Ability of oysters to denitrify Bay surprises scientists
One acre of the restored Shoal Creek reef could remove 543 pounds of nitrogen in a year.
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On a warm summer morning, Lisa Kellogg and her colleagues were racing with the tide.
While the ocean water was low on the seaside of Virginia's Eastern Shore, her crew was scooping up the exposed oysters and sediment and placing them onto circular trays.
They took care to ensure that any worms and other organisms burrowing in the sediment were part of the move, as well as any tiny mussels and clams latched onto the oyster shells.
"As best we can, we are trying to put back what was there when we showed up," said Kellogg, a researcher at the Virginia Institute of Marine Science.
Then she stood on the edge of the tray to drive it into the sediment, making it as undistinguishable from the surrounding reef as possible. The hope was that when researchers returned to collect the tray in a month or so, it would appear as untouched as nearby reefs.
Kellogg's goal was to find out whether the oysters, and the vibrant community they support, would replicate a surprising discovery she made two years earlier on the Choptank River.
Working on a reef in Shoal Creek, which had been restored by the Oyster Recovery Partnership in Maryland, she found an acre of reef could remove 543 pounds of nitrogen through denitrification in a year. That's the highest denitrification rate of any natural system documented in the Chesapeake, and one of the highest ever reported in a marine environment anywhere.
"These rates are gigantic," said Jeffrey Cornwell, of the University of Maryland Center for Environmental Science, who collaborated on the study. But, he cautioned, "that is just one site."
Oysters have long been appreciated as powerful filterers that can clear algae from the water as they feed. But the fate of the nitrogen consumed with that algae has long been uncertain.
Some of it is absorbed in the oyster shell and flesh where it is stored for varying amounts of time. But that storage isn't necessarily permanent as some of the nitrogen can be returned to the water when the oyster dies and its shell breaks down.
Denitrification, in contrast, is considered the gold standard for nitrogen removal. It converts the nutrient into harmless nitrogen gas, the most common element in the atmosphere. It's the process used at modern wastewater treatment plants to remove nitrogen from human wastes.
Denitrification takes place when bacteria living in the presence of free oxygen use aerobic processes to convert ammonia (a form of nitrogen) into nitrate. Then, bacteria living in anoxic (no oxygen) conditions use anaerobic processes to convert that nitrate into nitrogen gas, which returns to the atmosphere.
Normally, denitrification takes place in the sediment where oxygenated and anoxic conditions are often found in close proximity. Earlier lab studies of denitrification associated with oysters had shown that a portion of the nitrogen in their excreted wastes was denitrified in the sediment, although the rates were not particularly high. But those studies just placed oyster wastes on sediment in the lab and measured the result.
The Choptank work sought to characterize what was happening in the wild, and not just in the sediment, but also the entire reef. Kellogg, using a technique developed by Cornwell and Michael Owens, also of the Center for Environmental Science, was able to capture a portion of the reef community and the surrounding water column in a plastic container. Then, the scientists were able to measure the amount of nitrogen taken out of the water.
The difference was dramatic. "What happens in a square meter of oyster reef might be 30– or 40-fold more intense in terms of nutrient processing than in adjacent sediments," Cornwell said.
The reason, the scientists think, is related to the complexity of the reef itself. Oyster reefs consist not only of oysters, but a wide variety of mussels, clams, arthropods and other organisms that live on the oysters or in the surrounding sediments. In the Choptank, the scientists found more than 24,000 organisms of 1 millimeter or larger growing on a healthy square meter of oyster reef.
The reef creates a huge amount of surface area that provides tiny pockets of oxygenated and anoxic conditions — and the associated nitrifying and denitrifying bacteria — in close proximity. Similarly, the active sediment community around the reef is filled with worms and arthropods that are churning through the sediment, increasing contact between oxygenated and anoxic areas.
The massive filtering power of the oysters, combined with that of other filter feeders dwelling on the reef, pulls algae out of the water. The algae is digested, and the ammonia in the oysters' excrement is then acted upon by bacteria in the sediment and microhabitats created on the reef.
In effect, the entire reef community acts as an organic denitrifying machine. "You vastly increase the number of microhabitats available for both the nitrifying bacteria and the denitrifying bacteria, and you provide them with huge amounts of organic material to work with," Kellogg said.
She and her colleagues estimated that if all 4,250 acres of restorable oyster habitat in the Choptank were rehabilitated, it could remove 48 percent of nitrogen inputs to the river through denitrification — more than enough to meet its cleanup goals.
That suggestion has generated interest in using oysters as a nutrient control strategy, although Kellogg and Cornwell caution that their Choptank estimates are filled with important caveats.
