What is Blue Carbon?


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Sustainable Ocean Alliance is accelerating ocean solutions around the world. Here are their stories.

Blue Carbon Ecosystems 

Blue Carbon is usually defined as the organic carbon captured and stored by vegetated coastal ecosystems (salt marshes, seagrass meadows, and mangroves). These ecosystems sequester or absorb carbon dioxide from the atmosphere through photosynthesis and by trapping carbon-rich particles coming from elsewhere. Mangroves and salt marshes specifically remove carbon from the atmosphere at a rate 10x greater and store 5x more carbon per acre than tropical forests (Source: NOAA). The conservation and restoration of blue carbon ecosystems are crucial in the fight against climate change. When these ecosystems are damaged or destroyed, the carbon stored in their soils is released back into the atmosphere; the total amount of stored carbon is roughly equivalent to a whole year of anthropogenic emissions. Protecting these ecosystems keeps this carbon locked away; restoring and expanding them allows more carbon to be sequestered.

Other species like kelp, calcifying organisms (coral, bivalves), and algae do cycle carbon dioxide, but their net impact varies.

Phytoplankton, for example, remove carbon dioxide through photosynthesis, but have very rapid turnover within the marine food chain as they are consumed or otherwise die and sink to the bottom of the ocean. Only ~.2Gt C per year of carbon is stored by phytoplankton -- roughly the same amount captured by mangroves, even though they cover less than a .05% area of the ocean. Similarly, some kelp does sink to the ocean floor and is buried in the sediment, but more often it is eaten and cycled through the food chain, with a somewhat negligible, and certainly debatable, impact on atmospheric carbon dioxide. 

Ultimately, blue carbon ecosystems, when paired with community engagement, can improve ecosystem health and mitigate carbon change. But their processes are complex, and even the most generous estimates of carbon sequestration aren't enough to avoid the temperature increases predicted by the IPCC

Other benefits of blue carbon include preserving unique ecosystems like estuaries and their endemic species, as well as supporting the coastal communities that preserve them through employment, sale of carbon credits, and providing coastal protection and food security. SOA supports blue carbon projects with Ocean Leadership Microgrants.


Mangroves | Seaweed, Seagrass, Kelp | Algae & Phytoplankton | Salt Marshes & Estuaries | Shellfish & Bivalves | Megafauna | Coral | Carbon Credits


What are mangroves?

Seaweed is the common name for countless species of marine plants and algae that grow in the ocean as well as in rivers, lakes, and other water bodies. While seagrass may look similar to seaweed, they are very distinctly different and unlike seagrass, they do not have root systems. There are three main types of seaweed- red, green, and brown. A mangrove is a woody tree or shrub that lives along sheltered coastlines. Mangroves support fisheries by providing spawning grounds for commercial fish species,  filter pollutants and contaminants from coastal waters contributing to healthy water quality, and protect coastal development and communities against storms, floods, and erosion (Source: Smithsonian, Blue Carbon Initiative). 

Where are mangroves found?

Mangroves grow in sheltered tropical and subtropical coastal areas across the globe. In general, this is an area between latitudes of 25 degrees north and 25 degrees south.

Mangrove_DistributionImage: World map of the mangrove distribution zones and the number of mangrove species along each region (Source: Habitat requirements for mangroves. Deltares, 2014).

How do mangroves sequester carbon?

As mangroves grow, they take the carbon from carbon dioxide and use it as the building blocks for their leaves, roots, and branches. When leaves and older trees die, they settle to the seafloor along with their stored carbon, creating a carbon sink within the soil (Source: Smithsonian: Mangroves).

Further reading: Huxham et al. "Mangroves and People"

Seagrass, Seaweed, Kelp

What are seagrass, seaweed, and kelp?

Seagrasses are marine flowering plants with roots, stems, and leaves that form extensive underwater meadows that create complex, highly productive, and biodiverse habitats (Source: UN Environment Programme).

Seaweed is the common name for countless species of marine plants and algae that grow in the ocean as well as in rivers, lakes, and other water bodies. While seaweed may look similar to seagrass they are distinctly different; unlike seagrass, seaweed has no root systems and therefore no carbon storage in sediments. There are three main types of seaweed: red, green, and brown. Kelp is a type of macroalgae, or seaweed, specifically brown seaweed.

Source: Christian Gloor

Where are they found?

Seagrass is found in shallow salty waters globally, from the tropics to the Arctic circle. They exist in 159 countries on six continents, covering over 300,000 km2, making them one of the most widespread coastal habitats on Earth (Source: Smithsonian). Seaweeds are found all over the world in many different kinds of bodies of water while kelp is found in cold, coastal marine waters around the globe. It grows near the shore in rocky or eroding conditions. 

