TALLYING HEALING VALUE OF OYSTERS
New interactive computer model helps show how filter feeders clean a waterway — and how they might best be deployed
It’s not news that oysters improve waterways by filtering out suspended solids, algae and nutrients, leaving cleaner water behind.
But how many oysters would it take, and where should they be placed, to do the best job and justify the cost of building a reef?
For the first time, a new interactive computer model is offering some answers.
“The idea is that you can use this tool both to plan restoration — where do you want to put your oysters, how many oysters should you put out, in what densities, to get whatever level of benefit you’re looking for,” said Mark Brush at the Virginia Institute of Marine Science in Gloucester Point.
Brush developed the computer model along with VIMS colleague Lisa Kellogg and Jeff Cornwell of the University of Maryland Center for Environmental Science.
The team just completed a proof of concept study focusing on Harris Creek in Maryland, site of the largest oyster reef restoration project on the planet.
Among their findings: About 350
acres of restored reefs are able to filter the full volume of the creek in less than 10 days during the summer months, and have the potential to remove a million pounds of nitrogen from the Chesapeake Bay in over a decade.
“That’s just a remarkable amount,” said Mark Bryer, Chesapeake Bay Program director for The Nature Conservancy, which funded the project along with the Oyster Recovery Partnership.
Remarkable as it is, it pales next to historical figures, he said.
Before the bay got heavily developed, it teemed with enough gigantic oyster rocks that the bivalves were able to filter the entire volume of the bay every three days.
Oysters were a lucrative commercial fishery for decades before overfishing, pollution and two deadly invasive parasites crashed the population by the mid-1970s.
In fact, oyster reefs throughout the world dramatically diminished.
“They are one of the most imperiled habitats on the planet,” said Bryer. “We’ve lost more than 85 percent, closing in on 90 percent, of the original extent of oyster reefs globally.”
The oyster stock in the Chesapeake is rebounding in recent years through stricter state management and active efforts to build out existing reefs and create new ones.
“We realize we’re not going to get back to the way things were when John Smith first sailed up the bay in the early 1600s,” said Bryer. “But this really marks a dramatic change in the ecosystem and shows that people can actually make a lot of progress in terms of bringing back oysters in natural reefs.”
Harris Creek is one of 10 waterways in Maryland and Virginia targeted for reef restoration as part of a 2014 regional bay agreement. Virginia rivers on the list are the Lower York, Great Wicomico, Lafayette, Lynnhaven and Piankatank.
In a project that ended in 2015, more than 2 billion oysters were planted in Harris Creek. But researchers decided they needed a better sense of its cost-benefits.
“We started asking questions: Do we know how much pollution could be removed, potentially,
“This study helps people understand the benefits that you can get from oyster reefs, which are hidden, in many cases, out of people’s sight.” — Mark Bryer, Chesapeake Bay Program director for The Nature Conservancy
from a restoration project?” Bryer said. “And the answer was, no, we really don’t.”
So they partnered with VIMS and the University of Maryland to find out.
Brush, Kellogg and Cornwell coupled an estuarine ecosystem simulation — which Brush already had developed to compute the amount of water masses, nutrients and suspended particles that move through a tidal waterway — with an oyster sub-model that computes the amount of water, algae, suspended solids and nutrients that oysters filter or recycle.
They validated their Harris Creek Oyster Restoration Model and its findings with data collected by permanent sensors and field crews.
According to VIMS, the model showed that reefs in the creek led to an 11.9 percent drop in chlorophyll concentration, which is a proxy for algal cells, nitrogen and phosphorus, and a 15.1 percent drop in sediment and suspended particles.
The oysters removed nitrogen chiefly through a process called denitrification, or converting it to a gas harmless to marine life.
One interesting finding is that it’s not just oysters that clean up the water: Other creatures that live in reef habitat, such as mussels and sea squirts, contribute more than 40 percent of a reef’s filtration.
Researchers see their model as a valuable tool to help resource managers better estimate the benefits of oyster restoration in meeting their Total Maximum Daily Loads of phosphorus, nitrogen and sediment into the Chesapeake watershed.
With the launch of the Harris Creek model, said Brush, “the next step is to go broader.”
If they can secure more funding, they can apply the model to new systems fairly quickly.
Kellogg also is part of an expert panel looking into getting oyster reefs federally approved as a “best management practice.”
But approval is an incremental process that has to consider potential negative feedbacks, including algae growth stimulated by the nitrogen and phosphorus that oysters secrete back into the water.
“We’re working on it,” said Kellogg. “We’ve been working on it for a while. But when you really get into the weeds, it’s a very complicated task.”
Bryer likens restoring oyster reefs to investments in manmade infrastructure, such as roads, bridges and dams.
“We make investments in them because we can see the benefits from them,” Bryer said. “This study helps people understand the benefits that you can get from oyster reefs, which are hidden, in many cases, out of people’s sight.”
“Even if it’s hard to see and hard to measure,” said Kellogg, “oyster reefs do have really quantifiable benefits when they’re restored. We’re finally getting to the point where we can actually put numbers on the services that they’re providing.”
To see the Harris Creek Oyster Restoration Model, goto bit.ly/2zn8Oco. Dietrich can be reached by phone at 757-247-7892.