Oysters and clams clean up dirty water
Scientists have found a clever and tasty solution to beat pollution: Eat it.
From an ill-lighted, paper-strewn office in the Marine Policy Center at the Woods Hole Oceanographic Institution, researcher Hauke Kite-Powell is refining an innovative method to clean up the nitrogen that is polluting New England’s coastal bays and inlets. Best of all, the solution he has chosen is inexpensive, readily available, and perfect with either chardonnay or sauvignon blanc.
“Shellfish are by far the most cost-effective strategy to control pollution,” says Kite-Powell. “It’s about maximizing all the variables to come up with a good policy.”
“Variables” is a bit of an understatement. Not only must a technology be effective and cheap, it must also be acceptable to the public. Of course, residents don’t have a problem with oysters and clams, but shellfish farms compete for space with recreational uses such as boating, swimming, and fishing. Calculating these costs and benefits means that Kite-Powell has some difficult numbers to nail down: How much space is needed to grow how many shellfish, and what amount of nitrogen will be removed?
Examining the problem
Any attempt to deal with nitrogen pollution must begin with an understanding of where it originates. Kite-Powell’s test site is Waquoit Bay, which lies east of Falmouth, Mass. Averaging only 4 feet (ft) in depth and about 500 hectacres (ha) in area, the bay drains a 3800-ha watershed. Kite-Powell estimates that only 25% of the bay’s water is exchanged with every tide. Since 1938, the amount of nitrogen entering the bay has more than doubled from 10,900 to 24,300 kilograms (kg) per year. The source of the pollution has also changed. In 1938, 77% of the nitrogen deposited into the water came from air pollution; however, as people built houses in the bay’s watershed and tourism increased, that dropped to 30%.
Today, the major source of nitrogen emanates from a different source—local residents flushing their toilets (48%).
“We could put everyone on central sewer systems or upgrade the septic systems so that they don’t send out pulses of nitrogen,” Kite-Powell says, but the expense is prohibitive. “Or we could change people’s application of lawn fertilizer, but that’s hard to do. And a big problem is the air deposition, and that’s something you can’t do anything about locally.” What shellfish provide, he says, is a way to clean water cheaply that can be regulated locally.
The genesis for this idea occurred with the invasion of zebra mussels, a native of the Caspian Sea region, into the Great Lakes. Today, these pests are altering the natural ecology of the lakes by displacing native species. What also struck scientists was the mussels’ ability to filter nutrients and the resulting effect on the water. “The water clarity shot up,” says William Walton, a fisheries and aquaculture specialist with Woods Hole. He points to studies showing that clarity increased in some parts of the lakes from 6 inches to 30 ft. “The converse example is the Chesapeake Bay, where the harvest records show that we are now at one-one-hundredth of the stock levels of shellfish that used to be there [in the 1700s]. The idea goes that because of this, we now have a muckier, greener estuary than in the past.”
Running the numbers
But how many shellfish are needed for a place like Waquoit, and how much space will they require?
Some of the work Kite-Powell is relying on to guide his research has come from the lab of Michael A. Rice, a professor of physiology and population ecology at the University of Rhode Island. He has calculated that every 100 oysters harvested for food also remove 28 grams of nitrogen from the watershed. Or as he puts it, “5000 oysters equal the nitrogen input from one tourist.”
Shellfish farmers have used these data to lobby for more close-to-shore water acreage for oyster and clam farms. Shellfish farms are regulated by local ordinances but may have to fight for space with wealthy residents who have shoreline vistas. These farms are usually hidden below the waterline, with only a few floating buoys, but at low tide much of the gear becomes exposed, creating aesthetic issues.
“You can only grow so many oysters in the bay, or you may reach a limit to what people will tolerate for use,” says Kite-Powell. “We estimate that for that bay, we could devote 6% of the area and get rid of half the nitrogen increase.” That means 6000 kg less nitrogen in the bay, a decrease that should stop the annual algal blooms that occur during the summer. Kite-Powell also expects that increasing the size of the shellfish farms will result in an economic benefit to the local economy even after the costs of lost recreation are subtracted. When he completes his study of Waquoit Bay, he anticipates that it will serve as a model for other beach communities looking for innovative tactics to keep their waters clean.
“It’s very expensive to do pollution reduction at the source,” says Kevin Kroeger, a geochemist with the U.S. Geologic Survey’s Center for Coastal and Watershed Studies. “The idea is that shellfish would be an additional means to reduce the pollution, not a solution in itself.”
Oysters and clams do their part by filtering out the algae and small plankton floating in the water, he says. “We do our part by eating the oysters.”
