The massive — and massively expensive — oyster restoration project in Maryland’s Harris Creek is yielding some pretty big pollution reductions, according to a new report.Nearly 2.5 billion hatchery-spawned oysters have been planted since 2012 over 351 acres of restored reefs in Harris Creek – an area roughly the size of the National Mall in the District of Columbia. The cost to date, including a second round of oyster plantings to cover thin spots on the reefs, is $28.3 million. (US Army Corps of Engineers)

Using a computer model to calculate the project’s water-quality impacts, researchers from the Virginia Institute of Marine Science and the University of Maryland Center for Environmental Science estimate that Harris Creek’s restored reefs are soaking up about 100,000 pounds of nitrogen annually that otherwise would be adding to the Bay’s pollution woes.

“We’re taking a lot of pollution out of the Bay through these oysters,” said Mark Bryer, Chesapeake Bay program director for The Nature Conservancy, which funded the report along with the Oyster Recovery Partnership. “And, presuming these oysters continue to do well over time, they’ll continue to provide that benefit.”

Despite the big nitrogen number, the reefs are producing only a modest improvement in water quality in the creek, a tidal tributary of the Choptank River on the Eastern Shore. That’s because pollution is washing into the creek from the Bay almost as fast as the reefs can filter it. But this report shows the value of doing more such large-scale reef projects, Bryer said.

Over the past seven years, federal and state agencies have planted nearly 2.5 billion hatchery-spawned spat, or baby oysters, on 350 acres of restored reefs in the creek. Advocates say such large-scale undertakings are the best hope for reviving the Chesapeake’s depleted oyster population, which has dwindled to 1 or 2 percent of historic levels through decades of overharvesting, disease and pollution.

Watermen and their political supporters, though, criticize the project’s cost — which has grown to $28.3 million — and question its outcome.

These hatchery-produced oysters were recovered from a restored reef in Maryland's Harris Creek during monitoring in 2015. A model developed by Virginia and Maryland researchers estimates that the creek’s restored reefs are capable of removing about 1 million pounds of nitrogen from the Chesapeake Bay over a decade. (Emily French/Oyster Recovery Partnership)

“How can you prove it’s been successful?” Ron Fithian, a Kent County commissioner and former waterman, asked at a recent meeting of the Maryland Department of Natural Resources Oyster Advisory Commission. “After 7 years, you can’t determine if Harris Creek was worth [the] expenditure.”

The restored reefs are in a sanctuary off-limits to harvest, and surveys in recent years have shown that the oysters there are thriving. It’s too early to tell, though, if the reefs are self-sustaining, let alone seeding neighboring waters with juvenile oysters.

But scientists and restoration advocates say the reefs already provide other ecological benefits, including habitat for fish, crabs and other marine life. The oysters also filter pollutants from the water, and the report by VIMS and UMCES researchers quantifies the water quality impacts for the first time.

“One of the reasons for this study is to start to illuminate the benefits that may go unseen from a project like this,” said The Nature Conservancy’s Bryer.

An oyster can filter up to 50 gallons of water a day, so restoration advocates see projects like the one in Harris Creek as natural wastewater treatment plants.

Nitrogen is one of the main pollutants that oysters filter out of the water. It enters the Bay in stormwater runoff that contains fertilizer or animal waste from farms, streets and lawns; in treated and untreated sewage; and in fallout from power plant emissions and vehicle exhaust.

Nitrogen feeds algae blooms, which in turn consume dissolved oxygen in the water as the aquatic plants die and decay. Fish, crabs and shellfish struggle to survive in the resulting oxygen-starved “dead zones.” Reducing nitrogen loads is a goal of the Bay’s “pollution diet,” formally called a total maximum daily load, that the U.S Environmental Protection Agency established for the Bay in 2010.

Oysters take in nitrogen as they consume algae and other bits of organic matter in the water. Some of the nitrogen in the algae is used by the oyster to grow its tissue and its shell, while the rest gets excreted and winds up in the sediment around the reef. There, in a complex chemical process known as denitrification, bacteria convert it into nitrogen gas, a form no longer able to feed algae blooms.

But measuring the oysters’ filtration in open water like Harris Creek is a huge and tricky undertaking, because it’s not a controlled environment. So, researchers Lisa Kellogg and Mark Brush from VIMS and Jeff Cornwell from UMCES collaborated on a computer model that could handle the complex calculation of all the variables involved in assessing what the reefs are doing to remove nutrients.

