Could an oyster reef do even more for water quality than hosting a bunch of water-filtering shellfish? That’s the question the Chesapeake Bay Foundation is trying to answer this summer in Maryland’s Severn River.
In collaboration with a pair of scientists, the Annapolis-based environmental group has placed 240 concrete “reef balls” in the Western Shore tributary of the Bay. They’ve been seeded with about 400,000 baby oysters, called spat, so tiny that they’re almost invisible to the naked eye.
The research team wants to see if the squat, bullet-shaped reef balls can alter water currents enough to keep fish and shellfish from being stressed or even suffocated by dead zones — places where dissolved oxygen drops to dangerously low levels in warmer weather.
“If it works, this would be a new way to utilize habitat to improve water quality,” said Allison Colden, the foundation’s senior Maryland fisheries scientist.
Oysters are champion filterers, each capable of siphoning algae and silt from up to 50 gallons of water per day. Reefs covered with bivalves also provide habitat — places to hide and feed — for fish, crabs, worms and other marine creatures.
Natural oyster reefs are formed by old and living oyster shells, which fuse together over time and create a rocklike surface where spat attach and grow. Oyster restoration projects often use stone, concrete and other hard materials to provide an artificial substrate for spat because there aren’t enough shells left to create reefs.
This experiment will test whether the structure of an artificial reef – one that sticks up off the bottom at least a few feet – can agitate the water enough to break up dead zones on the bottom.
In late April, the crew aboard the foundation’s oyster restoration vessel, the Patricia Campbell, used a crane to carefully place the reef balls in 60 clumps of four on Winchester Lump, an elevated spot in the river bottom where oysters once grew.
Andrew Muller, an oceanographer with the U.S. Naval Academy, and John Page Williams, the foundation’s senior naturalist, said they’ve documented repeated episodes of hypoxia, or low oxygen, on the Severn’s bottom at that spot.
“It’s something I call hypoxic squeezing,” Muller explained.
Nutrients in the water feed algae blooms, and when those microscopic plants die and sink to the bottom, their decomposition consumes the dissolved oxygen in the depths. In summer weather, the water at the river’s surface warms up, forming a layer that essentially prevents mixing with the colder, oxygen-deprived layer below.
“Fish, if they’re smart enough, get out of area,” Muller said. Otherwise, the situation can lead to fish kills. Oysters can’t escape, though. They can hold out for a while with little oxygen, but generally stop feeding or growing as they struggle to survive.
Williams said that water-quality sampling he’s done over Winchester Lump shows that oxygen levels can drop to 2 milligrams per liter about 20 feet down, which he called “borderline lethal for rockfish.”
Such dead zones can linger and grow, unless and until a summer thunderstorm or windy weather stirs the water, breaks up the layers and allows oxygen to mix all the way to the bottom. The research team hopes that in this case, the reef balls can prevent a dead zone from forming in the first place.
Two weeks before the foundation’s experiment began, Larry Sanford, an oceanographer and interim vice president for education at the University of Maryland Center for Environmental Science, placed an instrument on Winchester Lump to measure the currents all the way to the surface.
The device, an acoustic Doppler current profiler, also monitors turbulence in the water.
With the help of the Patricia Campbell’s crew, Sanford winched the gear off the bottom before the reef balls went down so he could take the instrument back to the Horn Point laboratory in Cambridge and download the measurements. He’ll return in July to redeploy it and learn if the water current and turbulence readings have changed.
“So what you would expect, if this is going to work, is that the currents are going to slow down, because there’s so much roughness on the bottom,” Sanford said. “And you’d expect there’d be a lot more mixing and turbulence in the water.”
The foundation frequently uses reef balls in oyster restoration projects. They’re fashioned with volunteer help, for a cost of about $200 apiece, Colden said. Before deployment, these reef balls were set with oyster larvae at the foundation’s restoration center in Shady Side, south of Annapolis.
“We’re trying to determine if we can take a marginal habitat in terms of summer and turn it into something that is higher quality habitat for both oysters and fish,” she said.
But, she added, it’s no panacea for the Bay’s water quality woes. “This is not to say we want to pave the bottom with reef balls,” Colden said. At best, they might help to improve oxygen levels in some local areas, she said, but they’re just a complement to larger-scale efforts to control polluted runoff and improve waste treatment.
“The solution for the dead zones is going to happen on the land,’’ she concluded.