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Oysters can’t improve Chesapeake Bay water quality
Most people know that when Europeans first arrived, oysters could filter a Bay-sized volume of water in a few days. Today, given the few oysters that remain, it takes a year. Filtering makes the water more transparent, allowing light to penetrate, but clearer water is not the most important issue. Could more oysters significantly improve Chesapeake Bay water quality by removing the nutrients nitrogen (N) and phosphorus (P) from the marine ecosystem? Scientists have agreed for over half a century that the Bay’s abysmal water quality is caused by pollution from excessive concentrations of N and P, mostly derived from inefficient crop fertilization.

The word “restoration” is purposefully being avoided. Given today’s acreage in heavily fertilized fields and lawns, extensive urbanization and an increasing population, we can never return to pre-Colonial nutrient discharge. It is impossible to restore the North American grassland prairies, just as it is impossible to restore the Bay ecosystem, or the Bay’s oyster population. Ecosystems can be improved, but he word “restore” should never be used.

Pollution must always be reduced at its source. A “sop up” strategy, while continuing to pollute, is never effective. What is the magnitude of the Bay’s nutrient pollution problem? EPA’s final TMDL requires that of the 250 million pounds of N delivered to the Bay each year, 56% of agricultural origin, 62 million pounds of N discharge must either be stopped or “sopped up” annually. That amount may or may not be sufficient to improve water quality satisfactorily.

Removing oysters and their shells from the water has been advocated as a “sop up” strategy. A recent study (DOI 10.1007/s10498-014-9226-y) proposed that oysters can significantly improve water quality in the Potomac River estuary. I disagreed (DOI 10.1007/s10498-014-9235-x). The Potomac River can be used as a surrogate for the entire Bay. A paper published in 1946 states “…in the late 1800’s it [the oyster harvest from the Potomac River] averaged approximately 1,600,000 bushels.” Given 300 market-sized oysters per bushel, 480 million oysters were harvested annually for a few years. Then the harvest crashed and in recent years it has rarely exceeded 5,000 bushels. One million market-sized (3 inch) oysters contain at most 150 kilograms of N, with sub-equal amounts in the shell and tissue. Today, the Potomac River receives about 30 million kilograms of N each year. Today’s oyster harvest only removes 0.00075% (225 / 30,000,000) of the nitrogen load. Even if we could harvest 480 million oysters annually again, only 72,000 kg of N would be removed from the marine ecosystem (480 million oysters * 150 kg N per million oysters). The maximum oyster harvest ever recorded, which is unlikely to be attained again, could only remove 0.2% of today’s N load (72,000 / 30,000,000).

In the absence of oysters, tiny algae (phytoplankton) proliferate because of the excessive amounts of N and P discharged into the Bay. They and other small organisms die or are eaten, settle out of the water and are deposited as organic material in the bottom sediment. Between about a third and half of that organic material is buried as sediment, effectively removing the nutrients from the marine ecosystem. Microbes decompose the remaining organic material, consuming dissolved oxygen and releasing ammonia (NH3) and phosphate back to the water column to nourish more algal growth, especially in summer. In the case of N an additional process takes place, denitrification, where nitrate (NO3-) is converted to diatomic N gas (N2) and removed from the marine ecosystem. Denitrification takes place everywhere there is anoxic sediment, or sediment rich in organic material that contains no dissolved diatomic oxygen gas (O2).

How do oysters alter these widespread processes? Oysters filter algae and other particles out of the water, extract nutrition from a small part of it, and discard the remainder as pseudofeces, or relatively large particles that settle quickly. Filtration makes the water clearer, allowing light to penetrate more deeply.

Some people claim that an oyster can filter 50 gallons of water each day. Although this sounds like a lot, it is trivial when the volume of Chesapeake Bay is quantified. The area of the Bay is 4479 square miles and the average depth is 46 feet. This translates into 43,000,000,000,000 (4.3 x 1013) gallons of water. If one oyster can filter 50 gallons per day, then it takes 860,000,000,000 (8.6 x 1011) oysters to filter all the water in the Bay every day. The maximum oyster harvest, at about the turn of the last century, was 8 million bushels, or only 2,400,000,000 (2.4 x 109) oysters. Even if we could resurrect oyster numbers to the days when Europeans first sailed into the Bay, the water would be no clearer because the current pollution load from fields and infrastructure is very much larger than it was from forests with a few scattered Native American settlements.

The pseudofeces particles that settle out of the water as the result of oysters’ feeding activities contain more organic material than typical sediment, including mucus from the oysters. Because the organic material is very reactive, all the processes of microbial decomposition increase, including ammonia release and denitrification.

