Blue Carbon Explained

Caitlin Dempsey

Updated:

Carbon storage is a term that many people may see in discussions of capturing atmospheric carbon dioxide. A lesser known related term is what is referred to as blue carbon.

Blue carbon refers to carbon dioxide (CO2) that is captured by the world’s oceans and coastal ecosystems.

The “blue” in blue carbon refers to carbon that is stored in watery environments. This includes include kelp forests, salt marshes, mangroves, and seagrass meadows, as well as the open ocean.

While the coastal areas represent only about 2% of the world’s land masses, these interface areas between the oceans and continents have an extraordinary ability to absorb and sequester carbon.

For example, tidal marshes can sequester carbon at an average rate of 1-2 tons per hectare.

A series of maps showing above ground carbon store for six tidal marsh areas in the United States:  Cape Cod, MA, Chesapeake Bay, MD, Everglades, FL, Mississippi Delta, LA, San Francisco Bay, CA, and Puget Sound, WA.
A 2018 studied looked at above-ground carbon storage in six coastal ecosystems: Cape Cod, MA, Chesapeake Bay, MD, Everglades, FL, Mississippi Delta, LA, San Francisco Bay, CA, and Puget Sound, WA. Maps: USGS, 2018, public domain.

Vegetated coastal ecosystems are a carbon sink which means they can absorb more carbon dioxide that they emit. Coastal ecosystems sequester more carbon per unit area than land-based forest areas, known as green carbon.

Blue carbon ecosystems

Blue carbon ecosystems can be broadly divided into two regions: oceans and coastal ecosystems. Coastal ecosystems include salt marshes, mangroves, seagrass meadows, and kelp forests.

Blue carbon diagram

Diagram showing the flow and storage of carbon in coastal wetlands.
Diagram showing the blue carbon cycle showing how underwater soils and sediments form carbon sinks. Diagram: OAA Climate.gov graphic adapted from original by Sarah Battle, NOAA Pacific Marine Environmental Laboratory, public domain.

Oceans

The majority of blue carbon is stored as dissolved carbon dioxide in ocean waters.

Salt Marshes

Coastal salt marshes are marshy ecosystems that are flooded and drained daily by ocean tides. Salt marshes are made up of mud and peat and serve as important estuaries for wildlife and fisheries.

A salt marsh pond in Plum Island, MA (on the left), alongside a tidal creek (on the right).
A salt marsh pond in Plum Island, MA (on the left), alongside a tidal creek (on the right). Photo: USGS, public domain.

Salt marshes are most common in the mid-latitudes. Salt marshes are found on almost every coast in the United States with about along the Gulf Coast in the southern region of the country.

In addition to being a carbon sink, salt marshes help to protect coastlines from erosion and storm surges.

Mangroves

Mangroves are groups of trees and shrubs that live in the coastal intertidal zone and thrive in saltwater environments.Mangroves have a complex root systems, known as prop roots, which allow them to anchor securely in the shifting, soft sediments of the coast.

Mangroves, which are tropical and subtropical coastal forests, possess one of the highest carbon densities of all terrestrial ecosystems.

Simple gray map with green shading to show the distribution of mangroves on a map of the world.
The global distribution of mangroves. Map: NASA/USGS, 2010.

Mangroves are mainly found in regions such as Southeast Asia, Central and South America, and the western and central parts of Africa.

Mangroves can be found throughout North America, stretching from Florida’s southernmost point along the Gulf Coast to Texas. One of the world’s most expansive mangrove swamps is found along Florida’s southwest coast.

Mangroves are ecologically significant for numerous reasons. Their extensive and elevation root system helps to stabilize coastlines, reducing erosion from storm surges, waves, and tides.

Red mangrove habitat, Everglades National Park. Photo: NPS
Red mangrove (Rhizophera mangle) habitat, Everglades National Park. Photo: NPS, public domain.

Their dense root systems are efficient at trapping sediments and contaminants from the water, enhancing water clarity and quality. By doing so, the mangrove root system enables the deposition and long-term storage of carbon-rich sediments as well as storing carbon in the tree species themselves.

Seagrass Meadows

Seagrass meadows are underwater plants found all around the world except for Antarctica.

There are more than 70 species of plants that form seagrass meadows: some are adapted to warm waters and other plants are adapted to cold waters.

Syringodium filiforme, manatee grass, is found in some seagrass beds at Padre Island National Seashore and Gulf Islands National Seashore. Photo: Joe Meiman/NPS, public domain.
Syringodium filiforme, manatee grass, is found in some seagrass beds at Padre Island National Seashore and Gulf Islands National Seashore. Photo: Joe Meiman/NPS, public domain.

For example, eelgrass is a variety of seagrass that flowers underwater in the temperate regions across the globe.  Turtle grass, manatee grass, and shoal grass are common to the warm waters of the Florida Keys.

Seagrass meadows are important contributors to blue carbon storage. Seagrass meadow plants store carbon in their stems as well as in the sediment that accumulates around the plants.

