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Reconsidering the Oyster

Reconsidering the Oyster

The delicious mollusk may also play an important role in restoring water quality.


If you eat, you’re probably increasingly aware of the environmental cost of the food that becomes your dinner: the methane associated with meat, the runoff associated with crops, the overfishing, the erosion. 

What if there were a crop that, instead of creating pollution, consumed it? Instead of despoiling the environment, improved it? And fed people in the bargain. Kind of like agricultural alchemy, almost literally a free lunch.

Consider the oyster.

Oysters are filter feeders. Filters, in their ordinary household context, remove impurities from whatever it is you’d like to be pure: water, oil, air. What oysters do for seawater isn’t far removed from what your Brita system does for tap water. 

Here’s how. (Be prepared; intimate details of digestion are never pretty.) 

The water comes in and runs over the oyster’s gills, which pick up the particles and let the water, particles removed, exit the animal. Some of those particles (grit, sand) aren’t edible, and get diverted out of the digestive system and excreted as pseudo-feces, a poop-like substance. The particles that are edible (algae and miscellaneous detritus) get eaten, and the remains excreted as actual, genuine feces. (Everybody poops.) Both the undigested and the digested remains are encased in mucus, and end up on the sea floor.

And here’s the important bit: In clearing the water of particles, the oysters also clear it of the nitrogen that’s bound up in the algae and detritus. When it comes to water quality, excess nitrogen is the enemy (in saltwater, that is; excess phosphorus is the culprit in fresh water).

When we think of water pollution, we tend to think of petroleum products or industrial solvents. But according to Ken Moraff, director of the EPA’s Office of Ecosystem Protection for New England, “Nitrogen and phosphorus,” harmless-sounding nutrients, “are the number one water quality problem in New England.” You’d think “nutrient pollution” would be an oxymoron, but it’s not.

Moraff explains that nitrogen feeds algae, and the algae create a variety of problems. Some, like beach closings, happen when the algae are alive. But the biggest problems happen when the algae die. “They release noxious gases. They decompose and suck up all the oxygen and you have fish kills. We’ve had massive fish kills in Narragansett Bay,” he says. If the oyster eats the algae, you nip that in the bud.

Nitrogen and phosphorus get into the water in a variety of ways. In places that are primarily agricultural, the major source is runoff from farms. Two years ago, algae blooms in Lake Erie forced the city of Toledo, Ohio, to warn its citizens not to drink the water, and the Des Moines, Iowa, water utility is suing three upstream counties, charging that they allow too much runoff from farm fields.

Closer to home, it isn’t always agriculture. My husband and I have a small oyster farm on Cape Cod, where the problem is people, not plants. Most Cape homes aren’t connected to sewer systems, and backyard septics slowly leech nutrients into the soil (everybody poops). From the soil, those nutrients make their way to the water—a process that can take decades.

Water is our business here on Cape Cod. People come here to look at it, fish from it, swim in it, boat on it, and they pay big money for the privilege. As our water goes, so goes our economy. “Everything that makes the Cape special is in danger of being lost,” says Moraff. And there are other coastal New England communities that can say the same. 

If the problem is nitrogen—and it is—and oysters filter out nitrogen—and they do—are we looking at a solution on the half-shell?

To figure that out, we have to take a closer look at what happens to nitrogen the oysters ingest.

Lisa Kellogg, a scientist at the Virginia Institute of Marine Science, studies just that, and explains that one of three things happens to every bit of nitrogen the oyster takes in. First, it can be incorporated into the meat or the shell of the oyster, where it’s trapped until the oyster either dies or is taken out of the water. Second, it can be released as ammonia (everybody pees), in which case it goes straight back into the water. 

The fate of nitrogen in the third option is a bit harder to pin down. When it’s excreted as feces or pseudo-feces, the key question is where it goes from there. In the best of all possible worlds, it gets broken down by bacteria and released as nitrogen gas, which bubbles up through the water and is released harmlessly back into the air, which is mostly nitrogen to begin with. But, for that process—called denitrification—to occur, there has to be a fortuitous combination of aerobic and anaerobic bacteria. 

Absent that combination, the feces and pseudo-feces either sink into the bottom, where the nitrogen can be buried in the sediment (a win, as long as it stays buried), or they break down and the nitrogen is released back into the water (not a win). 

When it comes to measuring all that nitrogen, there’s one number that’s pretty solid and widely agreed on: When you take an oyster out of the water, about 0.5% of its dry weight is nitrogen, which translates to about 0.2 grams of nitrogen for an oyster in the three- to four-inch range. Hard numbers on all the other nitrogen measurements are elusive.

That’s the backdrop against which scientists, regulators, oyster farmers and elected officials are asking the question: Can oysters make a meaningful contribution to removing nitrogen from our coastal waters?

The answer is a maddening, frustrating, irritatingly predictable “It depends.”

