Study highlights under-appreciated benefit of oyster restoration (2024)

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by David Malmquist, VIMS | May 23, 2013

Oyster reefs shown to buffer acidic inputs to Chesapeake Bay

Scientistshave identified many benefits for restoring oyster reefs to Chesapeake Bay andother coastal ecosystems. Oysters filter and clean the water, provide habitatfor their own young and for other species, and sustain both watermen andseafood lovers.

A newstudy co-authored by Professor Roger Mann of William & Mary's VirginiaInstitute of Marine Science adds another item to this list of benefits—theability of oyster reefs to buffer the increasing acidity of ocean waters.

The study, “Ecosystem effects of shell aggregations and cycling incoastal waters: An example of Chesapeake Bay oyster reefs,” appearsinEcology, the flagship journal of the Ecological Society of America.It is co-authored byGeorge Waldbusser of Oregon StateUniversityandEric Powell of the Haskin Shellfish ResearchLaboratory at Rutgers University.

Concernsabout increasing acidity in Chesapeake Bayand the global ocean stem fromhuman inputs of carbon dioxide to seawater—either through the burning of fossilfuels or runoff of excess nutrients from land. The latter over-fertilizesmarine plants and ultimately leads toincreased respiration byplankton-filtering oysters and bacteria. In either case, adding carbon dioxideto water produces carbonic acid, a process that has increased ocean acidity bymore than 30% since the start of the Industrial Revolution.

Amore acidic ocean concerns marine-life experts, who cite its corrosive effectson the calcium carbonate shells of oysters, clams, and other mollusks, as wellas its possible physiological effects on the larvae of fish and other marinecreatures. At current rates of increase, ocean acidity is predicted to doubleby 2100.

TheEcologypaperreports on the research team’s efforts to calculate past and present shellbudgets for Chesapeake Bay, with a goal of estimating how effective healthyoyster reefs might be in moderating ocean acidity, and whether today’s depletedreefs can withstand future acidity increases.

“Oystershells are like slow-dissolving TUMS in the belly of Chesapeake Bay,” explainsMann. “As ocean water becomes more acidic, oyster shells begin to dissolve intothe water, slowly releasing their calcium carbonate—an alkaline salt that buffersagainst acidity. An oyster reef is a reservoir of alkalinity waiting to happen.

Theteam’s calculations suggest that in 1870—before people began large-scaleharvesting of oyster meat and shells from the Chesapeake—the amount of oystershell exposed to Bay waters was more than 100 times greater than today, with anequally enhanced capacity to buffer acidity.

“Ourdata show that that oyster reefs likely played a key role in the pH budget ofpre-harvest Chesapeake Bay,” says Mann. “The amount of carbonate in the shellsof living oysters at that time was roughly equal to the total amount ofcarbonate dissolved in the modern Bay. If similar numbers of oysters were alivetoday, they could take up about half of the carbonate that rivers currentlycarry into Bay waters.”

Manypeople are familiar with the notion that the cloudy waters of the modern Baywould be clearer if over-harvesting and disease hadn’t drastically reduced theoyster population and its capacity to filter particles from the water. Mannsays, “Our study suggests a similar loss of ecosystem function, but in terms ofbuffering acidity rather than improving water clarity. This has significantecological ramifications, but hasn’t really been on anyone’s radar screen.”

Returning oyster shells toBay waters—a practice that began in earnest in the 1960s to restore reefs forfood and filtering—has helped buffer acidity in the Bay, but to nowhere nearhistorical levels. Today, scientists estimate that the Bay loses 100 millionbushels of oyster shell each year to harvesting and corrosion in Marylandwaters alone, despite the return of 20-30 million bushels of shell throughdredging and restaurant recycling.

The study by Mann and his colleagues estimates that oysters nowcontribute only 4% to buffering of acidity baywide, whereas they wereresponsible for 70% of all baywide buffering in 1870.

Lookingtowards the future, the team’s concern is that oyster reefs in the modernBay—fewer and smaller than their pre-harvest counterparts and featuring smalleroysters—may be unable to keep pace with the increasing acidity of Bay waters.

“Theshells of dead oysters degrade rapidly in estuarine environments,” says Mann,“with a half-life of only 3 to 10 years. For a reef to maintain the structureneeded to support future generations, oysters must grow fast enough and largeenough so that their rate of shell production exceeds that of shelldegradation.”

Theoptimal rate of shell addition, says Mann, “occurs with larger, older animalsthat contribute more shell carbonate per mortality event.” But, he adds, “theonset of diseasehas unfortunately reduced the life span and maximum sizeof Bay oysters, thus compromising the shell budget.”

“What’sworrisome about this is that the shell reservoir is getting smaller andsmaller,” says Mann. “Could we reach a tipping point where increasing acidityso overwhelms the decreased buffering capacity of dead shells that it thenbegins to significantly affect live oysters, further limiting their ability toadd shell to the alkalinity buffer? If so, we could end up with a negativefeedback loop and a worst-case scenario."