Since the onset of the Industrial Revolution 200 years ago, carbon dioxide levels in the atmosphere have increased by 35 percent. During this time, the world’s oceans and waterways also have been soaking up excess carbon and growing more acidic in the process. For oysters, clams, scallops and other shellfish with calcium carbonate shells, this increased acidity may have dire consequences, causing their shells to grow more slowly, grow not at all, or, in some cases, begin to dissolve.

Already under siege from overfishing, disease and poor water quality, the oyster population in the Chesapeake Bay today stands at 2 percent of what it was in colonial times. How oysters will respond to the growing acidification of the oceans in coming decades and what impact their demise could have on the environment remains a critical question for biologists. Oysters serve as filters for the Bay, and their reefs provide habitat for juvenile crabs and fish.

“We’re going to see acidified oceans and coastal ecosystems no matter what we do,” says Ecologist Whitman Miller of the Smithsonian Environmental Research Center on the Chesapeake Bay. “The $64,000 question is: What will the biological and ecological response be to these changing conditions?”

Larvae response

In a laboratory filled with small three-liter aquaria, each of which has been inoculated with 15,000 microscopic oyster larvae, Miller has begun tackling this question. He is feeding and caring for the oysters as he studies their response to varying levels of CO2 in their water.

Some of Miller’s aquaria contain water at levels of CO2 that reflect current atmospheric concentrations. The other aquaria contain more CO2, some at concentrations anticipated 50 years from now and others at concentrations anticipated 100 years from now. Miller is carefully monitoring the oysters’ growth, the amount of calcium in their shells and their rates of survival.

A few hours after emerging from their eggs, oyster larvae look a lot like tiny adult oysters with a thin shell and beating cilia that propel them through the water. They spend three to four weeks as free-swimming larvae, feeding on tiny algae, before settling down to begin the rest of their lives attached firmly to some hard surface.
During this larval phase, oysters incorporate aragonite, a specific crystalline form of calcium carbonate that they extract from the water, into their shells. As adults, they continue to build shell, but by using calcite, a slightly different form of calcium carbonate.

For shellfish, the problem begins when CO2 dissolves in seawater and creates carbonic acid that is rapidly transformed into carbonate and bicarbonate ions in the water. Increased acidity caused by higher CO2 levels tips the balance toward bicarbonate formation and away from carbonate. Less carbonate in the water means the oysters have less material with which to build their shells. If the water is acidic enough, oyster shells will even begin dissolving.

Already vulnerable

Miller is interested in the early life stages of oysters because he wants to know more about the vulnerability of these bivalves during this very crucial point in their life cycle. He intends also to investigate acidification’s effect on adult oysters and other calcifying species, eventually taking his experiments into the field.

“Perhaps in the future, bivalves and other calcifiers will begin to lose out to competition with noncalcifiers,” he says, “or interactions of low pH [higher acidity] with known stressors like disease will conspire to attack an already vulnerable organism.”

This may already be happening in the Chesapeake Bay, he says. “Oysters are holding on, but we know disease and overfishing have troubled them.” Miller suggests that higher levels of CO2 in the water may already be having an impact on oyster populations, but it is a subtle impact that scientists have yet to identify clearly.

Teasing apart how the gradually rising levels of CO2 absorbed into the water from the atmosphere is impacting coastal ecosystems presents quite a challenge. These environments are extremely complex, and many factors affect them. In addition to the CO2 that bays and estuaries absorb from the atmosphere, nutrient runoff from land and carbon input from marshes and many other sources contribute to the problem.

“Most researchers [working on impacts of atmospheric carbon dioxide] are looking at pelagic systems—open ocean systems—where carbon dioxide input and outputs are somewhat simpler and conditions are much more constant—as in, no complicating inputs from land, etc.,” Miller explains.

In other areas of the oceans, “experts are already seeing a grim picture of what acidification can do to corals, mollusks, foraminifera and some plankton. In some cases, they’re seeing the dissolving of coral structures,” Miller says. “We know we will see a loss of biodiversity along with loss of critical habitats created by coral reefs, but what about estuaries? Nobody’s looking.

“The Smithsonian Environmental Research Center is an ideal place to begin looking at acidification of estuaries because of our already rich history in long-term studies on the impacts of carbon dioxide on plant and marsh communities,”Miller adds.

When combined with the decades of environmental research that the Environmental Research Center has focused on atmospheric CO2 and its effect on the Chesapeake Bay watershed, “extending our CO2-global change studies into the water is a natural step, and one that is urgently needed.”