Blue carbon

Blue carbon is the carbon captured by the world's coastal ocean ecosystems, mostly mangroves, salt marshes, seagrasses and potentially macroalgae. Blue carbon is the carbon captured by the world's coastal ocean ecosystems, mostly mangroves, salt marshes, seagrasses and potentially macroalgae. Historically the ocean and terrestrial forest ecosystems have been the major natural carbon (C) sinks. New research on the role of vegetated coastal ecosystems has highlighted their potential as highly efficient C sinks, and led to the scientific recognition of the term 'Blue Carbon'. 'Blue Carbon' designates carbon that is fixed via coastal ocean ecosystems, rather than traditional land ecosystems, like forests. Although the ocean’s vegetated habitats cover less than 0.5% of the seabed, they are responsible for more than 50%, and potentially up to 70%, of all carbon storage in ocean sediments. Mangroves, Salt marshes and seagrasses make up the majority of the ocean’s vegetated habitats but only equal 0.05% of the plant biomass on land. Despite their small footprint, they can store a comparable amount of carbon per year and are highly efficient carbon sinks. Seagrasses, mangroves and salt marshes can capture carbon dioxide (CO2) from the atmosphere by sequestering the C in their underlying sediments, in underground and below-ground biomass, and in dead biomass. In plant biomass such as leaves, stems, branches or roots, blue carbon can be sequestered for years to decades, and for thousands to millions of years in underlying plant sediments. Current estimates of long-term blue carbon C burial capacity are variable, and research is ongoing. Although vegetated coastal ecosystems cover less area and have less aboveground biomass than terrestrial plants they have the potential to impact longterm C sequestration, particularly in sediment sinks. One of the main concerns with Blue Carbon is the rate of loss of these important marine ecosystems is much higher than any other ecosystem on the planet, even compared to rainforests. Current estimates suggest a loss of 2-7% per year, which is not only lost carbon sequestration, but also lost habitat that is important for managing climate, coastal protection, and health. Seagrass are a group of about 60 angiosperm species that have adapted to an aquatic life, and can grow in meadows along the shores of all continents except Antarctica. Seagrass meadows form in maximum depths of up to 50m, depending on water quality and light availability, and can include up to 12 different species in one meadow. These seagrass meadows are highly productive habitats that provide many ecosystem services, including sediment stabilization, habitat and biodiversity, better water quality, and carbon and nutrient sequestration. The current documented seagrass area is 177,000 km2, but is thought to underestimate the total area since many areas with large seagrass meadows have not been thoroughly documented. Most common estimates are 300,000 to 600,000 km2, with up to 4,320,000 km2 suitable seagrass habitat worldwide. Although seagrass makes up only 0.1% of the area of the ocean floor, it accounts for approximately 10-18% of the total oceanic carbon burial. Currently global seagrass meadows are estimated to store as much as 19.9 Pg (gigaton, or billion tons) of organic carbon. Carbon primarily accumulates in marine sediments, which are anoxic and thus continually preserve organic carbon from decadal-millennial time scales. High accumulation rates, low oxygen, low sediment conductivity and slower microbial decomposition rates all encourage carbon burial and carbon accumulation in these coastal sediments. Compared to terrestrial habitats that lose carbon stocks as CO2 during decomposition or by disturbances like fires or deforestation, marine carbon sinks can retain C for much longer time periods. Carbon sequestration rates in seagrass meadows vary depending on the species, characteristics of the sediment, and depth of the habitats, but on average the carbon burial rate is approximately 138 g C m−2 yr−1. Seagrass habitats are threatened by coastal eutrophication, increased seawater temperatures, increased sedimentation and coastal development, and sea-level rise which may decrease light availability for photosynthesis. Seagrass loss has accelerated over the past few decades, from 0.9% per year prior to 1940 to 7% per year in 1990, with about 1/3 of global loss since WWII. Scientists encourage protection and continued research of these ecosystems for organic carbon storage, valuable habitat and other ecosystem services. Mangroves are woody halophytes that form intertidal forests and provide many important ecosystem services including coastal protection, nursery grounds for coastal fish and crustaceans, forest products, recreation, nutrient filtration and carbon sequestration. Mangroves are located in 105 countries, as well as in the special administrative areas of China (Hong Kong and Macau), the four French overseas provinces of Martinique, Guiana, Guadeloupe and Mayotte and the contested area of Somaliland. They grow along coastlines in subtropical and tropical waters, depending mainly on temperature, but also vary with precipitation, tides, waves and water flow. Because they grow at the intersection between land and sea, they have semi-terrestrial and marine components, including unique adaptations including aerial roots, viviparous embryos, and highly efficient nutrient retention mechanisms. Globally, mangroves stored 4.19 ± 0.62 Pg (CI 95%) of carbon in 2012, with Indonesia, Brazil, Malaysia and Papua New Guinea accounting for more than 50% of the global stock. 