Understanding blue carbon ecosystems with Dr Craig Smeaton

Blue carbon is a commonly used term to describe coastal and marine habitats that store carbon, ranging from the deep seabed to shallow salt marshes. Although touted as highly efficient carbon sinks – removing carbon from the atmosphere at a far higher rate than terrestrial forests¹ – their genuine contribution to limiting climate change might not live up to the hype. Here, Dr Craig Smeaton, lecturer in physical geography at the University of St Andrews, delves into this debate, using data derived from his research to help shed light on the matter.

Diving into the unknown  

Blue carbon ecosystems are ubiquitous, spanning coastlines worldwide and extending into the depths of our oceans and seas. It is widely accepted that these environments store significant amounts of carbon, but how effective are they at sequestering more? Craig weighed in on the discussion, commenting: “Blue carbon habitats certainly have climatic benefits because of their rapid sequestration rates, but how much new carbon they sequester is relatively small compared to other natural stores, such as peatlands and forests. Additionally, the proportion of carbon actually sequestered from the atmosphere remains a big unknown. Unlike forests and peatlands, where carbon is pulled directly from the atmosphere, salt marshes, for example, also accumulate carbon from marine and terrestrial environments, while the ocean floor doesn’t directly sequester carbon at all. In fact, there’s no real atmospheric link; organic carbon from dead plants and animals sink to the ocean floor, where it is stored in the marine sediments.”

Sequester or safeguard?

As the ability of blue carbon ecosystems to sequester new carbon is still a hot topic, perhaps more emphasis should be placed on protecting these environments and the carbon already stored there. Of course, the benefits of developing new coastal habitats aren’t limited to carbon sequestration, but it seems there is a more urgent need to protect the known, rather than create something for a reason that is still subject to debate. Craig continued: “Coastal habitats – including salt marshes, seagrass beds and mangrove swamps – are highly vulnerable to sea level rise and anthropogenic encroachment, risking releasing the carbon stored within, as well as loss of biodiversity and flooding. Knowing how much carbon is stored in these environments therefore gives us more incentive to protect them. For example, we’ve estimated that approximately 5 million tons of carbon is stored in UK salt marshes alone. Before our work, this sort of data was essentially non-existent, but we now have a much better understanding, which can feed into realigning sites – basically, creating new habitats – and, more importantly, protecting existing ones.”

“There has also been increasing concern about the impact of bottom trawling on the seabed, and its potential to release stored carbon and, subsequently, carbon dioxide into the atmosphere. However, I think this is less of a problem than people think, as it depends on the reactivity of the carbon that is released. The seabed in deeper waters is comprised of sediment that has been floating around for hundreds of years, during which time it has broken down and lost its reactivity, and therefore poses a lower risk if disturbed. On the other hand, relatively still and deep bodies of water – such as the Black Sea and the fjords in New Zealand, Alaska, Chile, Canada and Scotland – contain incredibly reactive sediment. These areas act as giant traps, where sediment drops off quickly into deep, hypoxic water and is preserved, inhibiting biogeochemical processes and leaving it highly reactive. If you were to disturb these areas, there is a much bigger risk. Luckily, many of these waters are fairly remote, making them easier to protect and rezone as marine protected areas, which is something that the Scottish government is currently looking to do.”