and Hg generated from our study and the blue carbon data compiled by others to roughly estimate the Hg stock and flux in global blue carbon ecosystems. Finally, we proposed the term “Blue Hg” and “Blue Hg dilemma” to highlight the close linkage of coastal OC and Hg storage and cycling (Fig. 1c), as well as the trade-offs between OC sequestration with Hg risks from the carbon-focused NbS in coastal ecosystems (that is, blue carbon strategies).