Denitrification has proven to be difficult and contentious to quantify. Most scientists would agree with a statement by one author that the N removed by harvesting oysters “…is about 2/3 of the amount estimated to be removed annually as a consequence of the oysters’ normal feeding and deposition processes.” Denitrification and burial in sediment remove about the same amount of N from the water as does harvesting the oysters, so denitrification is, like harvesting oysters, not a way to remove significant amounts of N from the marine ecosystem.

Everybody agrees that oyster shells should go back into the water because they are the best substrate for more oyster strike. Given that the half-life of shell is less than a decade because of bioerosion and dissolution in increasingly acid water, there is no significant N removal by shell returned to the water.

How does N removal from the marine ecosystem by oysters compare to N pollution from crop fertilization practices? Given a chemical fertilization rate for grain of about 150 pounds of N per acre and a “N Use Efficiency” of about 65%, 100 pounds of applied N fertilizer are sequestered in the harvested grain. The remaining 50 pounds are released to the environment. Assuming that roughly half the 50 pounds of N not sequestered in the grain ends up in the Bay, and that the N removed from the ecosystem by harvesting oysters and by their “normal feeding and deposition processes” (sediment burial and denitrification) are about the same, growing twenty acres of grain causes about as much N pollution (500 pounds) as the amount of N removed by a million oysters (660 pounds). Given the many millions of acres used to grow grain in the Bay watershed, the previous conclusion is confirmed. “Sopping up” N pollution by harvesting oysters cannot measurably improve Bay water quality given such massive N pollution.

All the money we have spent upgrading wastewater treatment to reduce point source nutrient pollution has not resulted in significant improvements in Bay water quality. All the money we are now spending on stormwater, a smaller source of pollution, can’t possibly improve Bay water quality measurably. We must stop deluding ourselves (or being deluded) that anything other than a focused effort to significantly increase the efficiency of crop fertilization can have any measurable effect on Bay water quality. Two good places to start are to require that disposal of animal waste (poultry litter, sewage sludge and manure) by land application, which causes about half of agricultural nutrient pollution, be either banned or strictly phosphorus-based, supplying the phosphorus needs of the crop, and no more. And second, we must begin to replace conventional chemical fertilizers with controlled- (timed-, slow-) release products in order to increase conventional fertilization efficiency from about 65% to at least 80% (it is 30% for sludge). Neither of these changes would significantly impact the profitability of the average farm if controlled-release fertilizers were manufactured on an industrial scale.

All authors who have advocated nutrient removal by oysters temper their conclusion with statements like “We agree that traditional land-based nutrient-management measures should be continued. Much can be done to improve reductions from land-based sources, particularly from nonpoint sources…” and “It is important to recognize that oyster reef restoration is not a substitute for reduction in land-based inputs to the system” and “Consequently, natural and aquaculture-reared stocks of bivalves are potentially a useful supplement to watershed management” and “shellfish (oyster) aquaculture could remove eutrophication impacts directly from the estuary through harvest but should be considered a complement – not a substitute – for land-based measures.”

The authors of the paper previously cited responded to my criticism of their study (DOI 10.1007/s10498-014-9233-z) with many caveats and a failure to understand how Bay pollution can be reduced by changing the way crops are fertilized. In my opinion, money should be spent reducing pollution, not subsidizing oyster farming.

Some folks on Cape Cod are advocating growing oysters to “solve” their pollution problem, largely from septic systems. In my view, this is also unrealistic. Upgrading septic systems or installing small wastewater facilities that include tertiary treatment is the only solution to the pollution problem. A human excretes at most 4 Kg of nitrogen (N) per year. There is about 150 Kg N in a million oysters, so to “sop up” the N pollution each person excretes would require harvesting roughly 25,000 oysters (90 bushels) each year and not return the shells to the water. That is obviously preposterous, just as is true in Chesapeake Bay. Again, if money is to be spent, use it to reduce pollution.

We need to stop wasting time advocating that growing oysters is a “water-based solution” to Chesapeake Bay’s impaired water quality. Reference to oysters’ insignificant role in improving water quality merely detracts from the only meaningful action that will improve Bay water quality, namely, significantly improving crop fertilization efficiency.

As an example, Jim Perdue, acting as a self-serving “Merchant of Doubt,” advocated growing oysters in an attempt to deflect criticism of poultry litter as a major source of Bay pollution. My response, published in the Baltimore Sun pointed out that harvest of 140 million bushels of oysters (at the turn of the last century, the maximum harvest was about 8 million bushels) would be required just to offset the nitrogen pollution caused by the land application of poultry litter in Maryland. Never mind all the other sources of Bay nitrogen pollution!

