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Into the Blue: The Importance of Blue Carbon

We had the chance to interview Sophie Meyjes, a PhD student at the University of Cambridge, who is working on the ocean carbon cycle. Her work focuses specifically on better understanding plankton grazing and the impact of fish and fisheries on the oceanic carbon pump.

Sophie answers our questions to help us learn more about blue carbon, the ocean carbon cycle, its importance in fighting the climate emergency and why it should matter to businesses.

What is blue carbon?

Blue carbon refers to the carbon captured by the world’s coastal and ocean ecosystems such as seagrasses, salt marshes, mangroves and phytoplankton. At first coastal ecosystems were predominantly studied under the umbrella term blue carbon but in recent years this field has incorporated the study of open ocean ecosystems and the role they play in sequestering carbon.

The ocean is a major carbon sink, sequestering up to 20 – 30 % of global anthropogenic carbon emissions to date. Given this carbon sequestration capacity, it is key that we get a better understanding of blue carbon and how our oceans can help us mitigate climate change.

How do coastal and ocean ecosystems sequester carbon?

The process by which marine plants absorb and store carbon is similar to that of terrestrial plants and vegetation. However, those found in some coastal ecosystems can sequester carbon much more quickly and efficiently. This is due to their high metabolic rates, fast ability to filter carbon from water, and a large proportion of their biomass allocated to their roots, instead of the plant ‘body’. This produces excess carbohydrates that get efficiently stored in the ground where they can be sequestered.

The root structure of coastal plants and vegetation is also in sediment, which is continually being added to by incoming tides – therefore it never gets fully saturated by carbon, unlike terrestrial soils. This means these coastal ecosystems can store layers upon layers of carbon for thousands of years, whereas a rainforest sequesters carbon for decades or hundreds of years.

However, for carbon to be stored, and considered ‘sequestered’, this coastal vegetation has to be undisturbed. Its destruction will release the carbon they have stored in the sediments, back into the atmosphere. This is a real danger if we do not protect these already-fragile and climate-stressed ecosystems.

What is the ocean carbon cycle?

The ocean carbon cycle, or the oceanic carbon pump, refers to how carbon is stored deep into our ocean floor. It is a biological process, with carbon passing through living organisms and plants, that will move the carbon from the surface water down into the ocean floor where a small fraction can be sequestered. In surface water (where sunlight penetrates), phytoplankton will photosynthesize carbon from the water. They are then either:

  • Eaten: Phytoplankton are at the bottom of the food web and get eaten by a variety of species. When they do, the carbon they have photosynthesized is absorbed by the other species that ate them. And then that specie’s poo (or fecal pellets if you are feeling very scientific) will sink to the bottom of the ocean, storing carbon and eventually sequestering it in the ocean floor.

  • Survive and eventually die: When they die, the phytoplankton carcasses sink to the bottom of the ocean where eventually a small fraction is sequestered in the ocean floor bed. This also happens when other species die, from fishes to whales.

Once carbon reaches depths of over 1000m, it can stay sequestered in the ocean for hundreds of years. It is also worth noting that this cycle is not static since animals are not stationary. As they move up and down in the ocean depths they will be moving, transporting and disturbing the carbon cycle. This can sometimes lead to carbon sinking faster.

How are humans disrupting this ocean carbon cycle?

We disturb it in various ways. Firstly, humans are part of the food web. We consume the fish that have eaten the phytoplankton. When we do, the carbon they have stored can no longer be sequestered at the bottom of our ocean. It is actually released by humans as atmospheric carbon when the fish are landed, processed and consumed. Mariani et al., published a study in 2020 looking into this. They tried to quantify the carbon footprint of fishing and determine the impact of fishing on the biological carbon pump.

Secondly, through fishing (and especially over-fishing or destructive fishing practices) we are disturbing the food web in the ocean and therefore its ability to store and sequester carbon. For example, trawling, a fishing method that involves pulling a fishing net through the water behind one or more boats, impacts the sea bed and can disturb the carbon stored in it.

And finally, models have been developed predicting the ocean carbon pump could weaken due to warmer, more acidic oceans, as a result of increased anthropogenic carbon dioxide. With the warming and acidification of our oceans the biological composition and diversity of our oceans are changing. New species will thrive whereas others will go extinct. The species that are predicted to survive are unfortunately not as effective at sinking carbon. Additionally, the number of fish and animals in our oceans impact the amount of phytoplankton which again could affect the ability of our ocean to sequester carbon.

But there is still a lot we need to learn about our oceans and the carbon pump. After all, we do not actually know the total number of fish in our oceans, We know more about space than the blue depths! It is why there is such renewed interest in better understanding our oceans and their role in carbon sequestration and climate mitigation.

What solutions are currently being explored to enhance the ocean carbon pump?

Since we still don’t know so much about our oceans, researchers are trying to better understand the ocean carbon pump including how to enhance it and protect it. For example, the Centre for Climate Repair at the University of Cambridge is looking at nature-based solutions to remove greenhouse gases. They are looking specifically at how growing macroalgae, such as kelp, in our oceans and coastal areas, could help with carbon drawdown and climate mitigation. Recent IPCC reports also recognise that carbon removal solutions, such as enhancing the ocean’s ability to sequester carbon, are likely needed to achieve net zero emissions.

There is also a real need to conserve our oceans and protect coastal habitats that contribute to carbon sequestration. We need to be more active in protecting these habitats, not just for their carbon sequestration abilities, but because entire communities and people’s livelihoods depend on them.

There is also interest in geoengineering solutions to improve carbon storage and capture. It is very likely we will overshoot our climate targets and carbon mitigation will not be sufficient, hence why these carbon capture and storage solutions are being developed and tested. They are obviously not sufficient and we should continue to reduce emissions as much as we can. But they will play a role in addressing the climate crisis.

Why should business care about blue carbon?

Obviously, any business involved in the blue economy and specifically the fishing industry should care about their impact on the ocean and blue carbon. They will also have an impact on the carbon pump. With more businesses being required to disclose their carbon footprint, they may need to start considering blue carbon as part of their emissions.

But any business operating in coastal areas should also care about blue carbon and start assessing their impact, take active steps to protect these ecosystems and limit their disruptions of the ocean carbon cycle. They may already depend on the ecosystem services our ocean and coastal ecosystems provide or will face challenges and risks to their operations if these ecosystems are degraded and destroyed.

We have also not yet grasped the full impact of fishing on our oceans. We need more research and data to determine how we will fish going forward and to inform policies that help our ocean, economies and people. We are all part of the blue carbon pump and our oceans play an important role regardless of their carbon sequestration abilities. Our coastal and ocean ecosystems regulate our temperature, protect us against flooding, support our economies and form part of entire communities’ livelihoods.


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