What is blue carbon?

‘Green carbon’ includes all the carbon that is captured and stored above ground in trees, soil and terrestrial vegetation. ‘Blue carbon’ includes all the carbon that is captured and stored by the vegetation in coastal and marine ecosystems, including mangrove forests, seagrass beds, intertidal salt marshes, wetlands, and soils and sediments beneath this vegetation.

‘Blue carbon’ ecosystems are important net carbon sinks, sequestering atmospheric carbon. Many have expanded upon the basic definition of ‘blue carbon’ to include marine organisms and the ocean itself, including the deep sea, coral reefs, algae, fish and the carbon-capturing great whales, increasing the carbon sequestration potential of the entire marine ecosystem.

How much blue carbon is there?

The most efficient ecosystems for storing carbon are tundra and peatlands, followed by the three main ‘blue carbon’ ecosystems (mangrove forests, seagrass beds, salt marshes), forests (tropical, boreal, temperate) and grasslands (tropical savannah, temperate) (Alongi 2020).

Coastal ecosystems (mangrove forests, seagrass beds and intertidal salt marshes) cover approximately 49 million hectares globally, less than 2% of the total ocean area and account for approximately 50% of the carbon stored in ocean sediment (Blue Carbon Initiative 2019).

If the expanded definition of ‘blue carbon’ (the deep sea, coral reefs, algae, fish and the great whales) is considered, there is more ‘blue carbon’ available to sequester atmospheric carbon.

The International Monetary Fund (IMF) estimates an individual whale alive today sequesters around 33 tons of carbon dioxide on average, removing atmospheric carbon for hundreds of years, and that increasing global whale populations not only protects whales, but positions cetacean carbon sequestration as another tool to fight climate change (Chami et al. 2019).

‘Blue carbon’ and big fish

A recent study (Mariana et al. 2020) looked at the role big fish play in the ocean as potential carbon sinks. It examined global fish catch data to estimate how much ‘blue carbon’ has been extracted and released from the ocean over the last 70 years due to fishing and its fuel use.

Fisheries released more than 0.73 billion metric tons of carbon dioxide into the atmosphere, suggesting the carbon footprint of fisheries may be 25% higher than previously estimated. Leaving more big fish in the sea reduces carbon dioxide released into the atmosphere.

Large predatory fish including sharks, tuna and mackerel are 10 – 15% carbon and when those fish are taken out of the ocean (instead of being left in the ecosystem where they are either eaten by predators or allowed to die naturally, sinking to the ocean floor where they continue to store carbon), the carbon they sequester is released into the atmosphere.

‘Blue carbon’ ecosystems and marine biodiversity

Marine biodiversity is highest nearer to the coastline, in the shallow waters of the ocean ecosystem, where coral reefs (25% of all marine species), mangrove forests, seagrass beds and estuaries are found (the deep-sea ocean floor has high marine biodiversity also).

The pelagic zone extends horizontally from the shoreline (at high tide) and is divided into three main zones – the intertidal or littoral zone, the neritic zone (starting at the edge of the intertidal zone and ending at the continental shelf) and the oceanic zone.

The epipelagic zone extends vertically from the water surface to 200m below the water surface and is known as the photic zone, due to high availability of sunlight; the zone where photosynthesis occurs, and primary production is high.

Biodiversity is higher near the coastline due to the great diversity of primary producers, marine habitats and nursery areas that can be found here. Mangrove forests, seagrass beds and salt marshes are found in the intertidal or littoral zone. Mangrove forests are highly productive ecosystems, provide habitat for a diverse range of species, and nurseries for fish.

If most marine biodiversity occurs near the coastline, and high biodiversity means more robust ecosystems and greater resilience of marine species to withstand impacts (see Marine Food Webs & African Penguins under Impacts), then it makes sense to protect these ‘blue carbon’ ecosystems and the marine organisms that directly and indirectly rely on them.

Unsustainable fishing practices and ‘blue carbon’ ecosystems

Destructive fishing methods including bottom trawling, dynamite fishing, cyanide fishing damage valuable ‘blue carbon’ ecosystems (see Coral Reefs, Seagrass Beds & Mangrove Forests under Impacts) and removal of ‘blue carbon’ ecosystems for aquaculture (see Farmed Fish & Atlantic Salmon under Impacts) releases stored carbon into the atmosphere.

Historical mangrove forest loss was estimated to be around 20% (FAO 2007), with mangrove deforestation occurring at a rate of 2% per year (but the rate of loss has slowed over the last 20 years). Seagrass beds have lost approximately 30% of historical global coverage, declining at a rate of 1.5% per year. Intertidal salt marshes have lost more than 50% of historical global coverage and are declining at a rate of about 1 – 2% per year (Blue Carbon Initiative 2019).

Protecting and restoring ‘blue carbon’ ecosystems

Protecting and restoring global ‘blue carbon’ ecosystems to ensure high biodiversity, marine ecosystem resilience, sustainable fisheries and for carbon sequestration purposes has social, environmental and economic benefits, and is crucial for climate change management.

The Blue Carbon Initiative is leading the way in global ‘blue carbon’ restoration projects, while coastal communities around the world are working to restore their wetland, mangrove and seagrass ecosystems. Get involved in a local project or refuse to purchase seafood from aquaculture systems that have destroyed our ‘blue carbon’.

© 2016 – 2021 Seafood Free September

REFERENCES:

Alongi DM. Global Significance of Mangrove Blue Carbon in Climate Change Mitigation. Sci. 2020; 2(3):67. https://doi.org/10.3390/sci2030067

Ralph Chami, Thomas Cosimano, Connel Fullenkamp, and Sena Oztosun, Nature’s Solution to Climate Change: A strategy to protect whales can limit greenhouse gases and global warming, Finance & Development, December 2019, Volume 56, No. 4, https://www.imf.org/external/pubs/ft/fandd/2019/12/natures-solution-to-climate-change-chami.htm, accessed 06.07.2021

FAO. 2007. The world’s mangroves 1980 – 2005. Rome. 78 pp. https://www.fao.org/3/a1427e/a1427e.pdf

Gaël Mariani, William W.L. Cheung, Arnaud Lyet, Enric Sala, Juan Mayorga, Laure Velez, Steven D. Gaines, Tony DeJean, Marc Troussellier, David Mouillot, Let more big fish sink: Fisheries prevent blue carbon sequestration—half in unprofitable areas, Science Advances 28 Oct 2020: EABB4848, https://doi.org/10.1126/sciadv.abb4848

The Blue Carbon Initiative, https://www.thebluecarboninitiative.org/