- The more than 70 known species of seagrass play a key role in helping mitigate climate change by sequestering large amounts of carbon, as well as providing habitats for fish and invertebrates.
- But their importance in maintaining healthy oceans has long been overlooked, with seagrasses threatened by anthropogenic activities such as destructive fishing and water pollution, often to the extent that they can’t self-recover.
- Several initiatives around the world are trying to restore seagrass meadows, including an ongoing project off the coast of Wales in the U.K., and a successful one around the coastal bays of Virginia.
- Seagrasses are important nurseries for a rich diversity of marine life, including commercially important fish, which means their loss threatens the food security of many communities that depend on these fisheries.
A group of researchers is preparing to dive into the shallow waters of Dale Bay in Wales. They’re on a mission to monitor the restoration of the local species of seagrass, Zostera marina, also known as eelgrass, as part of a program spearheaded in 2014 by Project Seagrass, a U.K.-based charity that works to restore seagrass meadows and turn research into conservation projects worldwide.
“We are essentially doing underwater gardening,” says Richard Lilley, director and co-founder of Project Seagrass.
Seagrass meadows are ecosystems of flowerings plant that inhabit shallow waters from the tropics to the Arctic Circle, with the exception of Antarctica. With more than 70 known species of seagrass around the world, these meadows cover more than 300,000 square kilometers (116,000 square miles) and have been found in the waters of 159 countries.
But seagrass meadows are also under threat. Recently published research shows that at least 44% of the U.K.’s seagrasses have disappeared since 1936, and 39% since the 1980s. The situation might be much worse: it’s estimated that as much as 92% of U.K. seagrass ecosystems are degraded.
“When there is so little of it left, it can’t reach the threshold it needs to begin to self-recover,” Lilley says.
Like plants on land, the seagrass around the U.K. starts growing in the spring. And then in the summer, it starts producing its seed. To collect the seeds needed for the restoration project in Dale Bay, divers swam through some of the remaining seagrass meadows in other areas of the U.K., carefully snapped the seed pods off, and brought them back to the lab.
“We ended up with half a million seeds,” Lilley says.
Seeds were then scooped into hessian bags, placed on a rope 1 meter (3 feet) apart from each other, and laid on the seafloor in Dale. From that little cluster, the seagrass will expand to fill up each meter over the next seven to 10 years.
“By putting the seagrass there, you can completely change the ecosystem,” Lilley says.
According to the Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services, seagrass meadows around the world decreased by more than 10% every decade between 1970 and 2000. Several factors have contributed to its decline. Pollution from agricultural runoff and destructive coastal development can destroy seagrass connectivity. Moreover, chain moorings, dredging and destructive fishing directly damage seagrass meadows and destroy the communities of grazing animals living in these environments. Extreme weather events, rising temperatures and the acidification of the ocean, caused by human-driven carbon dioxide emissions, also endanger these habitats.
Underwater prairies under threat
One of the first efforts to restore seagrass started in Virginia in 1997, when surprised residents found patches of seagrass growing in the Atlantic coastal bays. For years, seagrass had never been spotted there. By 1933, in fact, residents believed the seagrass of these coastal bays had disappeared completely, wiped away by a mold slime disease and a hurricane. But somehow it returned.
Robert J. Orth, a professor at the Virginia Institute of Marine Science at the College of William and Mary, had been working since the 1970s in nearby Chesapeake Bay, where seagrass still survived. Together with researchers from the University of Virginia, Orth started planting back eelgrass seeds from Chesapeake Bay into the smaller bays.
“Every year from 2001 through today, we would harvest the seeds from Chesapeake Bay, and then let nature do the rest,” Orth says.
Researchers tossed the Chesapeake seeds from boats in the waters of South, Cobb, Spider Crab, and Hog Island bays.
“In 2004 we started seeing the actual spread,” Orth says. “Over the years probably we placed well over 70 million seeds.”
The program seeded 200 hectares (500 acres). Then, seagrass started spreading on its own. Now, some 4,000 hectares (nearly 10,000 acres) of seagrass sway underwater.
With the seagrass, the marine ecosystem flourished. Anchovies (Anchoa mitchilli) and silversides (Menidia menidia), pinfish (Lagodon rhomboides) and blue crabs (Callinectes sapidus) returned.
“We have about four times more fish in the seagrass meadows than outside [them] or before the seagrass meadows were restored,” says Karen McGlathery, a professor at the University of Virginia, whose role in the project is to understand the benefits of seagrass restoration from carbon sequestration to increase in fisheries.
