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Oyster reefs could provide 'dynamic' sea walls that outpace sea-level rise

Researchers at the University of North Carolina think they may have found a way to rehabilitate oyster reefs and protect coastal communities from erosion and sea-level rise.

The team -- led by Antonio Rodriguez, a coastal geology professor at UNC -- constructed 11 oyster reefs in intertidal areas on the North Carolina coast between 1997 and 2011. In a study published in Nature Climate Change yesterday, the team reported that the reefs had grown in size over the years at a pace that would match any future sea-level rise.

Oyster reef abundance has declined by roughly 85 percent globally in the last 100 years, according to the study, from overharvesting and disease. In the Chesapeake Bay -- a Mid-Atlantic tidal area formerly replete with oysters like North Carolina -- is less than 1 percent of its former size (Greenwire, Oct. 21, 2013).

But the reefs grown as part of Rodriguez's research could help rebuild these populations while also protecting coastal ecosystems from sea-level rise, erosion and hurricanes. The reefs could also provide important fish habitat and filter the water as they grow, controlling levels of pollutants like nitrogen in the water, Rodriguez said.

"It would be a dynamic structure out here that's in tune with environmental changes," he added.

Barriers that can grow

Oyster reefs have been studied for years, but this research is the first to suggest the reefs would act any differently from a normal sea wall. The problem with conventional sea walls is that after the sea level rises high enough, the waves come in over their top. That makes them relatively short-lived as coastal barriers, despite their substantial cost.

Oyster reefs, on the other hand, can grow by attracting oyster larvae that drift through the water. The larvae latch onto the existing wall, secured by sediment buildup, and continue growing. Rodriguez said previous studies hadn't accounted for the cumulative growth of all the oysters on the reef.

For example, four oysters growing 2 centimeters a year would have been recorded as 2 centimeters of growth. But in actuality, with the four oysters added together, the growth would have been 8 centimeters that year.

When that cumulative growth is accounted for, Rodriguez said, intertidal reefs could keep pace with future sea-level rise.

"They grow on top of each other, so their growth rates are added together." he added. "That's why the reef was growing so rapidly."

But oyster reefs would only be able to achieve this growth rate under certain, specific conditions.

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Location can be tricky

First, Rodriguez said reefs built farther out to sea, in subtidal areas that are constantly underwater, would possibly not grow as quickly. His team's study only focused on intertidal areas -- where the reefs would be above and below water as the tide ebbed and flowed.

Oyster reefs could also be hampered by other ecosystem stressors like ocean acidification or predation. Reefs that the team built near salt marshes didn't grow as fast as ones built close to the bare shoreline.

"We think that's because there's some interaction between the salt marsh and the reef," Rodriguez said. "Maybe some predators are coming out of the marsh and gobbling up oysters.

"There must be some distance where you no longer get that negative interaction with the marsh," he added.

And oyster reefs may grow more slowly if there aren't as many larvae in the water. The area of North Carolina coastline where the study was conducted is known for having high concentrations of oyster larvae, Rodriguez said.

George Waldbusser, an ocean ecology professor at Oregon State University, said in an email that he would have liked to have seen more attention paid to subtidal reefs -- which are far more prevalent -- in the study.

"I worry the interpretation is that all reefs grow this fast, and the limited studies indicate this is not true," he wrote.

Warming water may help

Intertidal reefs are rarely found north of the Virginia coast, according to Mike Wilberg, an associate professor at the University of Maryland Center for Environmental Science. In winter the oysters freeze to death if they are exposed above the water line during low tide.

With increased ocean warming on the Atlantic Coast due to climate change, however, these intertidal reefs could be able to survive farther north in the future, Wilberg added.

"One of the things that continues to amaze me is how our natural systems are able to respond to the stresses we put on them," he added.

Rodriguez acknowledged that there is much that still remains to be studied. Besides doing more research on subtidal reefs, he also wants to investigate how far oyster reefs could be built away from salt marshes. Ultimately, the result could be a sea wall that not only rebuilds oyster populations, cleans the surrounding water and provides a fish habitat, but actually grows in lock step with sea-level rise.

The discovery could even be potentially lucrative with the global sea wall market predicted to be worth $9 billion in the next 10 years (Greenwire, Feb. 18).

"They would provide a lot more benefits than just a concrete or rock wall," he said. "Those other structures would just drown."

Twitter: @henrygass | Email: hgass@eenews.net

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