“The idea … is basically that the sampling we’re doing is too expensive for any management agency to really undertake on their own,” explained Kellogg, a senior research scientist at VIMS in Gloucester Point, VA. Through the model, she said, she and her colleagues hope to provide a tool that natural resource agencies could use to gauge the ecological benefits of this and other reef restoration projects.

Initially developed in 2014, while the Harris Creek reefs were still being restored, the model has since been enhanced and updated. With funding from the National Oceanic and Atmospheric Administration, the scientists gathered more recent information about the creek’s water quality and the number of oysters planted there. Using that data, the model indicates that the restored reefs can filter all the water in Harris Creek — nearly 10 billion gallons — in less than 10 days during the warm summer months, when oysters are actively filtering.Oysters growing on restored stone reef in Harris Creek are covered with bulbous sea squirts, small marine animals that grow in clumps on reefs, pilings, jetties and other hard surfaces in shallow waters. Like oysters, they are filter feeders. The model developed by Virginia and Maryland researchers found that sea squirts and mussels growing on the restored reefs accounted for about 40 percent of the nutrient removal taking place. (NOAA)

The oyster population in Harris Creek increased from an estimated 4.5 million bivalves before the restoration project to about 134 million afterward, the report notes. As with wild oyster reproduction, many of the hatchery-spawned spat perished — eaten by predators, smothered by sediment or outcompeted and starved of food by the other shellfish crowded around them. But if the remaining oysters continue to thrive, the reefs could remove a total of 1 million pounds of nitrogen over the next decade, according to the model calculations.

And oysters aren’t the only ones removing nutrients from the water. The model figures that more than 40 percent of the nitrogen removal in the creek is being done by other filter-feeding marine animals that have found a home on the restored reefs — such as mussels and sea squirts, small bulbous creatures with little siphons, which grow in clumps on reefs, pilings, jetties and other hard surfaces in shallow waters.

Though the restored reefs could potentially remove all the nitrogen getting into Harris Creek from its tiny watershed — a little less than 10 square miles in a mostly rural landscape — it’s only reducing the overall amount of nitrogen coming into the creek by about 5 percent . That’s because the vast majority of nutrients come in from the Bay, and the pollution being removed is promptly replaced as tides flush more Bay water into the tributary. Even so, the scientists say it’s significant that the reefs are dealing with pollution from beyond the creek watershed.

“It’s a needle in the haystack when it comes to [cleaning up] the Chesapeake,” acknowledged Brush, an associate professor of marine science at VIMS. But to reach the Baywide cleanup goal, he added, “every little bit helps.”

While the Harris Creek project’s expense has been controversial, prompting Maryland officials to weigh a much smaller-scale effort planned for the St. Mary’s River, the report indicates that restored reefs have value to the Bay cleanup. Based on the construction cost to date, the nitrogen removed by the Harris Creek reefs cost roughly $300 per pound. That’s significantly more than what it costs per pound for most farm runoff control measures, but far less than it costs to capture nitrogen running off city or suburban streets, where storm drain retrofits can cost thousands of dollars per pound of pollution captured.

“Obviously, this is not an inexpensive experiment, but I think the project looks pretty good from what we see right now in terms of water quality,” said Cornwell, a research professor at UMCES Horn Point laboratory in Cambridge.

Brush, who had the lead role in crafting the model, said it’s designed to assess water-quality impacts of any reef project. The researchers hope to test that by applying it next to the Lynnhaven River in Virginia, recently selected for a 56-acre restoration project. Kellogg said that the Lynnhaven, near the mouth of the Bay, is far saltier than Harris Creek, and the population of filter-feeding creatures is somewhat different. Both factors could influence the rate of nitrogen removal.

“The real value is going to be down the line, when you apply models like this to more and more places,” Cornwell said.

The report was welcomed by Stephanie Westby, oyster restoration coordinator for the National Oceanic and Atmospheric Administration’s Chesapeake Bay Office, which provided funding to Kellogg and Cornwell to collect data from Harris Creek that was used in the model.

“We’ve long believed that the ultimate goal is not just to restore oysters, but to restore the eco-service function as well,” she said. “We’re very interested in anything that starts to quantify what that ecosystem service impact is.”

Bryer cautioned that building oyster reefs is no substitute for tackling pollution where it’s occurring, but he said that the study shows the value of restoring the Bay’s lost “natural infrastructure.”

“I think it should have a really great impact on how the state thinks about its restoration moving forward,” Bryer said.

“It’s not cheap,” he added, but “if we want to have clean water and abundant fisheries in the Chesapeake Bay, you have to pay for that.”

As originally posted, the story misstated the estimated cost per pound of nitrogen removed. A photo caption also mischaracterized sea squirts. The Bay Journal regrets the errors.