Oysters Need PhD

Piled Higher and Deeper: Study identifies tipping point for oyster restoration

. . . a study by researchers at William & Mary's Virginia Institute of Marine Science identifies a tipping point in oyster restoration efforts, wherein reefs rebuilt to reach a foot or more above the bottom develop into healthy, self-sustaining ecosystems, while those rebuilt at lower heights are quickly coated and then buried by sediment.

The study, the feature article in the November 6 issue of Marine Ecology Progress Series, is co-authored by former VIMS Ph.D. student Allison Colden along with VIMS professors Rob Latour and Rom Lipcius. It is the second featured article by VIMS researchers in the prestigious journal in the last month.

The researchers say their findings can help explain oyster loss and enhance oyster restoration efforts not only in the Bay but worldwide. Native oyster populations have declined around the globe during the last century due to overfishing, nutrient pollution, disease, and degraded habitat, with major economic and ecological impacts.

The team's research builds on several recent studies that identified reef height as a key predictor of restoration success or failure. But, says Lipcius, "no one had quantified the threshold elevation at which a reef's trajectory diverges, or identified the mechanisms involved."

To answer those questions, the scientists used oyster shell to build 24 experimental reefs across 4 sites in the Great Wicomico and Lynnhaven rivers, tributaries of Chesapeake Bay in Virginia. They built the reefs at 6 different heights, from 5 centimeters to half a meter above the bayfloor—a range of elevations typically used in oyster restoration or repletion activities in the Bay.

"Our video sampling showed that reef condition diverged almost immediately— within 2 months of construction," says Colden. "On the lower reefs, sediments quickly buried the shells, particularly around the reef margins. On the taller reefs, we saw emergent shell, lots of young oysters, and lower levels of sedimentation."

The tipping point or threshold between these two states was 0.3 meters, about a foot. Reefs built to this height or higher had oyster densities 3.5 times greater than lower reefs, and densities on the higher reefs generally increased through time. Densities on the lower reefs tended to decrease under a range of sedimentation rates, with most of these reefs eventually disappearing beneath a blanket of sediment.

"We found that the relationship between oyster density and reef height was non-linear," says Lipcius. "Rates of sediment deposition declined exponentially with increasing reef height, while rugosity increased."

The ability of oyster to filter water, and deposit sediment around them in tremendous, but with enough relief, the current will sweep away the feces and psuedofeces, and leave the oysters unburied.

Of course, this means that properly prepared oyster restoration sites need to be left unharvested, since harvesting knocks down the relief. This will not make watermen happy.