Seagrass Role in Carbon Sequestration (Part I)

Nov 21, 2012 › Seagrass Recovery › www.seagrassrecovery.com

Recent studies have emphasized that conserving and restoring seagrass meadows may reduce greenhouse gas emissions and increase carbon stores.^5,17 Seagrasses occur over a wide distribution range along the shores of every continent, except Antarctica, to maximum depths down to 50 meters, depending on water clarity.⁸ The total area of the Earth covered by seagrasses has not been cataloged to-date; however, different estimates are available. The current documented area of seagrass coverage is approximately 177,000 km², but this is acknowledged as an underestimate since many regions with large seagrass cover have not been fully monitored.^2,7 Recent estimates most commonly used in literature are between 300,000 km² and 600,000 km², with the global area potentially suitable to support seagrass growth estimated at 4,320,000 km².^{1,3,4,6,12,13} That’s equivalent to between 74,131 and 148,263 acres of seagrass or roughly 56,160 to 112,320 football fields of estimated global cover. Considering the global area potentially could support over one billion acres of seagrass, the aforementioned numbers are just a drop in the bucket.

Not only are seagrass meadows highly productive ecosystems that play a key role in supporting high biodiversity they also provide an enormous source of carbon to the detrital pool. Some of this carbon gets exported to the deep sea, where it provides a supply of organic matter in sometimes extremely food-limited environments.^14,15 Most of the organic carbon produced by seagrasses is stored within the sediments making these areas hot-spots for carbon sequestration.^4,14 Seagrass sediments are organic-rich, with an average organic concentration of 4.1% and can be characterized by their capacity to sequester and store considerable amounts of carbon in their sediments (known as blue carbon).⁵ Seagrasses remove carbon dioxide from the atmosphere and incorporate it into organic matter; in other words, they convert sunlight and carbon dioxide efficiently into organic form. Seagrasses are also responsible for approximately 10% of the yearly global carbon sequestration in marine sediments even though they only occupy less than 0.2% of the ocean surface.^4,5,8,9

Recent studies have shown that coastal seagrass beds store up to 83,000 metric tons of carbon per square kilometer, predominantly in the sediments beneath them.^5,17 As a comparison, a typical terrestrial forest stores about 30,000 metric tons per square kilometer, mostly in the form of wood.^5,17 The total global carbon pool in seagrass beds is estimated to be as much as 19.9 billion metric tons.^5,9 Seagrass meadows have occupied coastal environments over thousands of years and can combine a high metabolic capacity to act as carbon sinks with the capacity to accumulate large carbon pools in the sediments. These pools are able to form thick carbon deposits or mattes, raising the seafloor by about 1 mm per year^.5,9 The thickest documented sedimentary deposit has been reported to be 11 m thick from the Posidonia oceanica meadow at Port Lligat, Spain, in the NW-Mediterranean Sea which is equivalent to an accumulation of approximately 0.18 tons C m-2, that’s over 6000 years of seagrass growth at that site.^5,11 Other seagrass deposits up to several meters in thickness, have also been reported at other sites in the Spanish Mediterranean, Shark Bay (Western Australia) and Florida Bay in the United States.⁹

Unfortunately, seagrass meadows are declining globally at a rapid rate, with about 5% of seagrass meadows lost annually.2 This decline is mostly due to dredging and degradation of water quality which has accounted for at least 1/3 of the area present lost since World War II, which means the existence of an important blue carbon sink is at stake.^2,12,14,16 The long retention times of carbon in these sedimentary deposits is quite unique and qualifies seagrass meadows as one of the most carbon-rich ecosystems on the planet. That is why it is fundamentally necessary to understand the processes for the capacity of seagrass meadows to capture and store carbon so we can manage these ecosystems in support of strategies to mitigate climate change. Necessary management strategies should include both conservation and reforestation of seagrass meadows.^5,14 There is also a critical need for targeted global conservation efforts that include protection, monitoring, management and policy regimes for seagrass habitats, as well as targeted educational programs informing regulators and the public of the value of seagrass meadows.^5,14 Without proper management and conservation practices and increased global awareness, seagrass meadows will continue to decline. But if restored, these ecosystems have the capacity to effectively and efficiently sequester carbon, reestablishing lost carbon sinks in the ocean. ^5,17 Let’s take that as a challenge.

