Why are oysters important in estuary ecology?

In a 1988 scientific paper, Roger Newell, a researcher at the University of Maryland Center for Environmental Science, dramatized the importance of oysters to Chesapeake Bay based on the fact that a single adult oyster can filter up to 200 litres of water a day.  He made the then-astonishing argument that a century before, oyster stocks could have filtered the entire Bay in less than six days. In 1988, the Bay's considerably depleted oyster population would have taken more than 300 days to do the same. Today's further diminished stocks would likely take even longer. Recent work undertaken by researchers at the Australian National University point to an identical  situation in Port Stephens, though the reasons for oyster depletion appear to differ somewhat. The following article is especially relevant for future planning in Port Stephens.

Though Newell's calculations are based on a number of simplifying assumptions, his argument seized the attention of many in the Chesapeake region and has been instrumental in changing the way we think about oysters in the Bay. His paper has been cited in countless articles on the ecological value of oyster restoration; it has been used by journalists, nonprofit organizations and educators and has led to major changes in Bay oyster policy.

In Maryland, for example, where oysters are still largely managed for the commercial fishery, their ecological role is now recognized as an important thrust of resource management and replenishment activities.

In its Chesapeake 2000 agreement, which covers goals for the next decade, the Chesapeake Bay Program the multi-state and federal Bay restoration effort has called for a minimum tenfold increase of oysters by 2010. Can this be done? And if so, what will it actually mean for sustainable oyster populations for the fishery, let alone for the ecosystem?

Bay oysters, after all, are still plagued by MSX and Dermo, parasitic diseases that have been devastating oysters and limiting many attempts at oyster restoration. While research efforts show much promise in developing disease-tolerant strains, getting these oysters into the Chesapeake in large enough numbers over the next decade will require a considerable investment.

It will also take more a clear idea, says Newell, of what the goals of oyster restoration are. Just planting oyster reefs to meet the tenfold goal may not be enough. To make use of the oyster's filtering capacity, we need to develop a strategic plan, he says, one that identifies those areas in the Bay where oysters and reef habitats can provide the most benefit by filtering algae from the water. "So many shellfish feeding on algae," says Newell, "can help improve water clarity," and if we choose our locations wisely, he adds, "those oysters could also help in restoring submerged bay grasses."

It is the Bay's dark, murky waters a combination of dense phytoplankton growth and suspended particles, the result of nutrients, shore erosion and the scouring of bottom sediments by waves and currents that prevent light at the surface from reaching the bottom waters that grasses inhabit. Submerged aquatic vegetation (SAV) throughout the Bay system is limited by light it is the major reason that grasses now cover less than 15 percent of the bottom than they did 50 years ago.

By integrating plans for oyster reef restoration with nutrient reduction efforts on land, it may be possible to use oysters for bringing SAV back. Placed in the "right areas," reefs could not only offer grass beds protection from pounding waves, but would help clear nearby waters, enabling sufficient light to reach the grasses.

Oysters and Algae

Others besides Newell had recognized the great potential of oysters for clearing algae from the Chesapeake and what their loss has meant for water quality. Max Chambers, a commercial aquaculturist on the Eastern shore, argued that "the Bay is polluted because the oysters are gone." In a paper written in 1988, he called for "bolstering the [oyster] filter feeders in the Bay for the purpose of 'from the bottom-up clean-up'" and went on to quote a retired biologist from the UMCES Chesapeake Biological Lab, Klaus Drobeck who, in 1970, told him, "If for no other reason, we need to breed and reproduce oysters just to keep the Bay clean. Are we to think an oyster's only value is for food?"

Newell's assumptions have been subject to criticism because they did not take into account important factors that could have significantly altered his conclusions. For example, oysters of course release feces, pellets of nutrient-rich excreta that could be chemically recycled back into the water for uptake by algae; they also produce pseudofeces, pellets of undigested particles that include sediment. What is the fate of these biodeposits? Do they remain inert in the sediments? Do microbial processes recycle them into the water for uptake by algae so that, in effect, there is no ecological gain? Over the past five years, Newell and his colleagues at the UMCES Horn Point Lab have been working to answer these questions.

With support from Maryland Sea Grant, Newell, along with researcher Jeff Cornwell, has conducted laboratory studies to examine how the recycling of nutrients in oyster biodeposits would affect the production of algae and, therefore, water quality. The answer, says Newell, depends on whether oxygen is present (oxic conditions) or absent (anoxic conditions) in the bottom environment where oysters dwell.

