Seagrass ecosystem services, threats, and “blue carbon” restoration potential

In near-coastal zones almost anywhere in the world, you will find seagrasses: thin, green plants growing in meadows on the ocean floor. Imagine swimming along, observing these wonderfully unassuming organisms. Despite their somewhat dull appearance, seagrass meadow ecosystems provide important benefits to humans and to the planet. Though these vital ecosystems are under threat globally, conservation and restoration measures could ensure the future provision of their ecosystem services and play an important role in “blue carbon” sequestration. 

Seagrasses are a type of flowering plant, which evolved from terrestrial plants that re-colonized shallow marine ecosystems over 100 million years ago . Found along the coasts of nearly all continents, seagrass meadows are part of greater coastal marine ecosystems widely considered to be among the most productive ecosystems in the world. Complex collections of living and non-living features, seagrass meadows are crucial for ocean health as well as for human well-being. They are also the focus of a growing field known as “marine natural capital.” More often referred to as ecosystem services, these are services provided by nature that enhance human well-being. Seagrasses are an excellent example of this concept. They improve water quality and clarity by trapping sediment and acting as a natural water filter. Their roots stabilize the shoreline and offshore soils while their blades modulate wave action, improving coastline stability by helping to prevent erosion. They improve coastal resilience to floods and storms. As photosynthesizing primary producers, they are part of the backbone of the marine food web and nutrient cycles. Providing essential habitat and nurseries for many marine species, they are crucial for healthy fish populations and for biodiversity more broadly. 

As understanding of seagrass meadow ecology has grown in recent years, yet another ecosystem service has come into focus. Seagrasses have been identified as major carbon sinks, capable of sequestering large quantities of carbon. Plants naturally uptake carbon, using CO2 from the atmosphere to photosynthesize. The idea behind nature-based carbon sequestration is to leverage this natural phenomenon, allowing highly productive photosynthetic ecosystems to uptake carbon and sequester it in plant tissues and sediments. For seagrasses, blue carbon sequestration functions through the capture and storage of carbon from ocean water. Around 2 percent of the carbon captured is sequestered in the blades or leaves, while 98 percent is sequestered more stably in below-ground roots and sediments. Though much of the global interest in nature-based carbon sequestration currently rests on terrestrial ecosystems such as forests, there is growing interest in marine and coastal carbon sinks. Known as vegetated coastal ecosystems (VCEs), they include salt marshes, mangrove forests and seagrass beds. Termed “blue carbon,” VCEs have been shown to exhibit remarkable carbon-sequestering capabilities. Though only making up about 0.2 percent of marine ecosystems, they are responsible for close to 50 percent of the carbon sequestered in marine environments. Globally, mangroves, salt marshes and seagrasses each sequester approximately 10 petagrams of carbon in the surface meter of soil. Though somewhat variable due to local environmental factors, this is equivalent to over ten billion tons of carbon.  

Despite the myriad services they provide, seagrass meadow ecosystems are under threat worldwide. Primarily human-caused, stressors include coastal development, dredging, overfishing, introduction of invasive species, soil degradation, and agricultural, industrial and wastewater runoff. Additionally, changes in abiotic environmental factors due to climate change, such as water temperature, weather and rising sea levels are significant stressors to seagrass ecosystems.  It is estimated that seagrass beds are declining at a rate of about 7 percent per year globally. If left unchecked, the destruction and death of seagrass meadows can turn these vibrant ecosystems from carbon-sequestering sinks to carbon-releasing sources.  

To design effective conservation and restoration plans, it is essential to have an understanding of the current – and potential – amount of carbon sequestered in a specific ecosystem. Valuing seagrass ecosystems and their ecosystem services is an important step in conserving and restoring these ecosystems for their blue carbon potential. An emerging field, environmental accounting is the monetary valuation of ecosystems and their services. It serves as an important tool used by environmental managers and policy makers. Done in both terrestrial and marine ecosystems, environmental accounting evaluates the benefits produced by an ecosystem and the costs of losing its functions. It also often takes into account human exploitation of those resources. A recent study led by Dr. Elvira Buonocore of Parthenope University of Naples quantifies the marine natural capital stocks inside Punta Campanella, an approximately 5.8-square-mile Marine Protected Area in the Mediterranean. Researchers found the highest eco-exergy density value, or stored genetic information and chemical energy potential, in seagrass beds of Posidonia oceanica. They estimated the economic value of the seagrasses within this MPA study unit to be 3.05 million euros (3.44 million USD at time of writing). For such a small MPA, this is an impressive sum; it is easy to see why blue carbon has garnered so much excitement and traction in global scientific and climate policy communities. 

Though undoubtedly an exciting frontier in science and climate policy, there are challenges to widespread adoption of blue carbon accounting methods. Looking at estimations of carbon sequestration across sites, a recent study led by Dr. Inés Mazarrasa of the Instituto de Hidráulica Ambiental de la Universidad de Cantabria found that many site-specific factors influenced the carbon sequestration capabilities of seagrasses. These included species composition, water depth, degree of exposure to the force of waves, amount of carbon entering the ecosystem, and soil composition. The study looked at whether similar environmental characteristics – termed by the authors as “bioregional and geomorphic settings”– could be good predictors of soil carbon stocks. Some trends did emerge, but were for the most part seen only at a local level. These results suggest that computed means based on similar regions would not be good predictors of soil carbon content. Such a finding presents a challenge to blue carbon accounting: at this time, it would be necessary to conduct site-specific estimates using local environmental factors to properly assess carbon stocks. Based on this research, further work is needed to continue to refine blue carbon accounting. 

While blue carbon accounting may face challenges, if a policymaker’s ultimate goal is to offset atmospheric carbon and ensure future ecosystem services, then it is vital to prioritize seagrass protection and restoration in conservation plans. The silver lining is that unlike coral and other fragile coastal ecosystems, seagrasses are resilient – invasive when they first colonized marine ecosystems millions of years ago, these humble plants know how to survive in the face of a changing climate. In increasingly unstable marine ecosystems, resiliency is essential. At this moment in time, seagrasses need humans for restoration and protection from degradation; in the near future, however, it is likely humans will need seagrasses for the ecosystem and carbon sequestering services they provide.