Caribbean seagrasses provide services worth $255B annually, including vast carbon storage
Discussions of valuable but threatened ocean ecosystems often focus on coral reefs or coastal mangrove forests. Seagrass meadows get a lot less attention, even though they provide wide-ranging services to society and store lots of climate-warming carbon. But the findings of a new University of Michigan-led study show that seagrass ecosystems deserve to be at the forefront of the global conservation agenda, according to the authors. It’s the first study to put a dollar value on the many services—from storm protection to fish habitat to carbon storage—provided by seagrasses across the Caribbean, and the numbers are impressive. Using newly available satellite data, the researchers estimate that the Caribbean holds up to half the world’s seagrass meadows by surface area, and it contains about one-third of the carbon stored in seagrasses worldwide. They calculated that Caribbean seagrasses provide about $255 billion in services to society annually, including $88.3 billion in carbon storage. In the Bahamas alone, the ecosystem services provided by seagrasses are valued at more than 15 times the country’s 2020 gross domestic product, according to the study published online June 21 in the journal Biology Letters. “Our study is the first to show that seagrass beds in the Caribbean are of global importance in their areal extent, in the amount of carbon they store, and in the value of the economic services they provide to society,” said study lead author Bridget Shayka, a doctoral student in the U-M Department of Ecology and Evolutionary Biology. “The findings underscore the importance of conserving and protecting these highly threatened and globally important ecosystems, which are critical allies in the fight against climate change.” One way to prioritize seagrass conservation would be to include those verdant undersea meadows in global carbon markets through projects that minimize loss, increase areal extent or restore degraded beds. The idea of selling “blue carbon” offset credits, which monetize carbon stored in coastal and marine ecosystems, is gaining traction for several reasons. For one, many island nations that have already been impacted by climate change—through increasingly intense hurricanes or rising sea levels, for example—have large areas of valuable coastal ecosystems that store carbon and that provide other services to society. Blue carbon (the name refers specifically to carbon stored in coastal and open-ocean ecosystems while “green carbon” refers more broadly to carbon stored in all natural ecosystems) offset credits could be a way for wealthier countries to compensate for their contribution to human-caused climate change while at the same time benefiting the economies of impacted countries and helping to conserve coastal ecosystems, which are among the most impaired in the world. Threats to seagrass meadows include coastal development, chemical pollution, recreation, shipping and climate change. “Because seagrass ecosystems are both highly important for carbon storage and sequestration, and are highly degraded globally, they represent an important burgeoning market for blue carbon,” said marine ecologist and study senior author Jacob Allgeier, an associate professor in the U-M Department of Ecology and Evolutionary Biology. “Yet, to date, a fundamental impediment to both evaluating seagrass and promoting it in the blue carbon market has been the lack of thorough seagrass distribution data.” For their study, the U-M-led team used newly available seagrass distribution data collected by the PlanetScope constellation of small DOVE satellites. They classified Caribbean seagrass ecosystems as either sparse or dense and estimated the amount of carbon in plants and sediments using data from Thalassia testudinum, the dominant seagrass species in the region. The researchers then calculated a conservative economic value for the total ecosystem services provided by seagrasses in the Caribbean and for the stored carbon, using previously published estimates for the value of services including food production, nursery habitat for fishes and invertebrates, recreation and carbon storage. Grouper, queen conch and lobster are among the commercially harvested animals that rely on Caribbean seagrass. Green sea turtles, tiger sharks and manatees also depend on it. To estimate the dollar value of the carbon stored in Caribbean seagrass beds, the researchers used $18 per metric ton of carbon dioxide equivalents, borrowed from California’s cap and trade program. In addition to Caribbean-wide estimates, the researchers calculated values for individual countries in the region: The Bahamas has the largest share of Caribbean seagrass (61%), providing total ecosystem services valued at $156 billion annually, including $54 billion in carbon storage. Cuba ranks second in areal seagrass coverage (33% of the Caribbean total), with a value of $84.6 billion per year for all ecosystem services, including $29.3 billion for carbon storage. The dollar value of the carbon in seagrasses around Cuba is equivalent to 27% of the country’s 2020 GDP. “Importantly, the degradation of seagrass beds often leads to erosion and sediment resuspension, which can create a positive feedback of increased seagrass loss and the release of C stored in sediments,” the authors wrote. “Blue carbon finance thus represents a potential mechanism by which the global community can invest in conserving and protecting these vital ecosystems.” More than 60 species of seagrasses grow in shallow coastal waters around the world. They evolved from land plants that recolonized the oceans 70 to 100 million years ago. In a separate paper accepted for publication in the journal Proceedings of the Royal Society, Allgeier and colleagues show that the construction of artificial reefs in the Caribbean can help protect seagrass ecosystems from human impacts, including nutrient pollution and overfishing. Seagrasses use photosynthesis to pull carbon dioxide from the atmosphere, then store the carbon in plant tissues. The seagrasses are quickly inundated by sediments, slowing decomposition. As a result, more than 90% of the carbon stored in seagrass beds is in the top meter of sediment. Caribbean seagrasses and associated sediments store an estimated 1.3 billion metric tons of carbon, according to the new study. That’s a big number, but it’s just 1.09% of the carbon contained in above- and below-ground woody biomass in the Amazon, and just 1.12% of the carbon in the biomass and soils of the world’s temperate forests, according to the new
What is blue carbon?