How does each sequester carbon?

Like plants on land, seaweeds and seagrasses photosynthesize to absorb CO2 and grow biomass. The majority of the carbon benefit of seagrass occurs in below ground storage. Algae and kelp serve most effectively as as carbon sinks when they sink to the bottom of the ocean and are buried, but the magnitude of this impact varies and is up for scientific debate.

Screen Shot 2022-01-27 at 5.26.21 PM

Image: Pathways for sequestration of macroalgae carbon into the deep sea. This figure was adapted from Krause-Jensen and Duarte, 2016. Source: Harvard University).

Seagrass meadows store large amounts of carbon in the biomass and sediment below. Specifically, seagrass absorbs carbon about 35 times faster than trees and absorbs 10%  of the ocean's carbon annually (Source: Second World Ocean Assessment). The lack of oxygen within the seagrass sediments allows for increased preservation of organic carbon, leading to the formation of large carbon deposits within the sediment that when left undisturbed, can remain for thousands of years. The carbon stored in the above-ground living biomass (for example, leaves) is subject to grazing, harvesting, or decomposition, and is considered a short-term carbon sink. Most of the carbon sequestered by seagrass meadows is stored in the sediment (Source: UN Environment Programme).

Algae & Phytoplankton

What are Algae and Phytoplankton?

Algae are a diverse group of aquatic organisms with the ability to produce oxygen through photosynthesis (the process of harvesting light energy from the sun to generate carbohydrates). Phytoplankton are microscopic marine algae, or microalgae, that contain chlorophyll and photosynthesize. They provide a food source for a wide range of marine organisms.

phytoplankton.1600x0Where are they found?

Most phytoplankton is found at shallower depths of the ocean where sunlight is able to penetrate, which allows the microalgae to live and grow (Source: NOAA).

How do they sequester carbon?

Phytoplankton is photosynthetic, meaning they draw carbon from their atmosphere to grow. However, their benefits of carbon sequestration can turn negative when their population size remains unchecked. In an unbalanced ecosystem with too many nutrients available, phytoplankton can overpopulate, and form harmful algal blooms (HABs) that can produce toxic chemicals negatively affecting marine life, birds, and humans (Source: NOAA). Phytoplankton are a primary food source for many marine fauna, so their sequestration is more accurately captured as cycling; for example, when whales eat phytoplankton they ingest carbon that may or may not be stored for the long term, depending on where the whale ends up at the end of its life. And no doubt, there are more compelling reasons to save the whales than their ability to store carbon!

algal_bloomImage: Algal bloom in the Arabian Sea, 2017

Salt Marshes & Estuaries

What are Salt Marshes & Estuaries?

Estuaries are the bodies of water where the river meets the sea. The mix of freshwater from the rivers and the saltwater from the sea create brackish water, housing many unique plants and animal communities such as small fish, shellfish, migrating birds, and shore animals along with a plethora of nutrient-dense plankton and bacteria. The organisms that inhabit this ecosystem are incredibly stress-tolerant, able to withstand a wide range of environmental conditions. However, they also have to face serious threats from anthropogenic activities. Not all estuaries are connected to the ocean; freshwater estuaries are where a river flows into a freshwater lake (Sources: NOAA: What is an estuary?”, National Geographic).


Salt marshes are a common habitat within estuaries. Along with seagrasses, salt marshes line estuaries and filter water flowing into the ocean while acting as a storm buffer. They are coastal wetlands with soil composed of deep mud and peat that are flooded and drained by salt water brought in by the tides (Source: NOAA: What is a salt marsh?).

Where are they located? 

Estuaries and salt marshes occur worldwide, particularly in areas of middle to high  latitudes, and thrive along protected shorelines. Estuaries in particular are found along global coastlines, with the exception of the Great Lakes. There are over 1,300 estuaries worldwide. You can view a map of all recorded estuaries around the globe here: Global Estuary Database 

How do they sequester carbon?

As with terrestrial ecosystems, most carbon is stored within the soil. As these wetland plants grow and photosynthesize, they take in carbon dioxide from the atmosphere and convert it into organic matter, in the same process that mangroves and seagrass use to store carbon (Source: AGU Journals). Salt marshes and estuaries are crucial and highly effective carbon sinks, with research showing stored carbon dating back 2,500 years (Source: NCCOS). However, these ecosystems are being threatened by sea-level rise due to climate change.

Shellfish & Bivalves

What are shellfish and bivalves?