These habitats provide critical ecological functions, including nutrient cycling and sediment stabilization, alongside their role in carbon sequestration.

Kelp forests

Kelp forests are another significant component of the blue carbon equation, though they’re sometimes overlooked when compared to more commonly discussed ecosystems like mangroves, salt marshes, and seagrasses.

Kelp forests are underwater areas with a high density of kelp, large brown algae that can grow at remarkable rates – in optimal conditions, some species can grow up to half a meter per day. These habitats are predominantly found along colder, nutrient-rich coastlines, including those of the Pacific Northwest, Southern Australia, South Africa, and Northern Europe.

Kelp seen in Monterey Bay, California.
Kelp forests are often called the rainforest of the sea. They support wide varieties of marine life. Kelp seen in Monterey Bay, California. Photo: Tania Larson, U.S. Geological Survey. Public domain.

Similar to terrestrial forests, kelp forests engage in photosynthesis, absorbing atmospheric carbon dioxide (CO2) and releasing oxygen. This process allows kelp to sequester carbon, hence contributing to the overall concept of blue carbon.

However, the dynamics of carbon storage in kelp forests are different from those in other blue carbon ecosystems.

Unlike the long-term carbon storage seen in sediments of mangroves, salt marshes, and seagrass meadows, kelp primarily stores carbon in their biomass.

Kelp tissues also store gas in pouches called bladders. These bladders keep kelp afloat as the macroalgae float out to sea. As the kelp plant is broken down, the gas-filled sacs burst and the plant sinks to the deep ocean floor, sequestering the carbon.

A research paper published in 2016 estimated about 173 million tons of carbon is sequestered by macroalgae with 90% of that total occurring when the plant matter is carried out to the deep sea.

Although kelp forests might not be as effective as other blue carbon ecosystems in terms of long-term carbon storage, they are critically important for marine biodiversity, providing shelter and food for a multitude of marine organisms. Additionally, they help to reduce ocean acidification locally by absorbing CO2 from the surrounding water.

A climate mitigation tool

Because of blue carbon’s ability to sequester a high density of carbon, these areas have the potential play a significant role in mitigating climate change.

Coastal population increase, coastal habitat destruction, and sea level rises have all contributed to the loss of the world’s coastal ecosystem loss.

Research that looked into the additional carbon storing potential of restored blue carbon ecosystems found 841 (621–1,064) tetragrams of CO2e per year could be removed from the atmosphere by 2030. That represents a reduction of about 3% of total global emissions (based on 2019/2020 data) per the study authors.

Restoring coastal wetlands and other blue carbon ecosystems could help increase carbon sequestration and offset some of the effects of climate change.

References

Byrd, K. B., Ballanti, L., Thomas, N., Nguyen, D., Holmquist, J. R., Simard, M., & Windham-Myers, L. (2018). A remote sensing-based model of tidal marsh aboveground carbon stocks for the conterminous United StatesISPRS Journal of Photogrammetry and Remote Sensing139, 255-271. https://doi.org/10.1016/j.isprsjprs.2018.03.019

Duarte, C. M., Losada, I. J., Hendriks, I. E., Mazarrasa, I., & Marbà, N. (2013). The role of coastal plant communities for climate change mitigation and adaptation. Nature climate change3(11), 961-968. https://doi.org/10.1038/nclimate1970

Giri, C., Ochieng, E., Tieszen, L. L., Zhu, Z., Singh, A., Loveland, T., Masek, J. and Duke, N. (2010) Status and distribution of mangrove forests of the world using earth observation satellite data. Global Ecology and Biogeography, DOI: 10.1111/j.1466-8238.2010.00584.x.

Krause-Jensen, D., & Duarte, C. M. (2016). Substantial role of macroalgae in marine carbon sequestration. Nature Geoscience9(10), 737-742. https://doi.org/10.1038/ngeo2790

Macreadie, P. I., Anton, A., Raven, J. A., Beaumont, N., Connolly, R. M., Friess, D. A., … & Duarte, C. M. (2019). The future of Blue Carbon science. Nature communications10(1), 3998. https://doi.org/10.1038/s41467-019-11693-w

Macreadie, P. I., Baird, M. E., Trevathan-Tackett, S. M., Larkum, A. W. D., & Ralph, P. J. (2014). Quantifying and modelling the carbon sequestration capacity of seagrass meadows–a critical assessment. Marine pollution bulletin83(2), 430-439. https://doi.org/10.1016/j.marpolbul.2013.07.038

Macreadie, P. I., Costa, M. D., Atwood, T. B., Friess, D. A., Kelleway, J. J., Kennedy, H., … & Duarte, C. M. (2021). Blue carbon as a natural climate solution. Nature Reviews Earth & Environment2(12), 826-839. https://doi.org/10.1038/s43017-021-00224-1

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About the author
Caitlin Dempsey
Caitlin Dempsey is the editor of Geography Realm and holds a master's degree in Geography from UCLA as well as a Master of Library and Information Science (MLIS) from SJSU.

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