The reef Kellogg studied had more than 100 oysters per square meter — an extremely high number compared with most restoration projects. Guidelines adopted by state and federal agencies last year set a goal of 50 oysters per square meter for Bay oyster restoration projects, though as few as 15 living oysters per meter can quality as a success.
Whether such low densities provide significant nitrogen removal rates is unknown.
Further, Cornwell noted, the Choptank study took place in one, 12-foot-deep location. Whether those denitrification rates are the same in shallower water — or in intertidal areas where oysters are exposed during part of the day — are also unknown.
"We don't know very much about the potential range of denitrification rates," Kellogg said. "We suspect that what we measured in the Choptank is at the high end."
But the intriguing results have stirred support from the National Oceanic and Atmospheric Administration Chesapeake Bay Office, The Nature Conservancy, the city of Virginia Beach and others to help sample additional sites.
A quick look in Virginia's Lynnhaven River suggested a relationship between the density of oysters and their denitrification potential.
Last summer, Kellogg and her colleagues began more rigorous studies on a shallow water reef in Onancock Creek on the Bayside of Virginia's Eastern Shore, as well as on the intertidal zone of the seaside, where oysters are exposed above the water for a portion of each day, then inundated as the tide comes up.
On the seaside, the scientists went to exposed reefs during the low tide and transplanted oysters and the associated reef community onto semi-buried trays. Some trays had zero oysters, some 250; others had numbers in between. Those densities were replicated multiple times in each site.
On the Onancock, trays of hatchery oysters of varying densities were incorporated into shallow-water reefs.
Later, at various times, the trays were planted, crews returned and latched a plastic cover on top of the trays, capturing the reef above, as well as all the water around the reef. They were transported to the VIMS Eastern Shore Laboratory, where the nitrogen removal in the water could be measured.
But the oysters in the Onancock, which initially showed promising results, died for unknown reasons. Results from the intertidal reefs are still being examined, Kellogg said, but show denitrification there "is going to be a way more complicated story" — perhaps not surprising for a reef that spends a portion of each day out of water.
The fate of the Onancock oysters provides a cautionary note about overly relying on oysters for nutrient control. Relatively few oyster restoration projects in the Bay succeed, and when they are lost, most of the denitrifying benefits are lost, too. "We know that oysters being there, and being alive and functioning, is critical," Kellogg said.
She noted that oysters don't prevent nitrogen from getting into the water; they only remove it once it's there. Water quality in the Choptank tends to be worse upstream of areas with potential oyster habitat. Bringing back oyster bars would be of little help in improving those upstream areas.
"Nitrogen has to be in the water before oysters can remove it," Kellogg said. "So that nitrogen has already had an impact upstream from the oysters."
And the scale of restoration needed to make a significant impact is far beyond anything that's been accomplished so far in the Bay. The Bay's largest oyster restoration project, now under way in Harris Creek, a Choptank tributary, is expected to cover only 300 acres and could cost $30 million.
But, Kellogg said, denitrification by oyster reefs could provide a backstop to help keep nitrogen from reaching the mainstem of the Bay where it is a major contributor to large zones of oxygen starved water in the summer. "Oyster reefs can serve as sort of a safety net before that nitrogen hits the mainstem of the Bay," she said. "But you should primarily appreciate oyster reefs for other things, like habitat."
Diseases may be tied to sudden decline in water quality
While their potential role in future Bay restoration is unclear, the recent work highlighting the denitrification potential of oyster reefs might shed light on how the Chesapeake got to be in such bad shape, said Jeffrey Cornwell, a scientist with the University of Maryland Center for Environmental Science.
When nitrogen enters the Chesapeake, it spurs the growth of algae blooms. When there's more algae than can be consumed by fish and zooplankton, the excess sinks to the bottom where it is decomposed by bacteria in a process that draws large amounts of oxygen from the water.
Water monitoring data shows that in the 1980s, the amount of oxygen-starved water in the Bay began to increase relative to the amount of nitrogen entering the estuary. Put another way, each pound of nitrogen on average resulted in a slightly greater amount of oxygen-depleted water than had been the case prior to the 1980s.
Exactly why things began getting worse isn't clear. But Cornwell noted that the number of oysters in Maryland waters dropped sharply in the 1980s as they were hit by two diseases, MSX and Dermo, which decimated the population.
The loss of the oysters, and their denitrification capacity, could explain why water quality suddenly started getting worse, even as pollution inputs stayed roughly the same. "This seems to connect disease, potentially, to major environmental change," Cornwell said. "It's compelling in terms of timing."
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