Shellfish is the common name for aquatic invertebrates containing exoskeletons, usually eaten as food. Shellfish include mollusks (clams, mussels, oysters, scallops, octopus, or squid), crustaceans (shrimp, crabs, lobsters), and echinoderms (starfish, sea urchins, sea cucumbers, etc.) with their diet consisting primarily of phytoplankton and zooplankton. Bivalves are shell-dwelling, filter-feeding mollusks and rank among the top commercially important shellfish. Because bivalves are filter-feeders, they act as bioindicators of their aquatic environment and can tell us about the heath of the ocean in which they reside. 


Image of Green-lipped mussels (P. viridis). Photo by Julian Sprung. 

Where are they located?

Shellfish and bivalves are mostly found in saltwater aquatic habitats around the globe with some inhabiting freshwater ecosystems as well.

How do they sequester carbon?

Shellfish, unlike plants that photosynthesize, are respiratory animals, meaning they inhale oxygen and exhale carbon dioxide. However, the shell of a shellfish absorbs carbon as it grows. The shellfish secretes Calcium Carbonate (CaCO3) to form its shell which means a percentage of its shell contains carbon (Source: OA-ICC). Bivalve mollusks like oysters and mussels also contribute to the formation of reefs by aggregating together. This indirectly contributes to sequestering carbon as reefs act to provide habitat for numerous other species and bury carbon (Source: Proc Biol Sci. 2017). 

Other Sources of Carbon Cycling Megafauna

What is megafauna?

Megafauna are very large animals, typically weighing more than 44 kilograms with some exceptions. Most have no natural predators (besides their young) and their population is regulated by bottom-up food availability (Source: True Nature Foundation). Many megafauna species are extinct, however, some examples of megafauna species today include elephants, hippos, gorillas, sharks, loggerhead turtles, giant squids, and marine mammals such as whales.

Where are megafauna located?

Megafauna species inhabit many different habitats all over the world. However, most megafauna today are natively found in the southern hemisphere, specifically in the Asian and African regions with their ranges and population shrinking over time. Marine megafauna however can inhabit oceans all over the globe. 

How can megafauna sequester carbon?

One example of how megafauna actively sequester carbon is shown through whales. Whales play a crucial role in climate change mitigation by increasing the ocean’s ability to absorb carbon dioxide as well as acting as a carbon sink. Whales increase the level of nutrients on the ocean surface through their swimming and diving movements, allowing for increased water flow, as well as when they release feces. The nutrients promote the growth of phytoplankton which photosynthesize and absorb carbon (Source: Blue Climate Solutions, Pershing et al. 2010).

whale_carbon_sinkCoral Reefs

Are coral reefs blue carbon ecosystems?

The simple answer is "sort of". Calcification actually typically releases carbon, but research also suggests that coral reefs work in conjunction with seagrass meadows and otherwise contributes to healthy functioning of the seascape. The reefs act as a barrier for the seagrass meadows, protecting them from waves and storm conditions that would otherwise degrade and erode the seagrass ecosystem. Coral reefs protecting the meadows means preventing potential carbon releasing from the sediment due to wave action, thus demonstrating coral reefs’ vital, although indirect, role within blue carbon ecosystems (Source: Frontiers In Marine Science). 

Carbon Credits

What are carbon credits?

Carbon credits are credits that entities, usually companies, can buy to offset their carbon footprint or greenhouse gas emissions. For example, if a company cannot avoid emitting carbon, they can give money to another party that actively works to reduce or remove carbon from the atmosphere, thus compensating for their carbon emissions (debt), or even creating a carbon-neutral status (Source: World Economic Forum). Offsetting carbon emissions can be a valuable tool but only in conjunction with actively reducing emissions. 

How do they work?

Carbon credits are “packaged” into three categories, 1) emission reduction through efficiency measures, 2) active removal of emissions through carbon capture, and 3) avoiding emissions (for example, not clear-cutting rainforests).

In the past, a lack of monitoring or a standardized system has led to corruption within this process. A new report from UN Special Envoy for Climate Action and Finance Mark Carney drafted a blueprint for transparency within the carbon credit trading market based on verifying valid CO2 reductions. The report states, in order to be eligible to be traded, “carbon credits "must be based on projects that have been independently validated and monitored throughout their lifecycle” (World Economic Forum). As of May of 2020, the US Treasury has issued a new policy requiring companies to verify the amount of carbon captured in the credits they invest in.

How can I buy carbon credits?

You can purchase carbon credits directly from the individual organization's website. Some examples of organizations that certify and/or sell credits are: VERA //  PlanVivo // ACES // Katingan Project // Nori Carbon Removal Marketplace


Do you have a blue carbon project in need of support? Apply for a grant from SOA.

Further reading on Blue Carbon: Huxham et al 2018 | Carbon in the Seascape


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