Largest Oyster Restoration Project in the Chesapeake Bay
How Oysters Improve Water Quality
Parts of six states and Washington, D.C., drain into the Chesapeake Bay—an area that’s home to 17 million people. One of the biggest threats to water quality in the Bay is stormwater runoff, and with increasing urban and suburban development in that watershed, the threat is increasing.
Stormwater runoff from developed areas carries a lot of nitrogen into the Bay. Nitrogen is not toxic, but it is a potent fertilizer, and it causes algal blooms. When the algae die, they sink to the bottom and decompose. That robs the water of oxygen and can cause dead zones in the Bay.
Filter-feeding oysters consume algae, and if there were enough of them, they would help control algal blooms. Oysters aren’t a panacea, and they wouldn’t keep the Bay healthy on their own—managing runoff at the source is also important. But a restored oyster population would mean billions of tiny water filters doing their part to keep the water clear and oxygen levels high in the Chesapeake Bay.
Many partners are collaborating on the Harris Creek oyster restoration project, and each brings their particular expertise. The partners include:
It was a clear and unseasonably cold spring morning as the 30-foot aluminum survey boat Potawaugh skimmed along the surface of the Chesapeake Bay. Outfitted with sidescan and sub-bottom sonar, it can reveal the composition and three-dimensional structure of the seafloor and even peer through it to see the next layer down. With the sleek, gray vessel’s multiple sonar systems clicking away, the boat seemed perfectly named. Potawaugh is a native Algonquin word for dolphin.
Steve Giordano, a biologist with the NOAA Chesapeake Bay Office’s Habitat Assessment Team, piloted Potawaugh to Harris Creek, a tributary of the Chesapeake on Maryland’s eastern shore that is the site of the largest oyster restoration project in the Bay. Biologists hope that the 377 acre site will soon be home to a self-sustaining population of Crassostrea virginica, the species of oyster that once filled the Chesapeake with vast, layered reefs so great that their peaks emerged from the water at low tide like small Pacific volcanoes.
Only about 1 percent of that historic oyster population remains, the rest having succumbed to centuries of overharvesting, pollution, and disease. Today, the bottom of the Chesapeake is mostly mud, and this presents a challenge to restoration ecologists. You build an artificial reef by dropping millions of cubic yards of hard substrate into the water. But put that material down in mud and it will settle into the bottom and disappear.
Acoustic Seabed Mapping
Reef-building material, like all construction material, is expensive. “Any resources that we’re putting in the water out here,” Giordano said, “we have to be sure to put it in the right place.”
And that’s where the Potawaugh and the Habitat Assessment Team come in. Acoustic seabed mapping is their specialty, and they’re here to identify sites that are suitable for reef construction.
Giordano lowered the sidescan sonar unit into the water. It was a meter-long, bright yellow torpedo tethered by a cable, and it ran in the wake of the boat. Inside the cabin, a series of wall-mounted computer screens displayed a live sonar image. Different bottom types showed up in different colors—orange for sand, red for rock, and blue for oyster shell—that together formed a complex pattern of bands and ripples superimposed on a map.
We were looking for shoals of hard sand in water deep enough that a constructed reef wouldn’t present a hazard to navigation. The next phase of reef construction would happen at those sites.
Oysters Clean the Water and Provide Habitat
After mapping a section of bottom, we checked on a reef that was constructed two years before. Stephanie Reynolds Westby, NOAA’s Chesapeake Bay oyster restoration coordinator, hooked a small dredge to a winch. She dropped it in the water, let it drag for a bit, then brought it onboard. Up came the two-year-old oysters, and they didn’t look like what you’d find at your local seafood market. They were cemented together in a heap, the sharp ends of their shells pointing outward like serrated blades.
Westby liked what she saw. “See these spaces?” she said, poking a muddy finger into the gaps between the bound-together oysters. “These provide habitat for all types of creatures we care about.” Blue crabs and striped bass are just two of the species that spend their juvenile phase on the oyster beds, where the nooks and crevices offer shelter from predators.
In addition to providing habitat, filter-feeding oysters also clean the water. “There used to be so many oysters here that they filtered the entire volume of the Bay about every three days,” Westby said. Today, the same volume is filtered about once a year—not nearly fast enough to keep the water clear.
The Largest Oyster Restoration Project Yet
At the start of their lives, oysters spend two weeks as floating larvae. Then they need to find a hard surface to settle down and grow into adult oysters. But because most of the Chesapeake’s historic reefs are gone, and the bottom is mostly mud, many larvae never find a place to set.
By constructing reefs and then seeding them with young oysters, ecologists are attempting to kickstart the population. With time, new generations of oysters will settle on older ones and, as the layers accumulate, the reef will expand and eventually resemble a natural reef.
That’s the hope, anyway. But Harris Creek does have one thing going for it that earlier restoration projects did not. It’s big. When complete, the project will sport 377 acres of reef within a 4,400-acre sanctuary. Until a few years ago, a twenty-acre project would have been considered large. And size matters. A high-density network of reefs like the ones being built on Harris Creek increases the chances that oyster larvae find a good place to settle down.
Westby tossed the two-year-old oysters she was examining back into the water, then she hosed down the deck. “We’re trying to do restoration at a scale that matches the scale of the problem,” she said.