A heaping shoal of oysters might have peeked above the water two centuries ago near what is now the Francis Scott Key Bridge. Such a shoal could explain why the U.S. Army built the hexagonal Fort Carroll in that same area in the late-1840s to defend the busy harbor.
But the shellfish have not thrived in the Patapsco River since.
It's difficult for them to reproduce there — the process requires saltier water and the population hasn't been able to withstand decades of industrial and urban pollution.
Now, Chesapeake Bay advocates are testing a strategy to give oysters a second chance in the Patapsco.
Shells carrying 3 million speck-sized baby oysters clattered off the Chesapeake Bay Foundation boat Patricia Campbell this week — and onto a recently laid bed of stone between Fort Carroll and the Key Bridge.
The hope is that the oysters, like others planted in nearby beds in recent years, will at least survive and grow, filtering sediment and pollutants from the river, and creating food and habitat for other creatures.
Perhaps, they could do even better.
"One thing we want to find out is if they will reproduce," said Terry Cummings, who ran the foundation's Baltimore Initiative until his retirement last year. "The jury is still out on that one."
The oyster planting was the realization of a project the bay foundation and the Waterfront Partnership of Baltimore launched two years ago. The Great Baltimore Oyster Partnership pledged to seed the Patapsco with 5 million oysters by 2020.
Those groups, along with the Maryland Department of Natural Resources and the Living Classrooms Foundation, have deposited smaller numbers of oysters on natural beds around Fort Carroll for years.
But the seeding completed Monday is a larger experiment that could be reproduced elsewhere around the region if it's successful. The oysters were deposited on a man-made reef that extends the bivalve's footprint in the river by about an acre.
The project is separate from more substantial oyster restoration work in lower portions of the Chesapeake, largely on the Eastern Shore, designed to boost both the bay's health and the state's seafood industry.
In this case, the investment is purely ecological. Oysters cannot be harvested from the Patapsco because pollution makes them dangerous to eat.
Yet they serve as the bay's natural cleaning system, pulling sediment and nitrogen from water as they filter out and eat plankton. They excrete nitrogen pollutants directly onto the bay bottom, where microbes convert them into gas that bubbles up and is harmlessly released into the air, said Allison Colden, the bay foundation's Maryland fisheries scientist.
That process helps clear bay waters so sunlight can reach underwater grasses, which along with the oyster bars themselves serve as vital habitat for fish and crabs.
With those sorts of benefits in mind, the Maryland Port Administration provided about $90,000 to lay crushed stone on top of the soft riverbed last year. It came from a pot of money the state agency dedicated to environmental projects, to offset the impact of it placing material dredged from the harbor at a site in Masonville Cove, port spokesman Richard Scher said.
The artificial reef serves as the hard substrate oysters need to thrive.
The juvenile bivalves now occupying it came from the University of Maryland Center for Environmental Science's hatchery at its Horn Point Laboratory near Cambridge. The port administration and the Abell Foundation provided about $12,000 to grow the larvae and obtain a literal boatload of shucked oyster shells for them to grow on.
Ecologists and volunteers gathered and cleaned the shells from a recycling program and a Virginia seafood company. After letting the larvae mature in tanks for about a week and a half, they mixed them with the bare shells, letting the microscopic creatures become what is known as spat on shell.
About a month into their lives, the baby oysters tumbled to the bottom of the Patapsco.
They join a smaller number of oysters that are "gardened" in cages around the harbor. Through the oyster partnership, about 150,000 oysters are transplanted to the natural reefs each year. That project is focused largely on public education and outreach, though, officials said.
Biologists aren't sure what will happen next. Oysters thrive in saltier brackish waters, but there they also encounter diseases that, combined with overfishing, have cut their numbers to an estimated 1 percent of historic levels in the bay.
"Up here you're not going to see disease," Colden said. "The trade-off is they're going to grow more slowly."
An influx of fresh water running off from a storm could kill them off, and it's unclear if the water will often be salty enough for them to reproduce, she said.
Biologists and watermen alike are rooting for their success.
Jim Mullin, executive director of the Maryland Oystermen Association, said a positive outcome could inspire more oyster restoration work north of the Chesapeake Bay Bridge.
The industry has met resistance to such projects because they are considered a risky investment. But Mullin thinks it's worth it.
"It's one more acre that's got some seed on it," he said. "If it's in an area that traditionally hasn't had oysters on it and there's an opportunity for oysters to catch and grow there, that's a good thing."
Bay foundation staff meanwhile aren't sure how many oysters already live around Fort Carroll or how they are faring, Cummings said. The small numbers of shellfish they dredge up for schoolchildren on field trips look healthy and growing, he said.
The new oyster bed will serve as a good testing ground to learn more. The Patricia Campbell dropped the spat on shell at varying densities so biologists will be able to tell the conditions under which the bivalves grow best.
Even if they can't reproduce, they should boost the river ecosystem, said Adam Lindquist, director of the Waterfront Partnership's Healthy Harbor Initiative.
Within earshot of the rumbling traffic of the Key Bridge, cormorants, egrets, herons and gulls have taken over Fort Carroll, abandoned since the 1920s. Marine sonar systems show clusters of menhaden or other bait for rockfish or other large predators lurking around the reef.
The new reef should provide fresh food and habitat for fish and crabs, which in turn sustain the birds.
"By planting in the Fort Carroll oyster sanctuary, we are giving oysters a second chance in the Patapsco River," Lindquist said.