An abundant oyster population could improve Bay water quality by spurring microbial processes that permanently removing large amounts of nitrogen from the Bay, according to a new study.
While it has long been recognized that oysters were powerful filterers of algae in the water, it is the first study to show that they could also help to remove nutrients in the process, rather than recycling them back into the water as many thought.
A newly published paper by Roger Newell and colleagues at the University of Maryland’s Center for Environmental Science (UMCES), suggests that bacteria in the sediment around oyster bars biologically remove at least 20 percent of the nitrogen in oyster wastes through the same process used in modern wastewater treatment plants.
“It’s mind-boggling what the potential would be if we had a large oyster population in the Bay,” Newell said. “But unfortunately, we don’t.” Indeed, today’s adult oyster population, estimated to be less than 1 percent of historic levels, is at an all-time low and has little impact on the Bay’s water quality.
But oysters are increasingly seen as one of the most important species in the Bay. Their filtering ability clears the water for grass beds, and their reefs provide important habitat for numerous aquatic species.
Recognizing their importance, the Bay Program’s Chesapeake 2000 agreement called for achieving a tenfold increase in the oyster population by 2010. State and federal agencies, nonprofit groups and others are planning to spend as much as $100 million on oyster restoration in the next decade.
But the new findings suggest that — if the Bay had a healthy oyster population — it could play a more important role in Bay restoration than previously thought by removing excess nutrients from the water.
“Heretofore, we have talked about oyster restoration clearing up the water,” said Donald Boesch, president of UMCES. “We knew that was a facet of dealing with the nutrient overenrichment, but the full circle was not closed.”
Earlier work by Newell showed that the oyster once had a tremendous capacity to filter the Bay — he’s the scientist who made the widely cited estimate that the historic oyster population had been able to filter a water volume equivalent to that of the entire Chesapeake in three to six days, while it takes today’s depleted population about a year to filter the same amount.
But filtering algae from the water does not necessarily get rid of the nutrients they contain. Many scientists have speculated that most of the nutrients in the algae were returned to the water, where they would simply fuel more algae growth or other nutrient-related problems.
Newell agrees that many of the nutrients actually digested and excreted into the water by oysters are recycled.
But oysters in today’s algae-filled Bay are sloppy eaters: Most of what they filter from the water is never digested, but is instead formed into pellets known as psuedofeces which are expelled, uneaten, from the oysters and quickly settle to the bottom.
In a study published in the September Journal of Limnology and Oceanography, Newell and his colleagues, Jeffrey Cornwell and Michael Owens, placed sediments from the Bay in water tanks simulating real-world Chesapeake conditions. They loaded the tanks with a paste made of pelletized phytoplankton cells to simulate psuedofeces, and measured changes in nitrogen concentrations.
They found that the sediments in tanks that simulated shallow-water oyster habitat were able to remove about 20 percent of the added nitrogen.
Nitrogen removal takes place as a result of aerobic (oxygen-using) and anaerobic (non-oxygen-using) bacterial processes in the sediment. The aerobic bacteria convert any remaining nitrogen in oyster wastes to nitrate (nitrification), then the anaerobic bacteria converts the nitrate to inert nitrogen gas (denitrification). Nitrogen gas, unlike other forms of nitrogen, is generally not usable by other organisms, including most phytoplankton, and therefore does not contribute to nutrient enrichment problems.
It is the same principle that works in wastewater treatment plants using biological nutrient removal technology where wastewater alternately flows through tanks with aerobic and anaerobic bacteria to remove nitrogen.
The sediments in shallow water around oyster bars contain both types of bacteria. Aerobic bacteria live in oxygenated water and in top sediments, while anaerobic bacteria live in areas just below the sediment surface. In contrast, denitrification does not take place in deeper parts of the Bay because the water is depleted of oxygen, so there are no aerobic bacteria to mediate the crucial first nitrification process.
Fecal waste from other bottom-dwelling bivalves, such as clams, could also spur denitrification, but they would never transfer as much algae to the bottom as oysters, which have much higher feeding rates, Newell said. “It’s truthfully only oysters which have this huge capacity to produce psuedofeces. Other animals do it, but nowhere near this extent.”
The reason, Newell said, is that oysters are uniquely adapted for life in turbid estuarine environments. An adult oyster can filter 2 gallons of water per hour. That was important prior to European colonization, when fewer nutrients entered the Bay, resulting in less algae, the main food source for oysters. As they filtered, the oysters would select algae for ingestion and reject silt in their psuedofeces.
Today, by contrast, the Bay’s nutrient-laden water is filled with much more algae than the oysters need. “But the poor little things haven’t figured that out yet,” Newell said. “They still feed as if there was little food out there, so they are sorting away but are not only rejecting silt, but because their stomach is only a certain size, they are also rejecting a lot of phytoplankton.”
As a result, Newell said, psuedofeces expelled by oysters today contain large amounts of undigested algae, filled with nitrogen that can be acted upon by bacteria in the sediment.
In the paper, Newell suggested that the 20 percent figure could be low because the laboratory experiment removed all burrowing benthic organisms, which feed on biodeposits, from the sediment. The action of those organisms tends to increase the interaction between the aerobic and anaerobic zones, thereby increasing the potential for denitrification. Newell and colleagues are performing follow-up experiments funded by Maryland Sea Grant in the Bay itself.
The findings suggest that if a big oyster population could be restored, it could play an important role in helping to achieve the big nitrogen reductions needed to help clean up the Chesapeake.
Newell and colleagues made a “back of the envelope” calculation suggesting that historic oyster populations in the Choptank River, which once covered about 5,000 acres, might have had the capacity to remove 30 percent of all the nitrogen entering river today — if they were still around.
As it is, today’s population has been devastated by the oyster diseases MSX and dermo. High salinities that have prevailed during recent dry years have allowed the diseases to thrive — scientists believe the population has actually fallen since the tenfold increase goal was set in 2000.
Some scientists believe that major new oyster restoration efforts, including new hatchery efforts that will rear hundreds of millions of oyster spat for release each year, will help spur a recovery. If so, Boesch said, the new results could play into management decisions.
“There may be implications in terms of where we position oyster reefs for maximum effect,” he said. “There may be implications for the architecture of reefs in restoration projects that could make them a greater, or lesser, nutrient sink.”
Newell said the denitrification role of the oyster may also play into the debate over whether the disease-resistant foreign oyster, Crassostrea ariakensis, should be used in the Chesapeake. “Maybe the benefits to other species from improved water quality for sea grasses or whatever is worth the risk of having an exotic species of oyster,” he said.
But even a restored oyster population would not be a “silver bullet” for the Bay’s water quality problems, Newell said. During times of the year, including the early spring when nutrient loads into the Bay can be high, oysters are dormant because of cold temperatures.
Further, there is little oyster habitat in some places, such as the Upper Bay, so there is little opportunity to filter nutrients from places such as the Susquehanna River.
“The way I view it, oysters are a very useful adjunct to other activities, such as sewage plant upgrades and runoff controls, to reduce nutrient enrichment,” Newell said. “You need to do everything you can do to control nutrients on the land, but then, once the nutrients get into the water column, what do you do to get them out? One way may be managing your oyster resource to maximize the survival of oysters just for their filtration capacity, and not so much for their harvest capacity.”
While the Bay region faces an uphill fight to restore its oysters, Newell said other regions could learn from the Chesapeake’s experience.
“I think this is a lesson we all need to recognize,” he said. “We all think that these animals are out there just for us to utilize, but they do have this function in an ecosystem that we mess with at our peril.”