New hope for degraded oyster reefs along the eastern seaboard

The eastern oyster (Crassostrea virginica), also known as the Atlantic oyster, is a species of true oyster native to the eastern seaboard of the U.S., as well as to parts of eastern Canada and South America. It is an important commercial seafood that has been harvested for thousands of years but has also been severely overexploited in the past. 

Eastern oysters are filter feeders that draw water across their gills and filter out the edible organisms along with detritus. They release the cleaned water back into the environment and are known to keep bodies of water clear of suspended sediment. However, the removal of millions of tons of individual oysters over the past two centuries has reduced populations to only a small fraction of what they were prior to the arrival of European colonists. 

Commercial harvesters did not only remove the oysters, but also the banks of accumulated shells that the oysters form as they cement themselves to the broken and disused husks of dead individuals. Shells were ground up and used in mortar, as road fill and as a source of lime in fertilizer. Gradually, the reefs that had been built along sand bars and muddy tidal flats over thousands of oyster generations were dredged out such that today only around 15 percent remain, worldwide.

However, the future of oyster reefs is not all doom and gloom. University of Virginia researchers, in partnership with The Nature Conservancy, have published the results of a 15-year study in which they assess the possibility of restoring reefs to their former functional glory. Their findings, published in Conservation Letters, demonstrate that restored reefs can match natural reef oyster populations in about six years and continue to hold strong thereafter.

“Our study shows that restoration can catalyze rapid recovery of an imperiled coastal habitat and help reverse decades of degradation,” said the study’s lead author, Rachel Smith, a postdoctoral researcher in the Department of Environmental Sciences at UVA.

Since the 1990s, conservationists and government agencies have been restoring reefs by recycling oyster and clam shells from natural deposits, aquaculture farms and restaurant discards; the shells are used to form the substrate for the reefs, in the hopes that larval oysters will settle and grow on them. Unfortunately, many of these projects have not been rigorous enough to allow scientific conclusions, or they have terminated within two years or less, which is too short a time to see measurable results. 

In the current study, the researchers monitored changes on 70 reefs, at 16 different sites in coastal Virginia. They studied the oyster populations for 15 years in order to answer the question of whether reefs that have become degraded and “functionally extinct” can be restored to health.

“In the past, answering this question has been hampered by unknown timelines for recovery, undefined benchmarks for success and uncertainty about whether restored ecosystems can match natural ones,” said study co-author Max Castorani, an assistant professor in the Department of Environmental Sciences. “Although preventing habitat loss should be the first priority, our new findings show that restored oyster reefs can catch up within a decade.”

Smith and Castorani worked with scientist Bo Lusk at The Nature Conservancy, which has been restoring oyster reefs in Virginia’s coastal bays since 2003. The scientists conducted their experiments at the National Science Foundation’s Virginia Coast Reserve Long-Term Ecological Research site, and were able to collect extensive empirical evidence of the relatively quick rebound of the oysters, as well as the recolonization of the reef habitat by small crabs that help support the broader food web.

“We also learned that as restored reefs matured and gained oysters, they became more stable over time, which suggests that restoration can increase ecosystem resilience,” Smith added.

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By Alison Bosman, Earth.com Staff Writer