 

References

¹Charpy-Roubaud, C. & Sournia, A. 1990. The comparative estimation of phytoplanktonic and microphytobenthic production in the oceans. Mar. Microb. Food Webs 4, 31-57.
²Duarte, C.M., et al., Assessing the capacity of seagrass meadows for carbon burial: Current limitations and future strategies, Ocean & Coastal Management (2011), doi:10.1016/j.ocecoaman.2011.09.001
³Duarte, C.M., Borum, J., Short, F.T., Walker, D.I., 2005. Seagrass ecosystems: their global status and prospects. In: Polunin, N.V.C. (Ed.), Aquatic Ecosystems: Trends and Global Prospects. Cambridge University Press.
⁴Duarte, C. M., Middelburg, J. J. & Caraco, N. 2005.Major role of marine vegetation on the oceanic carbon cycle. Biogeosciences 2, 1-8.
⁵Fourqurean, J.W., Duarte, C.M., Kennedy, H., Marbà, N., Holmer, M., Mateo, M.A., Apostolaki, E.T., Kendrick, G.A., Krause-Jensen, D., McGlathery K.J. and Serrano,O. 2012. Seagrass ecosystems as a globally significant carbon stock. Nature Geoscience, 5: 505-509.
⁶Gattuso, J.-P., Gentili, B., Duarte, C.M., Kleypas, J.A., Middelburg, J.J., Antoine, D., 2006. Light availability in the coastal ocean: impact on the distribution of benthic photosynthetic organisms and their contribution to primary production. Biogeosciences 3, 489-513.
⁷Green, E.P. and Short, F.T. (2003) World Atlas of Seagrasses. Prepared by the UNEP World Conservation Monitoring Centre, University of California Press, Berkeley, USA.
⁸Hemminga, M.A., Duarte, C.M., 2000. Seagrass Ecology. Cambridge Univ. Press, Cambridge.
⁹Kennedy, H., Beggins, J., Duarte, C.M., Fourqurean, J.W., Holmer, M., Marbà, N., Middelburg, J.J., 2010. Seagrass sediments as a global carbon sink: isotopic constraints. Global. Biogeochem. Cycles. 24 doi:10.1029/ 2010GB003848.
10Laffoley, D.d’A., Grimsditch, G., 2009. The management of natural coastal carbon sinks. IUCN, Gland, Switzerland, 53 pp.
¹¹Lo Iacono, C., Mateo, M.A., Gracia, E., Gasch, L., Carbonell, R., Serrano, L., Danobeitia, J., 2008. Very high-resolution seismo-acoustic imaging of seagrass meadows (Mediterranean Sea): implications for carbon sink estimates. Geophys. Res. Lett. 35, L18601. doi:10.1029/2008GL034773.
¹²Mcleod, E., Chmura, G.L., Bouillon, S., Salm, R., Björk, M., Duarte, C.M., Lovelock, C.E., Schlesinger, W.H., Silliman, B., 2011. A blueprint for blue carbon: towards an improved understanding of the role of vegetated coastal habitats in sequestering CO2. Front. Ecol. Environ. doi:10.1890/110004.
¹³Nellemann, C., Corcoran, E., Duarte, C.M., Valdés, L., De Young, C., Fonseca, L., Grimsditch, G., 2009. Blue Carbon. A Rapid Response Assessment. United Nations Environment Programme, GRID, Arendal.
¹⁴Orth, R.J., Carruthers, T.J.B., Dennison, W.C., Duarte, C.M., Fourqurean, J.W., Heck Jr., K.L., Hughes, A.R., Kendrick, G.A., Kenworthy, W.J., Olyarnik, S., Short, F.T., Waycott, M., Williams, S.L., 2006. A global crisis for seagrass ecosystems. Bioscience 56, 12; 987-996.
¹⁵Suchanek TH,Williams SW,Ogden JC,Hubbard DK, Gill IP. 1985.Utilization of shallow-water seagrass detritus by Caribbean deep-sea macrofauna: 13C evidence. Deep Sea Research 32: 2201–2214.
¹⁶Waycott, M., Duarte, C.M., Carruthers, TJ.B., Orth, R.J., Dennison, W.C., Olyarnik, S., Calladine, A., Fourqurean, J.W., Heck Jr., K.L., Hughes, A.R., Kendrick, G.A., Kenworthy, W.J., Short, F.T., Williams, S.L., 2009. Accelerating loss of seagrasses across the globe threatens coastal ecosystems. Proc. Nat. Acad. Sci. USA (PNAS) 106, 12377e12381.
¹⁷Wilson, H.W. 2012. Seagrasses Store More Carbon Than Forests Do. Science Teacher, 79.6; 26-27. Available from: OmniFile Full Text Mega, Ipawich, MA. Accessed November 2012.
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