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Carbon Dioxide Pollution Bulks Up Blue Crabs

JustinriesAcidification of the world’s oceans by carbon dioxide pollution could destabilize an age-old evolutionary arms race between predators and prey. Blue crabs could grow larger, while they creatures they eat, including oysters and clams, could have weaker shells and slower shell growth, according to Dr.Justin Ries, a marine geologist at the University of North Carolina, Chapel Hill.

“One of the oldest predator/ prey relationships in the ocean is between shell crushers and shell builders --for example, between lobsters and crabs, and their prey, clams, oysters, and mussels,” said Ries, author of reports on the impact of acidification on marine life.

“My experiment suggests that the predators are going to become stronger from the C02 based ocean acidification…while the prey become weaker,” Ries said in a recent interview.

The background is this: The world’s oceans are changing, with waters today about 30 percent more acidic than they were two centuries ago, before the Industrial Revolution, according to the National Oceanic and Atmospheric Administration. Scientists say this is because the burning of coal, oil, and gas released a huge amount of carbon dioxide into the atmosphere that was absorbed by the water.

The chemical reaction between this pollutant -- C02 -- and water creates carbonic acid, according to Dr. Ries and other researchers.

To discover how different forms of life will react to ocean acidification, Dr. Ries performed an experiment by growing 18 different species in laboratory tanks with varying levels of carbon dioxide.  The result: the shells of crustaceans, including blue crabs, grew faster and larger in lab tanks with higher carbon dioxide levels.

Meanwhile, creatures that crabs eat—such as clams and oysters – suffered because of the pollution, with their shells becoming thinner and more fragile and growing more slowly.

Will a C02-heavy future spawn supercrabs?  No, Dr. Ries said, because crabs with faster-growing shells may have significant flaws. All the extra shell growth in water with high levels of carbonic acid could be consuming energy crabs could use to build muscle, he said.

“So even though they build stronger shells, they’ve got less meat in them,” Dr. Ries said. “And if you’ve ever had blue crabs, you know the worst thing is a large crab without any meat in it.”

Moreover, if acidification ends up with the deaths of more clams and oysters that crabs eat, the crab populations could eventually crash, following a decline in populations of their food species, Dr. Ries said. Along with climate change and sea-level rise, this potential impact on the global ecosystem is another important reason the creation of federal laws or regulations to restrict the emissions of carbon dioxide into the atmosphere, Dr. Ries said.

Too much nitrogen and phosphorus pollution from sewage plants, fertilizers, and stormwater runoff can also cause low-oxygen zones that kill the clams and other prey that crabs eat, according to a Chesapeake Bay Foundation report.

The influence of acidification on the oceans and Chesapeake Bay ecosystem was the subject of a recent series of articles in Chesapeake Quarterly and a public radio program this morning on WYPR 88.1 FM in Baltimore.

MillerDr. Whitman Miller, a scientist at Smithsonian Environmental Research Center in Edgewater, Maryland, is using a monitoring device to study the carbon dioxide levels and acidity in the Rhode River, a Chesapeake Bay tributary.

Dr. Miller said some studies have suggested that acidic water dissolves the shells of some tiny drifting organisms: plankton.

 “Things like terrapods, sea butterflies, are very important at the base of the food chain,” said Dr. Miller.  “And so if we lose those, we are in real trouble.”

Without this plankton, some fish would be deprived of food.

The effects of acidification on the oceans, however, is complex and sometimes counter-intuitive. Dr. Miller recently co-authored a report with Dr. Thomas Arnold of Dickinson College and colleagues that found sea grasses grow faster with more carbon dioxide, which is true of some plants on land. But the researchers found that seagrasses exposed to extra carbon dioxide lack important natural chemicals called phenolic substances that protect them from disease and predation.

“Even though the grasses are growing faster, their chemical defenses are lowered,” making them vulnerable, Dr.  Miller said.

And there is also this disturbing possibility: Higher carbon dioxide levels in the atmosphere and waters could mean more explosive algal growth, because algae consumes C02 like any other plant, Dr. Ries said. “When algae die off, they rapidly decompose, and that can cause anoxic environments, as the decomposing matter consumes oxygen in the water column.”

It is not yet clear what acidification will mean for the Chesapeake Bay.  This is in part because acidity levels in estuaries vary widely with the rain and tides, Dr. Miller said.

However, the die-offs of juvenile oysters in West Coast hatcheries caused by acidification have raised potential future concerns for oyster aquaculture and restoration in the Chesapeake, Miller said.

“With these sort of massive failures in these hatcheries (in the West), people were very concerned and didn’t know what the problem was,” Dr. Miller said. “And only in the last two or three years have these hatcheries actually begun looking at the chemistry of the water.”

When scientists examine the issue, what they see is that the basic chemistry of the oceans appears to be shifting because of what humans are pumping into the sky. It is as if we’ve turned Earth’s basins into petri dishes in a vast experiment with fossil fuels. Soon the results of this experiment will roll in with the tide.

By Tom Pelton

Chesapeake Bay Foundation

(Photo at top of Justin Ries by Tom Kleindinst of Woods Hole Oceanographic Institute. Photo of Whitman Miller by Tom Pelton)





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