If the ocean had a doctor, she might want to prescribe some oysters to treat local outbreaks of ocean acidification. New research shows that groups of oysters could actually help prevent coastal waters from becoming more acidic, one problematic result of increasing levels of carbon dioxide emitted from burning fossil fuels.
When carbon dioxide dissolves in the oceans—which absorb about one-fourth of the carbon we burn each year—the water becomes more acidic. This is a problem for shellfish, coral and other creatures that create hard shells that are weakened by acid.
Oyster shells store calcium carbonate, a base (and the active ingredient in the antacid Tums).
A study published in the April issue of the journal Ecology found that in pre-colonial times there were roughly 100 times more oysters in the Chesapeake Bay and that these animals' shells likely helped regulate the acidity, or pH, of the bay, according to George Waldbusser, a marine ecologist at Oregon State University and study co-author.
He tells TakePart these oysters, and their shells, were about 100 times better at regulating the bay's pH. "I was surprised by the degree to which they could be contributing to the buffering capacity of water body," Waldbusser says.
The shells keep things basic by releasing their calcium carbonate when they dissolve, which counteracts acid created by carbon dioxide dispersed in the ocean. Shells have a half-life of about three to 10 years, according to the study.
Oysters take up carbonate released into the ocean by rivers. Waldbusser says that although rivers still supply carbonate, there are far fewer oysters now to hold on to it, take it up and keep it around.
Shells also foster the regeneration of oyster reefs, since they are the optimal spot for larvae to land, anchor and begin growing (oysters breed by releasing sperm and eggs into the water, where eggs are fertilized and become free-swimming larvae that quickly try and find suitable locations to settle down).
This is one reason why oyster shells had been planted in the Chesapeake by the state of Maryland to replenish oyster reefs from the 1960s until about 2005, when funds for the program ran out. Shells were obtained from dredging of marine sediments—but there are far fewer shells buried under the seafloor these days.
Waldbusser reports that in the southern Chesapeake Bay, acidity has increased three times faster than the open ocean. One reason for that is the proliferation of algal blooms, fed by fertilizer runoff and nutrients that spill into the bay.
During strong blooms, the tiny plantlike cells—which convert carbon dioxide and water into sugars and oxygen, via photosynthesis—use up all the carbon dioxide in the water.
But they don't stop there, and instead suck more of the greenhouse gas into the water from the atmosphere. When these algae ultimately die and break down, they release their carbon contents into the water, making it even more acidic than it was prior to the blooms.
Oysters are filter feeders, taking sediment and other substances out of the water column. When oysters are established in new areas, they can quickly improve the clarity of the water, allowing light to reach the bottom, which has a whole host of other good outcomes, like giving light to sea grass meadows.
While this ability is well-known, their buffering ability was previously less well-understood, and is likely about as significant, according to the study.
Oysters also create unique micro-environments where bacteria can convert nitrogen wastes into nitrogen gas, which then bubbles to the surface and is released into the atmosphere (which is, of course, mostly nitrogen).
Waldbusser said that shells may help create "hotspots where things are good, even if the surrounding waters aren't favorable." This affect is mostly local. "I don't think they'll fix the global ocean acidification problem. But I wouldn't let that discourage people—there are a lot of other great benefits to oysters."