"If we choose our locations wisely, those oysters could also help in restoring submerged Bay grasses."   Research has shown that low oxygen conditions are further exacerbated by bacterial decomposition of the dense algal growth that results from large volumes of nutrients running into the Bay. A century ago, perhaps, oyster stocks in the tributaries and along the flanks of the Bay might have consumed a vast majority of these algae; today, with so few oysters, much of this production remains uneaten. Ungrazed algae eventually die and sink into bottom waters; there they are metabolized by bacteria and other microbes, a process which further depletes oxygen.

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If oxygen levels are already low because of natural conditions, these bacterial processes can be the coup de grace, reducing all that remains. When sediments become anoxic, anaerobic microbial processes kick in, releasing ammonia, a form of nitrogen that algae can readily take up to sustain their further growth. If bottom waters are oxygenated, however, aerobic bacterial process occur, releasing nitrate and nitrite rather than ammonia. Such conditions foster denitrification, a microbial process that removes a proportion of nitrogen (20% in mesocosm studies done by Newell and Cornwell) by reducing the nitrate and nitrite into harmless nitrogen gas that algae cannot use. Rising through the water column this gas then escapes into the atmosphere.

According to laboratory studies undertaken by Newell and Cornwell, for oysters to clear algae from the water and not be responsible for recycled nutrients from their excreta, oxygenated sediments are key. "It's location, location, location," says Newell. If we are restoring oysters for ecological purposes, he says, we have to begin by choosing areas in which bottom waters are not starved for oxygen.

Bay Grasses Caught in a Catch-22

While too many nutrients and too much sediment from land runoff continue to be the bane of improving Bay water quality and restoring underwater grasses, turbid waters also result from sediment resuspension by currents and waves as well, says Evamaria Koch, a Horn Point scientist who has been studying the relationships between water flow and submerged vegetation.

A half century ago, when vast underwater fields of eelgrass, widgeon grass, coontail and many other species flourished throughout the Chesapeake, they protected sediments from being resuspended and coastlines from eroding. Leaves swaying in the water attenuated wave energy and slowed water currents, says Koch. Suspended particles settled out from the water. Today's diminished grass beds are often unable to dampen the wave and current energy that can tear up vegetation.

Grass survival is even more tenuous when epiphytic organisms (microscopic plants and animals) colonize grass leaves, further shading them from sunlight. Unless epiphytes are grazed, these combined assaults on grasses leave them weak and unable to obtain the light they need to survive.

Dense epiphytic growth on leaves and high suspended sediments in the water can also have a synergistic effect on reducing light availability to plants. Meredith Guarraci, a former student of Koch's, showed that as the epiphytic layer increases, so does the particle accumulation on the leaf surface. The combination of excessive epiphytes and suspended particles leads to even less light at the leaves than if the problem was only with nutrients. Scientists believe that for submerged grasses to flourish once more, they must grow densely in a large area so that they can help themselves by creating the light field they need to continue growing. But herein lies the catch.

In restoring underwater grasses so that they can affect water quality, plants need good enough water clarity to begin with. It is for this reason that stopping landborne nutrients and sediments at the sources on land before they get into the water has been the Bay Program's key strategy in trying to bring Bay grasses back. The Chesapeake Bay Agreement of 1987 called for slashing nitrogen and phosphorus by 40 percent (from 1985 levels). According to water quality models, curbing landborne nutrients at these levels will lower algal production sufficiently to have a host of positive feedbacks, especially for submerged vegetation. However, a key problem with this scenario is that slashing nutrients by 40 percent, as well as preventing erosion and runoff, is proving to be extremely difficult. And without the grasses already in place, says Koch, bottom sediments are eroded by tidal currents, waves and of course storms.

Stopping Runoff May Not Be Enough

There are many source reduction programs throughout the watershed that aim at stemming runoff from vegetated buffer strips, including trees, to sediment traps during new construction to no-till farming to improved waste treatment plants. And more are coming, such as nutrient management plans that all farmers must submit to the state by 2003 and nonpoint management plans mandated by EPA that will set total maximum daily loads (TMDLs) for each river.

