by Stephanie Smith
August 24, 2017
At the Chesapeake Biological Lab (CBL) in Solomons, Maryland, dozens of blue crabs are traveling into the future.
Not literally, of course—each of the crustaceans is safely nestled in one of the fish tanks that fill the lab where Hillary Lane Glandon conducts her work. But many of those blue crabs are experiencing the living conditions of the future, when climate change is predicted to significantly transform life in the Chesapeake Bay.
Warming temperatures are perhaps the best-known effect of climate change expected to alter the waters of the nation’s largest estuary. Less discussed is a process known as ocean acidification, or a gradual rise in the water’s acidity due to an increase in dissolved carbon dioxide.
“Though we don’t know it, we probably all have seen what happens when carbon dioxide dissolves in water,” explains Dr. Thomas Miller, director at CBL. Imagine the classic school science fair experiment: a dirty penny is dropped into a bottle of soda; several hours later, the penny is clean.
“It comes out clean because of the acidity that the dissolved carbon dioxide induces in the soda,” Miller continues. “In a sort of less extreme sense, that’s exactly what’s happening with ocean acidification. The carbon dioxide that’s produced by the burning of fossil fuels in our cars, in our power stations, in our heavy industry is released into the atmosphere. And about a third of that finds its way to dissolve in the water, and it changes the chemistry of the water, making it more acidic.”
The response of blue crabs to increased temperatures is fairly well-understood: as cold-blooded animals, the crustaceans are more active and grow more quickly in warmer waters. Less research had been done to understand the impacts of increased acidity on the species. For the past several years, Lane Glandon has been investigating not only how increased acidity could affect blue crabs, but how the combination of a rise in water temperature and rise in acidity could interact.
“One of the reasons that we decided to do both of those things, as opposed to just look at maybe temperature on its own or acidity on its own, is that when you have multiple things in the environment changing, they can interact in a way that’s not really predictable by just looking at one or the other separately,” Lane Glandon explains.
In the Chesapeake Bay, acidity levels—which are measured on a pH scale—can vary, but underwater life in the Bay has evolved to thrive in a specific pH range. In particular, increased acidity may make life more difficult for animals like oysters and mussels, which depend on the compound calcium carbonate to build their shells. In higher acidity waters, there may be less calcium carbonate for these organisms to use, or it may be more difficult for them to use the calcium carbonate that is available.
“Shells may become thinner or animals may grow less fast, because they have to invest more energy in depositing this protective material in their shells,” Miller says. “And if they have to spend energy depositing the material in their shells, they can’t spend their energy investing in becoming bigger, in growing.”
Like oysters, blue crabs build their tough outer shells using calcium carbonate—but in a notably different form.
“Oyster shells are made of calcium carbonate in the form of calcite, which is a very strong, hard material that allows the oyster to be very protected from the external environment,” Lane Glandon explains. “Crab shells also are made of calcium carbonate, but in a different form that has magnesium incorporated into it. This is important for the crab because it allows it to have a hard shell, but also it makes it more soluble so they can molt, which is how crabs grow.”
This difference in the compound’s composition could work in the crab’s favor as acidity levels increase: in the lab, crabs exposed to high acidity actually had more magnesium calcite in their shells. In fact, for the potential effects of increased acidity that Lane Glandon tracked, the blue crabs fared pretty well. Even for crabs exposed to high levels of acidity—8,000 micro-atmospheres, or ten times higher than typical acidity levels—researchers saw no change in growth, no change in food consumption and no effect on oxygen consumption.
“In general, the crabs were pretty resilient to changes we imposed on them,” Lane Glandon says.
In addition to her work in the lab, Lane Glandon worked to combine her research with predictions of future temperature increases in order to learn how conditions for blue crabs may change through the end of this century.
“We concluded that temperature’s going to be the primary sort of environmental factor that’s going to be affecting crabs in the future; acidity may not be such a big deal,” Lane Glandon says. “The question that we wanted to ask was, how might the overwintering behavior of crabs change in our system in the future, as temperatures warm?”
The results weren’t unexpected: wintertime for blue crabs in the Bay will most likely get shorter, decreasing from a current average length of between 100 to 115 days to—in the most extreme warming scenario—just 60 days. The effect this shorter winter may have on the overall blue crab population is unknown, but it could have significant effects across the estuary’s food web.
Essentially, warmer temperatures may allow blue crabs to grow more, be more active and need to eat more food, and ocean acidification may do little to hinder that growth. But animals like oysters, which blue crabs prey on, may be more vulnerable to the effects of climate change and ocean acidification, which could push the typical predator-prey interaction out of balance. And because crabs are a keystone species—meaning their survival is integral to the ecosystem—any change in blue crab populations could ripple not just through the food web, but through the entire Bay ecosystem.
“Changes to blue crab growth or food consumption or how effective they are as predators might have widespread effects on the Chesapeake Bay ecosystem,” explains Lane Glandon. “Not just on crabs and what crabs eat, but on all the other components that are therefore connected to crabs.”