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Oyster Reef Restoration: Coastal Protection Against Climate Change
Oyster reefs could be brought back as a natural coastal defense against the effects of climate change, experts in New York say.
Oyster Reef Restoration: Coastal Protection Against Climate Change
New York City
As the sun set behind the Statue of Liberty on Saturday, a conservation group worker opened a gate on a nearby island and revealed the ingredients for a potential oyster renaissance: stacks of “Reef Balls”, large domes made of oyster shells and concrete. These will soon be placed in tanks filled with free-swimming oyster larvae. Once the larvae attach to the domes and mature, the structures are submerged in the murky waters off New York City to revive a lost ecosystem.
Coastlines around the world were once protected by oyster reefs, vast masses of oysters bonded to rocks and each other. Overharvesting and habitat loss have destroyed about 85% of the world's oyster reefs over the past two centuries. But their revival could help make coasts more resilient to the effects of climate change, including violent storms and erosion, scientists say.
The Billion Oyster Project, a nonprofit organization in New York City, is using the engineering prowess of the bivalve to slowly build a living breakwater. After a decade of refining the process, the project is generating know-how for other efforts inspired elsewhere.
“The foundations are in place to rebuild these ecosystems, and there are significant environmental and social benefits to doing so,” says Melanie Bishop, a marine ecologist at Macquarie University in Sydney, Australia. But it remains unclear 1, whether the restored reefs can grow large enough to buffer the coasts; Oyster restoration advocates acknowledge that it will take countless transplanted oysters and many years before the reefs can provide a barrier against sea level rise.
Measurement of molluscs
Centuries ago, New York Harbor, which lies between the city's five boroughs — Manhattan, Staten Island, Brooklyn, Bronx and Queens — and parts of New Jersey, was home to a massive concentration of eastern oysters (Crassostrea virginica) that covered nearly 900 square kilometers. Demand for the succulent molluscs soared as the city's population boomed; at the same time, pollution in the harbor escalated, and by 1927 the oyster reef ecosystem had collapsed.
The Billion Oyster Project is seeding oyster beds at 18 locations around New York City. The goal is to initiate the process of reef building. If the oysters are able to reproduce on their own, they could one day form a structure that resists Hurricanes and extreme storms buffers and protects the coast from erosion into the rising sea says Asly Ventura, public relations coordinator for the project. Studies have shown that oyster reefs increase biodiversity 2 and improve water quality 3, which could create safe havens for other species as ocean conditions change.
The larval mollusks must settle on hard, stable surfaces in order to grow. To provide them with a home, project staff and volunteers mix crushed oyster shells donated by restaurants with recycled concrete and use the pulp to create hollow, dome-shaped structures with holes about three feet in diameter. Staff drop several of these domes at the project's reef sites each summer, with the goal of establishing large stocks of oysters at each site.
The campaign had mixed success. Of the 122 million oysters transplanted into the port by the end of last year, about half have died. “We expect a lot of mortality,” Ventura says. Oysters produce a lot of offspring because so many larvae die, she explains. In 2022 the organization reported that oysters reproduce naturally at about half of the installations.
Pollution could partially explain why the mollusks don't reproduce on their own in every location. After rainfall, a mixture of raw sewage and rainwater is discharged into the harbor. Remaining industrial waste contributes to poor water quality. Noise emissions could also be disruptive and affect the larvae's ability to find a suitable place to settle, as they use acoustic cues to locate existing reefs, Ventura says.
Ray Grizzle, a marine biologist at the University of New Hampshire in Durham who conducted scientific assessments for the project, says juvenile oysters primarily settle within 400 meters of their parent reef. Therefore, oysters may not easily self-reseed in locations far from existing reefs.
Grizzle's biggest concern is how pathogens, such as the parasites Haplosporidium nelsoni and Perkinsus marinus, might affect oysters long-term. “They now have a lifespan of about 3 to 5 years, whereas historically it was probably 10 to 20 years,” he explains. As lifespan is reduced, the population's ability to form durable vertical reef structures also decreases, he says. Nevertheless, the project is “moving in the right direction,” says Grizzle.
Reef resurgence
Other restoration projects are gaining momentum around the world. In Australia, biologists grew oysters simply by placing limestone rocks on a sandy seabed. Larval flat oysters (Ostrea angasi) naturally settled on the rocks in densities that far exceeded expectations 2, suggesting that the larvae travel from unknown remnant reefs or oyster farms.
"We didn't know we would get natural recruitment when we started," says study co-author Dominic McAfee, a marine biologist at the University of Adelaide in Australia. The effort is designed to support the marine shipping industry and has increased invertebrate biodiversity at the site.
In the North Sea, researchers place oyster larvae on the granite boulders at the foot of Wind turbines to strengthen structures and increase biodiversity.
Bishop says that oyster reef projects must be successful if they are carried out in areas where the problems that led to their decline are no longer present, they are designed to withstand predicted ecosystem changes, and they are monitored over a much longer period of time than two to three years. Although there is still a long way to go, “there is a lot of hope,” she says.
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Morris, R.L. et al. Ecol. Appl. 31, e02382 (2021).
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McAfee, D., McLeod, L.D., Carruthers, S. & Connell, S.D. Restor. Ecol. 32, e14169 (2024).
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Barr, J.M., et al. Estuary. Coast. 47, 789–804 (2024).