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Point of View: Dissolving our ocean bounty

Mark Green
By Mark Green
June 01, 2010

The effects of ocean acidification first became clear to me as a graduate student working in Long Island Sound in the early 1990s. I was looking at sediment samples, counting foraminifera - very small, single-celled organisms with shells made of the same calcium carbonate mineral as clams and oysters. I noticed that there were almost none of the little calcifiers in any of the samples taken during the winter. It wasn't that they were moving away, they were actually dissolving in the acidified sediment. [Scientists] were shocked.

This was big news. At the time, general scientific consensus was that calcium carbonate only dissolved in the deep ocean water; few believed this was chemically possible in near-shore waters. The study I published (along with several of my students) was one of the earliest to point to the effects of acidification on animals with small shells. And after showing that it was happening to foraminifera, we asked the same questions about other calcifiers, like clams and oysters.

It turns out that death by dissolution is one of the main causes of mortality in young larval clams in many bays and estuaries in the Northeast, and likely many other coastal regions worldwide.

Lab experiments, and more importantly, field observations, show the dissolving effects of ocean acidification are happening right now. And the problem is getting worse. Much of the carbon dioxide ( CO 2 ) released into the atmosphere from tailpipes and smokestacks ends up in the oceans, where it reacts to form carbonic acid. This acid eats away at the chalky shells (calcium carbonate) of many marine organisms.

I've photographed juvenile clams whose shells dissolved when they attempted to settle in mud that was too acidic. Clam larvae will dissolve in seawater with a pH of just 7.5, lasting as little as three days before they die (pH is a measure of acidity where a lower number means greater acidity).

In sediments, it is not uncommon for juvenile bivalves to settle into mud with even lower pH (7.0-7.4), where dissolution happens fast. The world's oceans as a whole aren't expected to reach those levels for another 100 years, but conditions more acidic than a pH of 7.5 already occur in some places.

Bay scallop, Eastern oyster and hard clam larvae all suffer delayed metamorphosis, slower shell growth and higher rates of mortality (in some cases a 10-fold increase) when the water has a pH of 7.8. Waters upwelling along the West Coast are well beyond that point already, causing serious problems for oyster hatcheries in the Pacific Northwest.

The trouble with acidification is that there's still a lot of uncertainty about how bad it will be as ocean acidity increases. Studying the effects in the Pacific Northwest and in bays and estuaries of the Northeast offers insight into where the oceans worldwide might be headed.

These two regions are already affected by high levels of CO 2 in upwelling water and sediments, respectively. It's difficult to predict exactly what will happen, but we're certain that as more and more CO 2 enters the ocean it will continue to become more acidic. My research is part of a growing body of scientific work that clearly shows the effects on animals (including some without shells) could be dire.

The only solution is prevention. It comes down to lowering emissions to a 
level that will no longer drive oceans toward acidification. Limits need to be based on sound scientific research so that marine 
ecosystems and fisheries remain viable. But it's important to keep in mind that there's no silver bullet. There will be tradeoffs with a global shift to lower CO 2 emissions, but those tradeoffs are worth it when you consider the alternative.

We can no longer afford to ignore the acidification of our oceans. It jeopardizes the ability of the oceans to support many of the organisms that we rely on 
for food.

We don't have the luxury of waiting to see what happens. We also don't have the luxury of letting this lapse into a political debate that loses sight of the real risk. Too much CO 2 in the atmosphere is irreparably damaging our oceans. We need to do something about that.

 

 Mark Green is a professor of environmental science at Saint Joseph's College of Maine. His research focuses on the effects of ocean acidification on larval and juvenile stage bivalves. He also has a small oyster farm, Peaks Island Shellfish Co., in Casco Bay, Maine. 

 

 

 

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