The list of things it depends on is as long as the list of variables in aquatic habitats. Sea floor, tidal flushing, salinity, temperature, pathogens, you name it. Some of those variables are better understood than others, but everyone I talked to stressed that oystering, like politics, is local. Scientists are running experiments up and down the East Coast to try and pinpoint what makes a successful oyster operation, and how much nitrogen that operation can remove, but those experiments tell us only a limited amount about how that same operation will perform somewhere else. 

Nevertheless, there’s firm scientific ground for optimism that oysters can help our nitrogen problem in some places. Firm enough that the EPA is making a substantial investment in oyster-related projects in New England. While the EPA’s Moraff acknowledges the uncertainties, he says the agency “wouldn’t be investing $400,000 if we thought it was a hare-brained scheme.”

As the EPA, academic institutions and municipalities experiment with oysters, one of the biggest questions they’re trying to answer is whether reefs (replicas of natural oyster ecosystems) or aquaculture (farmers growing oysters in bags and cages) is the more effective nitrogen solution. Because nothing in this realm is ever simple or clear-cut, of course each system has advantages and disadvantages.

Let’s take reefs. Before Europeans landed on our shores, oyster reefs were everywhere. Now there are none, or almost none, and the idea of restoring them has a visceral green appeal. Creating a new reef is a tricky business, but if it succeeds, it has benefits beyond the filter feeding of the oysters themselves. 

Curt Felix is a member of the Cape Cod Water Protection Collaborative, and has been involved in creating reefs in Wellfleet Harbor, where 95–99% of the natural reefs are gone. “If you restore an oyster reef, all sorts of other organisms move into that zone,” he says. The reef is habitat for fish, barnacles, worms, sea squirts and other sea life, and many of the reef denizens also filter water and contribute to nitrogen removal. In one two-acre restored area in Wellfleet, says Felix, they’ve seen a 70% reduction in nitrogen.

Kellogg is one of the scientists who have measured the denitrifying potential of oyster reefs, and she says that the highest rate measured experimentally is 56 grams of nitrogen per square meter per year. Usually, the rate is lower than that. Sometimes, it’s negative, if the reef affects the mix of bacteria in the sea floor below it so that denitrification is actually reduced. Just as a point of comparison, the nitrogen contained in the oysters on that square meter (131 of them) is in the neighborhood of 30 grams.

The denitrifying rate is just one of the uncertainties in reef restoration. To establish a reef, you need something called “cultch,” hard material that oyster larvae will choose to settle on. Usually, it’s oyster shell, although other materials can work, too. Since most sea floor doesn’t have that, the reef restorer has to put it there. That sounds straightforward, but soft bottoms and shifting sand can sometimes bury the restorers’ handiwork. That shifting sand also poses a risk to established reefs. In Wellfleet, Felix says, they had a reef going well for three years, but a stormy winter ended up burying half of it (he has hopes for its recovery). 

Burial isn’t the only threat. There’s disease (two oyster parasites, MSX and dermo, are imports, and didn’t exist in the heyday of oyster reefs). There’s failure to thrive (without a robust wild population producing larvae, it’s hard for young reefs to grow). And there’s poaching (oysters are delicious). 

Enter aquaculture, in which farmers introduce young oysters to the water, grow them to maturity and then take them out to sell. An oyster farm doesn’t have all the advantages of an oyster reef. For starters, it doesn’t create the same kind of ecosystem. According to Ray Grizzle, professor at the University of New Hampshire’s School of Marine Science and Ocean Engineering, although some farming systems provide a similar level of “nooks and crannies that fish and invertebrates can live in,” others don’t. An oyster farm also doesn’t replicate natural reefs but relies on equipment that some find unsightly. And, of course, aquaculture requires allocating a public resource—the water—for private gain.

When it comes to denitrifying, an oyster farm’s effect is hard to predict, according to Kellogg. In some cases it will increase, but in some it will decrease, and the variables aren’t well enough understood to predict with confidence which is more likely. It’s maddening.

When it comes to money, though, aquaculture brings a farmer with skin in the game. Reefs, the public has to pay for. Farms are moneymaking enterprises, and the farmer makes the investment in the expectation of profit. Every year, farmers put out oyster seed (what baby oysters, bought from a hatchery, are called). Every year, they harvest mature oysters, and we know that, as noted, 0.5% of each and every one of them is nitrogen. If you grow 500,000 oysters on an acre (a reasonable density), and they take two years to reach maturity, that means you take about 200 pounds of nitrogen out of the water every two years. If that same acre were a reef with the very highest level of denitrification, it would remove 540 pounds per year. But it might remove nothing at all. Or it might remove nitrogen for a year or two, and then die.