Marine conservationists say seagrass meadows boost biodiversity and help coastal communities thrive by providing food security and potential income. The invertebrates that live in the seagrass can be a fundamental source of protein for local communities. Seagrass meadows are also excellent fish nurseries, supporting 20% of the world’s biggest fisheries, according to a 2019 study. For example, the Atlantic cod (Gadus morhua), a commercially important species, spends its juvenile period in seagrass environments. Seagrass meadows are also excellent at storing carbon and protecting the coastline, reducing the potentially damaging effects of floods and storms.
Although policymakers have long overlooked the importance of seagrasses, their vital role is increasingly recognized. Today, they are included in the U.N. Decade on Ecosystem Restoration.
Excellent ‘blue carbon’ sinks
The project in Virginia also proved the potential of seagrass meadows for carbon sequestration, a role also stressed in the Paris climate agreement of 2015.
“Seagrass beds, along with mangroves and salt marshes … sequester large amounts of ‘blue carbon,’” Orth says.
Seagrasses sequester carbon in both their biomass and in the sediment below them. Seagrass vegetation also slows down the water current, which carries a lot of carbon-rich particles.
“When that water gets slowed down, those particles fall out and settle on the seafloor,” McGlathery says.
Seagrasses are estimated to be able to absorb carbon more than 40 times faster than forests on land. With differences from species to species, seagrass meadows are able to store up 18% of oceanic carbon, even if they cover only the 0.1% of the ocean floor. By keeping carbon locked up and preventing it from going back into the atmosphere, healthy and abundant seagrass meadows are vital to mitigating the climate crisis. Research shows that, at a global level, seagrass ecosystems could be able to stock as much as 19.9 billion metric tons of organic carbon — more than four times the total emissions from the U.S. in 2020.
But the climate crisis is also imperiling the very seagrasses that could be used as tools to cool the Earth.
“If a big heatwave comes through and those seagrass [meadows] are lost, the carbon that’s bound up in that soil will go back up into the atmosphere,” McGlathery says.
Losses of seagrass can have devastating effects, proving even more the need to protect these environments. For instance, historically, British seagrass meadows could have stored 11.5 million metric tons of carbon and supported approximately 400 million fish, according to the recently published research that shows the massive loss of seagrass in the U.K.
Connectivity and healthy seascapes
Seagrass are not isolated, but strictly interconnected with the other coastal ecosystems: tidal marshes, coral reefs, mangrove forests, kelp forests, and oyster and mussel beds. Herbivorous grazers such as green turtles, manatees, dugongs and other animals, including invertebrates, rely on seagrass as feeding grounds.
“Animals move between all these environments, and only by having a healthy seascape, actually our oceans are going to recover,” Lilley says.
The experimental project in Dale Bay aims to restore 2 hectares (5 acres) of seagrass meadow. This August, the Project Seagrass team will dive into the waters of north Wales, where another restoration project is taking place. They say they hope to also kick-start new restoration sites in other areas of the U.K.
But the scope of Project Seagrass also goes beyond the country’s coasts. Its scientists work in Indonesia, Malaysia, the Philippines, Thailand and Timor-Leste under the scope of the International Climate Initiative to share knowledge by engaging local NGOs and communities in seagrass conservation practices.
“Seagrass is important in each of these five populations. When that part of the ocean becomes unhealthy, it has a direct effect on them,” Lilley says.
In addition to providing food security and income for local communities, seagrass meadows often play some role in bringing fish to our markets. Healthy seagrass creates a positive ripple effect that cascades throughout the entire ecosystem.
“We talk about habitat connectivity and wildlife corridors on land,” Lilley says. “It’s the same thing in the sea.”
Citations:
Green, A. E., Unsworth, R. K., Chadwick, M. A., & Jones, P. J. (2021). Historical analysis exposes catastrophic seagrass loss for the United Kingdom. Frontiers in Plant Science, 12. doi:10.3389/fpls.2021.629962
Unsworth, R. K., Bertelli, C. M., Cullen-Unsworth, L. C., Esteban, N., Jones, B. L., Lilley, R., … Rees, S. C. (2019). Sowing the seeds of seagrass recovery using hessian bags. Frontiers in Ecology and Evolution, 7. doi:10.3389/fevo.2019.00311
Unsworth, R. K., McKenzie, L. J., Collier, C. J., Cullen-Unsworth, L. C., Duarte, C. M., Eklöf, J. S., … Nordlund, L. M. (2019). Global challenges for seagrass conservation. Ambio, 48(8), 801-815. doi:10.1007/s13280-018-1115-y
Fourqurean, J. W., Duarte, C. M., Kennedy, H., Marbà, N., Holmer, M., Mateo, M. A., … Serrano, O. (2012). Seagrass ecosystems as a globally significant carbon stock. Nature Geoscience, 5(7), 505-509. doi:10.1038/ngeo1477
Banner image: Divers pick seagrass seeds in shallow waters. Image courtesy of Lewis Jefferies/WWF-UK.