Restoring oyster reefs can buffer ocean acidification

A new study adds another item to this list of benefits of restoring oyster reefs to Chesapeake Bay and other coastal ecosystems—the ability of oyster reefs to buffer the increasing acidity of ocean waters.

The study, Ecosystem effects of shell aggregations and cycling in coastal waters: An example of Chesapeake Bay oyster reefs, was co-authored by Professor Roger Mann of the Virginia Institute of Marine Science (VIMS) and appears in Ecology.

A more acidic ocean concerns marine-life experts, who cite its corrosive effects on the calcium carbonate shells of oysters, clams, and other mollusks, as well as its possible physiological effects on the larvae of fish and other marine creatures. At current rates of increase, ocean acidity is predicted to double by 2100.

The Ecology paper reports on the research team’s efforts to calculate past and present shell budgets for Chesapeake Bay, which they estimated by comparing the amount of oyster shell lost to dissolution and harvesting with the amount produced by the extant oyster population.

Their goal was to estimate how effective healthy oyster reefs might be in moderating ocean acidity, and whether today’s depleted reefs can withstand future acidity increases.

“As ocean water becomes more acidic, oyster shells begin to dissolve into the water, slowly releasing their calcium carbonate—an alkaline salt that buffers against acidity. An oyster reef is a reservoir of alkalinity waiting to happen,” Mann said.

“Our data show that that oyster reefs likely played a key role in the pH budget of pre-harvest Chesapeake Bay,” says Mann. “The amount of carbonate in the shells of living oysters at that time was roughly equal to the total amount of carbonate dissolved in the modern Bay. If similar numbers of oysters were alive today, they could take up about half of the carbonate that rivers currently carry into Bay waters.”

Returning oyster shells to Bay waters has helped buffer acidity in the Bay, but to nowhere near historical levels. Today, scientists estimate that the Bay loses 100 million bushels of oyster shell each year to harvesting and corrosion in Maryland waters alone, despite the return of 20-30 million bushels of shell through dredging and restaurant recycling.

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

“What’s worrisome about this is that the shell reservoir is getting smaller and smaller,” says Mann. “Could we reach a tipping point where increasing acidity so overwhelms the decreased buffering capacity of dead shells that it then begins to significantly affect live oysters, further limiting their ability to add shell to the alkalinity buffer? If so, we could end up with a negative feedback loop and a worst-case scenario.”

FiS.com, 13 May 2013. Article.


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