Still, despite all these runoff reduction programs, there may still be too many nutrients and sediments to stop at the sources this is especially likely as development in the Bay watershed continues, and with it the conversion of permeable pasture and forested lands to impermeable built and paved structures.

Furthermore, airborne deposition can dump a good deal more nitrogen into the Bay system than was originally thought in the 1980s as much as 30 percent of the total in any given year. Resource management agencies in the Chesapeake have no control over many of these sources some portion of which originate in power plant exhaust towers in the midwest, others from automobile exhaust from a number of locations that come down in rain and snow on the east coast. It is for these reasons that the Chesapeake Bay Commission and the head of EPA shifted from a commitment to reducing nutrients by 40 percent to reducing controllable sources of nutrients by 40 percent.

If restraining runoff was not enough of a problem, the diminished natural capacity of the Bay's wetlands to absorb sediments and nutrients is another. Extensive losses are the consequence of sea-level rise and, again, development.

"Oysters and grass beds together could be crucial in doing what source reduction alone cannot do."       Disappearing wetlands may be having an immense impact.  Marsh vegetation such as tall grasses serve not only to trap sediments in land runoff, they remove nitrogen in groundwater as well.

If the Bay is to be returned to a semblance of its former integrity, grasses must flourish once more any hope of successful restoration depends on controlling runoff. But controls may not be sufficient, especially in areas where water clarity is so dismal that grasses cannot even get started. That is where oysters come in, says Roger Newell. "Oysters and grass beds together could be crucial in doing what source reduction alone cannot do."

Oysters and Bay Grasses to the Rescue?

Monie Bay is part of the National Oceanic and Atmospheric Administration's National Estuarine Research Reserve System, a program that partners the federal government with states in order to protect estuaries that will also serve as natural field laboratories for programs of research and education.

Though underwater grasses were once abundant in Monie Bay, they began disappearing in the late 1960s, according to studies by Robert Orth and Ken Moore at the Virginia Institute of Marine Science; today, they are virtually all gone. Oysters, too, were once abundant but they also fell, first to overharvesting and then to disease. Is there a connection between the loss of grasses and the loss of oysters? Newell and Koch believe that there may be a strong interdependence.

Monie Bay is surrounded by agricultural land and therefore receives a good deal of runoff. We think that when oysters were abundant, Newell says, their filtering capacity helped clear the water of algae so that grasses got enough light to grow, reproduce and help maintain sufficient clarity of water; however, with the decline and eventual loss of oysters, the waters darkened as a result of algal growth. With shore erosion already high, they believe, algae and suspended sediments together just shut off light from reaching the bottom. Underwater vegetation didn't stand a chance turbidity was simply too high.

They hypothesize that grass beds in Monie Bay declined in part because of the loss of oyster populations and their filtering capacity; they also hypothesize that as grasses declined in extent and density, they were less able to trap sediment, which thereby "permitted" larger amounts of sediment from shore erosion and bottom resuspension in the water column.

Because Monie Bay has no underwater vegetation, Koch is comparing turbidities and particle suspension in an unvegetated area with an adjacent area that has submerged grasses; these comparisons will enable her to measure the positive feedback that grass beds have on water clarity. At the same time, Newell is conducting experiments in the lab that measure how oysters feeding under different environmental conditions affect water clarity. Working with Hood, their goal is to develop a mathematical model that will quantify the actual increase in light penetration based on the biomass of oysters. "The model," says Hood, "will predict increases in light penetration resulting from oyster feeding." There are numerous examples worldwide of bivalves such as oysters, clams and mussels feeding on algae so voraciously that they significantly improve water clarity.

If the research in Monie Bay proves successful, it will provide resource managers with a predictive technique to link water clarity with oyster biomass and grass density for restoring vegetation. Still, models are tools they are not the real world. What will it take to put them to work so that we can see some demonstrable results?

We have to scale up sufficiently, says Newell. "We can do that," he says, "though it will take a good deal of funding." More importantly, he says, "although scientists are now beginning to understand that oysters were once a keystone species in the Chesapeake, the challenge for Maryland managers and politicians is to implement actions that will actually increase the abundance of oysters for their ecological value." A willingness by many to begin exploring just what such new thinking might mean for the future of Bay oysters and for the Chesapeake itself, which  takes its name from a native American tribe called Chesepiooc, an Algonquin word that means Great Shellfish-Water People.