Because a farmer is actively managing the site, there’s more certainty in aquaculture than in reefs. Although growing oysters is by no means a sure thing (my husband and I know this first-hand, and have the P&L to prove it), oyster farmers have a history of taking harvests out of the water, year after year. Aquaculture also has ancillary benefits, which should help justify the allocation of public resources for private gain. Oyster farms create jobs and pay taxes, and the industry has an impact on the local economy when growers buy equipment, supplies, waders, trucks, boats, gas, seed and an endless series of gloves, which oyster-handling destroys almost immediately. 

Even so, it’s easy to see the appeal of a reef. The public pays for it, and the public reaps the benefits in that reef’s ecosystem services. The oysters are left in the water to reproduce and keep those services coming. But Grizzle is quick to disabuse anyone who dreams of restoring reefs to pre-Columbian abundance. Today’s oysters face diseases unknown in these waters in reefs’ glory days. “Oysters are dead after three to five years,” says Grizzle. Which means that the reef has to get a natural set (a wild population of the next generation) within that time for the reef to survive. “Our reefs are hanging on,” he says. “They’re low density, but making it.”

Kellogg is more optimistic about reefs farther down the coast. MSX and dermo don’t survive in less salty water, and some Chesapeake reefs are being sited with that in mind. “In lower salinity waters you can get oysters that live longer,” she says, but points out that those sites may make recruitment—getting larvae to settle and grow on the reef – more difficult. 

Kellogg and Grizzle, and everyone else I talked to, agreed that neither reefs nor farms alone are optimal, and that some combination is probably best.

Which brings us back to the original question: Can oysters—in some combination of reefs and aquaculture—remove enough nitrogen to make a dent in the problem? 

In some places, probably not. The more water there is, the more oysters it takes to filter it. Grizzle points out that an average oyster filters 10 to 50 gallons a day (in summer—oysters go dormant when the water is cold), and, where he grows oysters, in Little Bay, New Hampshire, it would take an awful lot of them to replicate the filtering of the original wild reefs, which cycled through the entire bay’s volume “in a week to a month,” he says. “In most cases, the amount of water that an oyster farm or population of farms filters is a small percent of what’s in the bay.” 

In other places, however, it just might work. One study done by NOAA and the EPA looked at four Massachusetts estuaries and estimated that nitrogen could be reduced up to about 15% by cultivating oysters on 0.5 to 1% of the estuaries’ area. In Mashpee, Massachusetts, according to Moraff, there’s one area that the town is trying to clean up solely with oysters and clams. But that’s clearly not the norm. “The nitrogen problem is so severe in some places that we’re probably going to have to pull out all the stops,” he says. “Oysters aren’t going to solve the problem. No one thing will solve the problem. We need to do everything.” Kellogg, too, emphasizes that putting filter feeders in the water doesn’t mean we can ignore land-based nitrogen management. Decreasing what goes in is critical. 

As more coastal communities look to shellfish to help solve this problem, another difference between reefs and farms comes to the fore: public attitudes. It’s hard to find anyone who’s anti-reef, but not everyone loves an oyster farm.

It’s easy to understand why. If you owned a multimillion-dollar home with a beautiful beachfront, would you take it in stride if oyster trays, or floating gear, appeared 50 feet off your shoreline, and farmers showed up at every low tide? I’m not sure I would, and I’m an oyster farmer. 

Bob Rheault is a scientist, oyster farmer and head of the East Coast Shellfish Growers’ Association, and he says conflicts between shell fishermen and upland landowners have a long history.

Many years back, he says, a fight between soft-shell clam diggers and landowners got ugly. “The diggers were peeing off their boat and throwing f-bombs, but the homeowners were worse,” says Rheault. “They were shitting in a bucket of clams, buzzing the diggers with outboards, throwing cherry bombs.” And, although it doesn’t usually get quite that bad, Rheault has “seen this dynamic play out time after time.”

The problem is more than just user conflicts, he says. “It’s a social issue. We don’t drive yachts, we don’t wear the right gear, we don’t go to the right cocktail parties. We’re not their kind of people.” And there isn’t a cultural acceptance of aquaculture as part of the landscape. “In France, you see million-dollar homes with an oyster farm 50 feet off the beach. Nobody complains because it’s part of their culture.”

For shellfish farming (oysters, but also clams, mussels and seaweed) to expand in New England, it needs to become part of our culture, and there are signs that aquaculture’s ability to solve the nitrogen problem—which is everyone’s problem—might help with that. In my town we’re seeing attitudes change, and some opposition soften. 

Anyone who lives near New England’s coastline has seen the effects of nutrient pollution first-hand in the form of algae blooms and fish kills. Some of the tools we can use to solve the problem, like water treatment plants, are very expensive. Others, like composting toilets, are very unpopular. As we start to understand how, and how much, nitrogen can be removed by reefs and aquaculture, it may be time to reconsider the oyster. 

Dovetail Sake

Dovetail Sake

Portuguese Kale Stew with Oysters - courtesy of Edible Boston

Portuguese Kale Stew with Oysters - courtesy of Edible Boston