It’s tougher to see that happen in the wild, though. A recent field study by researchers with the Virginia Institute of Marine Science found that oyster farms in the lower Chesapeake Bay had only slight — but positive — impacts on water quality.

“We were expecting to see more effects of the oysters filtering the water than we saw,” said Jessica Turner, a Ph.D. student who was the study’s lead author. But, she added, “They’re not having any negative impacts either, and that’s definitely worth noting.”

Oyster aquaculture is a booming business in Virginia’s portion of the Chesapeake, yielding more than $14.5 million in sales in 2018. As oyster-growing operations have spread, the industry’s expansion has stirred controversy over leasing in areas offshore of waterfront homes. It’s also inspired competing claims about aquaculture’s impact on water quality.

Aquaculture proponents say the farmed filter feeders are helping clean up the Bay. Others, who don’t want oyster farms near their waterfront homes, contend that they hurt rather than help, with shellfish waste piling up on the river bottom and cages blocking currents that could flush coves and creeks.

Researchers have concluded that farmed oysters, like their wild counterparts, are removing nutrients from the water that otherwise would help feed algae blooms and the Bay’s summertime “dead zone.” As a result, the federal-state Chesapeake Bay Program considers certain types of oyster aquaculture as best management practices that could be counted toward states’ goals for cleaning up the Bay.

Still, it’s less clear whether oyster farms are having measurable impacts on the clarity or dissolved oxygen levels of the Bay or any of its tributaries.

The VIMS scientists monitored four aquaculture operations in 2017, from Windmill Point near the mouth of the Rappahannock River south to Broad Bay in Virginia Beach. Two were raising oysters in floating cages, while the other two kept the cages on the bottom. The area covered by cages ranged in size from a little more than a quarter-acre to nearly 10 acres.

Aboard a VIMS research vessel, the team towed two sensors between the rows of cages in each oyster farm, like a lawnmower crisscrossing a yard. They also checked the water just beyond the array of cages, collecting data on current speed and direction, chlorophyll a (an indicator of algae), turbidity and dissolved oxygen.

The results were not that striking. They found higher oxygen levels inside the large oyster farm in Broad Bay than outside of it, for instance, but no similar pattern at the other sites. They found lower turbidity inside the Broad Bay operation and one other farm, but nothing like that at the other two. In fact, it seemed that the differences they observed had more to do with the characteristics of each farm’s location than anything about their operation.

“We expected to see more signal from the oysters,” Turner said. “It’s not to say [they’re] not helping. It’s just hard to measure or quantify how much.”

Lisa Kellogg, a VIMS senior research scientist and co-author of the paper, said the lack of noticeable water quality impacts could be attributed to the relatively low density of oysters in the farming operations and the tiny footprint they had, compared with the water bodies where they were located.

Because the farms operated in open areas, researchers estimate that even under ideal conditions, when tides and currents were low, oysters were able to filter less than 10% of the water flowing past them.

“It’s a matter of the volume of water in the farm,” she said, being relatively small compared to the entire creek.

Even in Maryland’s Harris Creek, where more than two billion hatchery-reared oysters were planted on restored reefs covering 350 acres — an area the size of the National Mall in the District of Columbia — it’s yet to be shown conclusively that they’re cleaning up the water.

With the help of a computer model they developed, researchers with VIMS and the University of Maryland Center for Environmental Science calculated that the restored reefs in Harris Creek could remove about a million pounds
of nitrogen from the water over the next decade. They also estimated that the oysters, mussels and sea squirts that cover the reefs now can filter all the water in the creek — 10 billion gallons — in as little as 10 days.

The researchers took some field measurements in the creek to help them validate the model calculations and found some small-scale water quality impacts. Around a reef they studied, they saw distinct declines in the concentration of suspended algae and other tiny particles.

“As oysters are filtering, they’re actually cleaning out the water that goes over them,” explained Larry Sanford, a professor at the UMCES Horn Point laboratory. “The further down you go over the reef, the more you can see that signature of clear water.”

Although the water immediately over the reef was noticeably clearer than elsewhere, it grew murky again within a half-meter to a meter away from the oysters, Sanford said. Without a lot more fieldwork, he said, it’s impossible to tell if the restored reefs are affecting the creek as a whole.

“The preliminary results from our modeling studies suggest there is a measurable impact, but we haven’t measured it yet,” said Lora Harris, an associate professor at the UMCES laboratory in Solomons.

Eyes in space, however, may see what water-quality monitoring hasn’t been able to so far.

Preliminary analysis of data collected by satellites show that water clarity in Harris Creek and its neighboring tributary, Broad Creek, have gradually become less murky in recent years, said Jay Lazar, field program coordinator for the Chesapeake Bay office of the National Oceanic and Atmospheric Administration.

“You can see … over time, water clarity improves,” Lazar said. “Its sharpest and most distinct improvements are right there at the lower part of Harris.”

The satellite data span from 2009, before the restoration work began, to 2017, after it was finished.

“In layman’s terms, there’s something real there that we’re observing,” Lazar said. “The challenge is attributing a cause or reason to why we’re seeing it.”

Broad Creek’s improvement also begs closer scrutiny. It has not undergone any significant restoration work, and it remains open to commercial oyster harvest. Yet it routinely has some of the best natural oyster reproduction in Maryland’s portion of the Bay. Could the clarity there stem from a natural cause, rather than an abundance of oysters from restoration efforts?

Or could there be another factor? Lazar said satellite imagery also shows a recent surge in the extent and density of underwater grasses in that same area. Bay grasses need a certain amount of water clarity to grow but, once established, they help clear it up more.

“They’re connected,” Lazar said of the two creeks. “Quantifying how they’re connected and what’s responsible for the improvement, we’re continuing to struggle with that.”