2.96 ± 0.53 Pg of the global carbon stock is contained within the soil and 1.23 ± 0.06 Pg in the living biomass. Of this 1.23 Pg, approximately 0.41 ± 0.02 Pg is in the belowground biomass in the root system and approximately 0.82 ± 0.04 Pg is in the aboveground living biomass . Global mangrove canopy cover is estimated as between 83,495 km2 and 167,387 km2 in 2012 with Indonesia contaiingn approximately 30% of the entire global mangrove forest area. Mangrove forests are responsible for approximately 10% of global carbon burial, with an estimated carbon burial rate of 174 g C m−2 yr−1. Mangroves, like seagrasses, have potential for high levels of carbon sequestration. They account for 3% of the global carbon sequestration by tropical forests and 14% of the global coastal ocean's carbon burial. Mangroves are naturally disturbed by floods, tsunamis, coastal storms like cyclones and hurricanes, lightning, disease and pests, and changes in water quality or temperature. Although they are resilient to many of these natural disturbances, they are highly susceptible to human impacts including urban development, aquaculture, mining, and overexploitation of shellfish, crustaceans, fish and timber. Mangroves provide globally important ecosystem services and carbon sequestration and are thus an important habitat to conserve and repair when possible. Marshes, intertidal ecosystems dominated by herbaceous vegetation, can be found globally on coastlines from the arctic to the subtropics. In the tropics, marshes are replaced by mangroves as the dominant coastal vegetation. Marshes have high productivity, with a large portion of primary production in belowground biomass. This belowground biomass can form deposits up to 8m deep. Marshes provide valuable habitat for plants, birds, and juvenile fish, protect coastal habitat from storm surge and flooding, and can reduce nutrient loading to coastal waters. Similarly to mangrove and seagrass habitats, marshes also serve as important carbon sinks. Marshes sequester C in underground biomass due to high rates of organic sedimentation and anaerobic-dominated decomposition. Salt marshes cover approximately 22,000 to 400,000 km2 globally, with an estimated carbon burial rate of 210 g C m−2 yr−1. Tidal marshes have been impacted by humans for centuries, including modification for grazing, haymaking, reclamation for agriculture, development and ports, evaporation ponds for salt production, modification for aquaculture, insect control, tidal power and flood protection. Marshes are also susceptible to pollution from oil, industrial chemicals, and most commonly, eutrophication. Introduced species, sea-level rise, river damming and decreased sedimentation are additional longterm changes that affect marsh habitat, and in turn, may affect carbon sequestration potential. Both macroalgae and microalgae are being investigated as possible means of carbon sequestration. Because algae lack the complex lignin associated with terrestrial plants, the carbon in algae is released into the atmosphere more rapidly than carbon captured on land. Algae have been proposed as a short-term storage pool of carbon that can be used as a feedstock for the production of various biogenic fuels. Microalgae are often put forth as a potential feedstock for carbon-neutral biodiesel and biomethane production due to their high lipid content. Macroalgae, on the other hand, do not have high lipid content and have limited potential as biodiesel feedstock, although they can still be used as feedstock for other biofuel generation. Macroalgae have also been investigated as a feedstock for the production of biochar. The biochar produced from macroalgae is higher in agriculturally important nutrients than biochar produced from terrestrial sources. Another novel approach to carbon capture which utilizes algae is the Bicarbonate-based Integrated Carbon Capture and Algae Production Systems (BICCAPS) developed by a collaboration between Washington State University in the United States and Dalian Ocean University in China. Many cyanobacteria, microalgae, and macroalgae species can utilize carbonate as a carbon source for photosynthesis. In the BICCAPS, alkaliphilic microalgae utilize carbon captured from flue gases in the form of bicarbonate. In South Korea, macroalgae have been utilized as part of a climate change mitigation program. The country has established the Coastal CO2 Removal Belt (CCRB) which is composed of artificial and natural ecosystems. The goal is to capture carbon using large areas of kelp forest. Restoration of mangrove forests, seagrass meadows, marshes, and kelp forests has been implemented in many countries. These restored ecosystems have the potential to act as carbon sinks. Restored seagrass meadows were found to start sequestering carbon in sediment within about four years. This was the time needed for the meadow to reach sufficient shoot density to cause sediment deposition. Similarly, mangrove plantations in China showed higher sedimentation rates than barren land and lower sedimentation rates than established mangrove forests. This pattern in sedimentation rate is thought to be a function of the plantation’s young age and lower vegetation density.