The National Oceanic and Atmospheric Administration and U.S. Geological Survey recently published a study proposing that more oysters could significantly improve Potomac River estuary water quality. But the idea is unrealistic.
A 1946 paper states: “...in the late 1800s it [the oyster harvest] averaged approximately 1,600,000 bushels.” Given 300 market-size oysters per bushel, 480 million oysters were harvested annually. Then the harvest crashed and in recent years it has rarely exceeded 5,000 bushels.
We can never restore the ecosystem to its condition of the late 1800s. Given urbanization and chemically fertilized fields and lawns, we can never return to late 1800s nutrient levels.
One million market-size oysters contain, at most, 150 kilograms of nitrogen, with sub-equal amounts in the shell and dry tissue. Even if we could harvest 480 million oysters again, only 72,000 kg of nitrogen would be removed (150 kg nitrogen/ million oysters per 480 million oysters).
Today, the Potomac River receives about 30 million kilograms of nitrogen each year. So the maximum oyster harvest ever recorded could only remove 0.2 percent of today’s nitrogen load (72,000/30,000,000).
This is meaningless! If we take into account nitrogen removal from the ecosystem by denitrification and sediment burial, oysters remove about same amount of nitrogen from the ecosystem as harvesting does. But if oyster shells are returned to the water to serve as a substrate for more oyster strike — as they certainly should be — the nitrogen in the shell is not removed from the ecosystem. The required annual harvest remains preposterous.
The authors of the study state: “There is increasing recognition, however, that returns on investment in both point– and nonpoint-source controls are diminishing, that additional management will not lead to significantly greater reduction in nutrient loads.”
They are certainly correct for point source discharge from wastewater treatment plants, which has been reduced about as far as can reasonably be expected. But despite all the money that has been spent, Chesapeake Bay water quality has not improved. The reason for this sad fact is that, according to the EPA’s final Chesapeake Bay Total Maximum Daily Load, agriculture is responsible for nearly half of the Bay’s nitrogen load. Nothing substantive has changed in the way crops are fertilized.
Pollution always needs to be reduced at the source, and “sop-up” strategies (oysters in this case) are never effective. The authors are incorrect about nonpoint source pollution because they ignore “additional management” that would ban the disposal of poultry litter, sewage sludge and manure by land application. This practice causes half of all agricultural pollution, more than is derived from point source discharge from wastewater treatment plants. It could be stopped with the stroke of a pen and the waste used for biofuel. The authors also fail to advocate the phasing in controlled (timed–, slow–) release fertilizers that would increase the efficiency of nitrogen uptake by crops from about 65 percent to at least 80 percent (it is 30 percent for sludge).
Grow more oysters for the right reasons — they create habitat for other organisms and make great meals. Delusions about their role in improving water quality merely detract from the only action that will improve Bay water quality: significantly improving crop fertilization efficiency.