How to grow (“garden”) oysters
Growing (“gardening”) oysters not only provides great meals, but oysters in cages create habitat for many other organisms. If fertile (diploid, or having two chromosomes) oysters are grown, then larvae can be produced. When males and females are “ripe” in late spring or early summer, and one animal starts to release eggs or sperm, other oysters are triggered and gregarious (simultaneous) spawning can occur. Large numbers of fertile oysters contained in floats or cages encourages this strategy, just as was once true of oysters concentrated on reefs.

Choosing a method of containment is the first step, and there are three in common use: 1) bottom cages, 2) floats and 3) hanging a cage from a pier. Whatever the choice, the goals are to minimize flow obstruction (oysters grow fastest in strong currents) and maximize ease of handling and the necessary maintenance. Bottom cages are probably the worst choice because they sink into muddy bottom sediment and they are heavy and hard to lift out of the water when full of adult oysters. Oysters grow fastest in floats and many kinds have been devised. Taylor floats are rectangular PVC “rings” that support wire baskets. Flip-floats are wire “boxes” with two opposed floatation cylinders so they can be inverted to aid in cleaning. Floats must be tied properly to allow for tidal excursions (including storm tides!) and must be kept from bumping against pilings. Large floats are heavy when full of adult oysters, and some people use “cranes” or their boat lifts. Other people get in the water to tend or harvest the floats (not fun in winter), or float them to shore. Floats are unsightly to some people. Any kind of wire or plastic cage can be hung from a pier, horizontally so the oysters are spread out and don’t all end up at one end, and above the bottom and out of the mud. Small cages are easy to lift but they don’t hold many oysters. In winter, the torpid oysters must remain in the water. Most will survive as long as they are not disturbed and do not experience sub-freezing air temperatures, encasement in ice or burial in anoxic mud.

Most cages or baskets are constructed of welded, galvanized, vinyl coated wire, with a spacing of about one inch. This is fine for adult oysters, but smaller “seed” oysters must be supported by something with smaller openings. Bags or flattened cylinders of strong plastic are available in many mesh sizes and can be used as liners in the wire baskets or cages. The gardener must decide what size seed to buy. The smallest seed commonly available is 1/4 inch and typically sells for about $25 per thousand (about one full cup). It is important to plan ahead because 1000 seed oysters will grow into about 3 bushels of market-sized (3 inch) animals. The fine-mesh needed to retain 1/4 inch animals will foul quickly and reduce current flow. In order to avoid cleaning the bags frequently and to minimize the number of mesh sizes needed, some people choose to start with larger seed.

Oysters are notorious for growing at different rates, so it is important to move the large ones into as coarse a mesh as possible as they grow and separate them from the small ones. Sorting needs to be done every month or so during the growing season (April through November) if high growth rates are desired. If 1/4 inch seed is purchased, at lease one, and preferably two, coarser mesh liners should be used, and mesh sizes that would retain 1/4 inch, 1/2 inch and 3/4 inch animals would be good choices. The density should not exceed 3 deep if high growth rates are desired. Another reason for monthly maintenance is to remove crabs. Blue crabs can get through the mesh and then molt, grow larger and become unable to escape. If they are not removed, they can eat every oyster less than about 2 inches in size. The gardener should plan on maintenance at least every month or so, cleaning the mesh of fouling (air-drying in the shade for half a day can help), removing crabs and sorting the oysters into larger mesh. 1/4 inch seed oysters should grow large enough to eat after two summers at all but very low-flow sites.

Another decision a gardener must make is whether to grow fertile (diploid) oysters or sterile (triploid, with three chromosomes) animals. Because sterile animals do not reproduce, energy is not required to develop (watery) reproductive organs in summer and all their energy is expended on growth. In summer the tissue of sterile animals is not as watery as is true of fertile animals. If the gardener intends to eat oysters in summer, sterile animals are a good choice. Otherwise, grow fertile animals so they have a chance to reproduce. Although fertile animals are slightly less “meaty” than sterile animals, the oyster industry has thrived for centuries selling fertile animals. Some people believe that fertile animals taste better in late fall because they have “fattened up” after reproducing so they can survive through the winter.

It makes more difference where oysters are grown and how much they are maintained than what kind of oyster is grown. The results of a study comparing the growth of different strains of oysters can be found in the Winter 2013 Newsletter of the Tidewater Oyster Gardeners Association (TOGA) at www.oystergardener.org. There is also lots of additional information about growing oysters at this Virginia-based organization’s web site.