By Jenny Black The Carbon Cycle It is likely that we have all heard of carbon but what might not be known is the fundamental role it plays in our ecosystems. Carbon will naturally move between the atmosphere and the earth’s surface through interactions between organisms and ecosystems in a process known as the ‘carbon cycle’. In the carbon cycle, carbon can be a gas in the form of carbon dioxide (CO₂), or solid within organisms and rocks. On land, in what is known as terrestrial ecosystems, CO₂ can be captured, or sequestered, from the atmosphere and stored within plants in the process called photosynthesis. Here, plants use CO₂ and sunlight to create food for the growth of new plant tissue. This is a very basic description of the carbon cycle but what is not mentioned is the role of the ocean and marine ecosystems. Carbon exists in the sea in multiple forms (as seen in Figure 1). It can be dissolved in the water itself, stored by organisms in their bodies, buried in soil, or stored in rocks (Barnes, 2020). Carbon will move throughout these different stages and will be stored for different lengths of time depending on the stage. For example, carbon stored in plants and animals will be stored for the duration of that organism’s life. After it dies, some of the carbon will be released back into the atmosphere as CO₂ as bacteria breaks it down, and some will be buried within the sediment. Carbon that is buried within the sediment can be trapped and stored for much longer periods of time. In some cases, carbon stored in the plant tissue of rainforests can be retained for decades or centuries, but carbon stored within sediment can last for millennia (Nellemann et al., 2009). Figure 1: the oceans role in the carbon cycle. What is Blue Carbon? Whilst all marine life has a part to play in the carbon cycle, there are certain ecosystems within our oceans that act as major carbon stores, or carbon sinks, capable of capturing carbon in numerous types of forms. Think of it as the oceans equivalent to terrestrial rainforests that capture carbon and store it within their plant tissue and soils (Nellemann et al., 2009). The main blue carbon ecosystems include seagrass meadows, kelp forests and mangrove forests (Macreadie et al., 2019). Carbon that is captured and stored is called blue carbon (Nellemann et al., 2009). Figure 2 shows the three main blue carbon environments mentioned above. Figure 2: three key blue carbon ecosystems; Mangroves, Seagrass Meadows, and Kelp Forests (Nellemann et al., 2009). How is Blue Carbon captured and stored? There are two forms of carbon, organic and inorganic carbon. Inorganic carbon can be found in the environment as CO₂. The storage of atmospheric CO₂ in the oceans is complicated due to the barrier of water between the atmosphere and the blue carbon stores below. Because of this, CO₂ must dissolve into seawater to reach the blue carbon ecosystems below. After dissolving, CO₂ can be used by plants for photosynthesis, or it is mineralised into a hard carbonate by the likes of coral reefs and shell building organisms. Organic carbon on the other hand is found within the tissue of living organisms. Like terrestrial plants, some blue carbon ecosystems such as seagrass meadows can photosynthesise and store carbon within their plant tissue. What is unique about blue carbon ecosystems is that they can store both organic and inorganic carbon that they have sequestered and collected themselves, as well as storing carbon that has been sequestered by other terrestrial and marine ecosystems. Tidal and fluvial systems bring carbon particles from other marine and terrestrial suspended within its waters. Blue carbon ecosystems provide resistance to the flowing particles in the water column. They slow the speed of the water and capture the particles, encouraging them to be deposited into the sediment below. Dense root systems in the sediment provide protection preventing particles from being re-suspended. Over time, the carbon particles build up with old particles being buried below influxes of fresh material burying and protecting the carbon. This unique characteristic is the reason why blue carbon ecosystems can store ten times more carbon by burial than terrestrial ecosystems (McLeod et al., 2011). Figure 3 uses the example of a seagrass meadow of how different sources of carbon particles can be caught and stored within its sediment. Figure 3: seagrass and its role in the blue carbon cycle. Vital Ecosystems Blue carbon ecosystems offer many benefits to our oceans and coastlines including environmental protection, promotion of high biodiversity as well as carbon sequestration and storage. With governments and multi-national corporations acknowledging blue carbon ecosystems as the climate-combating solutions that they are, it is exciting to see blue carbon being given the recognition that it deserves. What is important for the future is continued protection and restoration of these environments. If we help to protect them, they will help us in our fight against climate change. Jenny is a masters student studying Aquaculture, Environment and Society in association with the University of the Highlands and Islands, University of Crete, Université de Nantes, and Radboud University. Her interest in blue carbon began with her Geology BSc dissertation where she investigated carbon storage within the soil of Scottish seagrass meadows. Since then, she has been focusing on the potential of blue carbon ecosystems and how they can be involved in carbon storage, environmental restoration, and food supply. References Barnes, D., (2020). What Is Blue Carbon and Why Is It Important?. Front. Young Minds. 8:154. doi: 10.3389/frym.2019.00154 Kennedy, H., J. Beggins, C. M. Duarte, J. W. Fourqurean, M. Holmer, N. Marb a, J. J. Middelburg. (2010). Seagrass sediments as a global carbon sink: Isotopic constraints. Global Biogeochem. Cy. 24: GB4026. doi:10.1029/2010GB003848 Macreadie, P.I., Anton, A., Raven, J.A. et al. (2019). The future of Blue Carbon science. Nat Commun 10, 3998 https://doi.org/10.1038/s41467-019-11693-w McLeod, E. et al., (2011). A blueprint for blue carbon: toward an improved understanding