Many Hues of Blue

One of the chief ways a healthy ocean can help mitigate the effects of climate change is through its ability to trap carbon, a process often referred to as “carbon sequestration.” 

Carbon trapped in ocean stores is referred to as “blue carbon”.  Scientists are interested in increasing and developing those stores and creating the potential for stockpiling additional blue carbon.  Carbon sequestration is important to our global environment because it reduces the release of carbon into our atmosphere, where carbon is known to contribute to the rising of global temperatures.  Further, blue carbon ecosystems have been reported to absorb greater levels of planetary carbon dioxide emissions than land-based forests. 

What is carbon and how does it contribute to increased global temperatures?

Carbon is a naturally occurring element that, when strung together, provides the backbone structure for all of earth’s living organisms.  Long carbon chains are broken down to provide animals with energy and then animals release the carbon into the environment as carbon dioxide.  Carbon dioxide is therefore a typical component of air. 

However, the burning of fossil fuels, auto emissions, and other human-controlled activities release excess carbon into the atmosphere, where it traps heat near the earth’s surface, raising the temperature of the earth, a phenomenon known as global warming.  In 2016, the Paris Climate Agreement warned that to avoid the impact of climate change, we must limit the rise in temperatures to not more than 1.5 degrees Celsius, the pre-industrial level (i.e. the level prior to 1850-1900).

How does the ocean trap carbon?

A chief way the ocean traps carbon is when plants carry out photosynthesis.  In this process, light energy is used to convert gaseous carbon dioxide into solid carbohydrate plant matter.  These carbohydrates either remain in the plant or are transferred to animals when animals eat the plants.  Animals could exhale this carbon dioxide, as mentioned above, or the carbon could become a structural feature of the animal and is then prevented from escaping into the atmosphere. 

Where is carbon stored in the ocean?

There is carbon in ocean water, but when we talk about sequestered carbon, we are not talking about carbon that is dissolved in the ocean water; water carbon will simply contribute to the formation of an acid, lowering the pH of the ocean and harming ocean life.  Unfortunately, much carbon is dissolved in ocean waters as a result of acid rain, and this rain is one aspect of pollution that needs to be controlled in order to maintain a healthy ocean. 

Sequestered carbon, on the other hand, is stored inside living plants as well as in dead and decaying plant, animal, and microscopic debris.  It is also stored in rocks and particulate matter.  This debris is found in a more permanent state in deep ocean sediments in the benthic zone, the area near and including the ocean floor.

What are the best donors of blue carbon?

Scientists investigate the best donors of blue carbon because those will help us learn to store carbon more effectively.  As scientific investigations continue, it is increasingly clear that a diversity of species and processes play a role in the sequestration of blue carbon.  There are, in a sense, many hues of blue.  Here we explore just a few.

We have mentioned the benthic zone, the sites where sediments accumulate.  It is hoped that carbon will end up in this zone because carbon is most effectively sequestered in the ocean floor.  Additionally, there are donor ecological sites and species of plant and animal life that contribute to these sediments. 

For example, it has been found that coastal wetlands contain seagrasses, tidal marshes, and mangroves that are rich in carbon. Therefore, many areas of the ocean shoreline have become a focus for maintenance and restoration.  More recently, a great deal of interest has focused on the role of macroalgae in carbon sequestration.

The role of macroalgae as a blue carbon donor

Microalgae are primarily kelp and other seaweeds, a diverse group that includes brown, red and green algae, as well as cyanophytes, also known as blue-green algae or cyanobacteria.  Macroalgae are the newest species to be considered as major contributors to blue carbon.  These species were previously thought to be restricted to rocky shores where there is not much sedimentation.  Scientists have shown, however, that macroalgal carbon material is actually exported from coastal ecosystems to sedimentary sites in the deep ocean, where it comprises a significant subset of sequestered carbon.

Illumination of the macroalgal contribution to blue carbon has prompted heightened research on their growth conditions and distribution.  For example, studies are underway to promote macroalgal aquaculture and farming conditions.  It has also been found that some macroalgal species have greater carbon storage potential than others, depending on the thickness of their leaf-like structures, or thalli (3,4).

As scientist continue to explore blue carbon and share their findings with policy makers, such findings, if heeded, will build a healthier ocean that provides greater service to humanity and the protection of our planet.


Fourth National Climate Assessment. Chapter 8: Coastal Effects. Retrieved July 17, 2020.

Krause-Jensen, D., Duarte, C. Substantial role of macroalgae in marine carbon sequestration.  Nature Geosci 9, 737–742 (2016).

Krause-Jensen, D., Lavery, Pserrano, O., Marba, N. Masque, P, Duarte, C.M. Sequestration of macroalgal carbon: the elephant in the Blue Carbon room. 2018.

Lovelock, Catherine, Frontiers in Ecology and the Environment. Assessing the risk of carbon dioxide emissions from the blue carbon ecosystem. 15:257-265 (2017)

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