“Blue Carbon” is the scientifically recognized term defining carbon stored by coastal ecological systems. These systems of seagrasses, mangroves, salt marshes, and seaweed, cover less than 0.5% of the seabed, are equal in size to about 0.05% of the biomass on land, and are responsible for over 50% of all carbon storage in ocean sediment. Through photosynthesis, carbon is captured in the plants and roots as the plants grow, ending up in the sediment where the carbon is stored (sequestered) for up to millions of years. One acre of seagrass can remove the carbon emitted from 4,000 miles of car exhaust each year.
There are about 72 species of flowering plants collectively called seagrasses that evolved from sea algae, moved onto land, and then, about 100 million years ago, transitioned back to the sea. Forming beautiful water-based meadows along coastal floodplains and in water up to 150’ deep, seagrass is a critical food source and habitat for wildlife, supporting a diverse community of fish, snails, sea turtles, crabs, shrimp, oysters, clams, squid, sea urchins, sponges, and anemones.
Seagrasses have been called “the lungs of the sea”, capturing and storing large amounts of carbon and releasing oxygen into the water as they build their leaves and roots through the process of photosynthesis—similar to how trees take carbon from the air to build their trunks. As parts of the seagrass die and decay, it collects on the seafloor and becomes buried and trapped in the sediment. The sediment forms sedimentary rock under the effects of time and pressure, and through actions of Earth’s tectonic plates, becomes buried in the upper crust; the carbon is thus effectively sequestered for decades and up to millions of years, resurfacing as volcanic spew (lava) and mountain outcrop eruption (magma).
But Blue Carbon ecosystems are being lost by 2-7%/year—a higher rate than even the rainforests. This loss adds to deadly atmospheric carbon by removing seagrasses’ carbon sequestration actions, and also reduces habitat that is vital for managing the health and viabilities of our climate, our coasts, our health, and a multitude of plants & animals.
Seagrass habitats are threatened by many anthropogenic (human) activities. Paved surfaces cause heavy runoffs of dirty, unfiltered water from storms and waste-waters. Increased fertilizer use from farms and housing developments, nitrous oxides from various fossil fuel burning activities—auto, factory, electricity generation, home heating—all cause excess nutrient accumulations flowing to the seas. These activities encourage algae blooms like red tide which deplete oxygen, cloud the sunlight, increase the water’s temperature, and release deadly toxins which kill animals, people, and the seagrass meadow.
Dragging weighted fishing nets over the meadows uproot the plants. Global Warming causes sea levels to rise and ocean acidification, reducing light infiltration, hindering photosynthesis thus causing a decrease in plant growth, health, and plant biodiversity. ( A “wasting disease” in the early 1930’s caused a large die-off of up to 90 percent of a seagrass species called eelgrass growing in temperate North America, causing an extinction of a snail species. ).
Another cause of seagrass depletion is the reduction of predatory actions upon the herbivore community, allowing these grass-eating animals unfettered access to seagrasses. This situation is mostly caused by over-fishing of the herbivore-eaters. For example, Chesapeake Bay Blue Crabs eat grazing snails. Over-harvesting the crabs allows the snails to flourish, destroying the seagrasses.
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 a 1/3 global loss since WWII. An increase in awareness and protections are required to eliminate these losses and to ensure the health and survival of not only these magnificent habitats, but of us too.
A global assessment, done by the National Academy of Sciences, involving 215 studies found that seagrass habitat has been disappearing at an increasing rate since 1940; 29% has disappeared since seagrass data began in 1879, with 25-50% in the past 55 years. Seagrass loss rates are comparable to those reported for mangroves and coral reefs, and place seagrass meadows among the most threatened ecosystems on earth.
So, vegetative coastal ecological systems are emerging as the most carbon-rich ecosystems in the world, and one of the most effective methods for long-term carbon storage: they bury carbon 35 times faster than tropical forests and contribute 50% of the total carbon buried in ocean sediments. Because of their remarkable speed and effectiveness to sequester carbon for millions of years, Blue Carbon storage should be a key strategy to combat Global Warming.
https://www.thebluecarboninitiative.org/library#Mangroves; https://www.pnas.org/content/106/30/12377; https://mousamwaylandtrustmaine.files.wordpress.com/2019/12/44722-atwood-et-al.-2015.pdf; https://www.fws.gov/verobeach/MSRPPDFs/Seagrass.pdf; https://ocean.si.edu/ocean-life/plants-algae/seagrass-and-seagrass-beds#element_37; https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4633871/