Ocean Acidification: Eating Away at Marine Ecosystems

by Haley McKey

When you think of climate change, you think of more heat waves and droughts, extreme weather, and melting ice caps.  But there’s another problem caused by carbon dioxide emissions, one which is less familiar to us, but no less catastrophic for our planet: ocean acidification.

How it happens:

Carbon dioxide (or CO₂, its chemical formula) is released by the burning of fossil fuels and the clearing of forests and natural areas.  Most CO₂ goes into the earth’s atmosphere, but some is absorbed by our oceans; in fact, about a quarter of the carbon dioxide emitted every year.  That comes to at least 5 million tons of CO₂ absorbed every day.

Algae and marine plants take up some of this CO₂ for photosynthesis, just as land plants do.  But a large amount of CO₂ simply dissolves into surface seawater.  This is what causes ocean acidification.  When CO₂ in the air is absorbed by the ocean, it bonds with water to form carbonic acid, the same stuff that gives an acidic bite to carbonated water and soda pop.  Like all acids, it releases positively charged hydrogen atoms, leaving behind the bicarbonate ion. The problem is that many marine creatures make their shells from a substance with a slightly different chemical composition – calcium carbonate – and the bicarbonate formed by the extra acidity is useless at best, and downright dangerous at worst.

Crustaceans like this sargassum crab need calcium carbonate to fortify their shells. Photo credit David S. Lee

How it threatens wildlife:

When the oceans have more bicarbonate and less carbonate, this interferes with the healthy growth of a variety of organisms, like mollusks (oysters and clams) and crustaceans (crabs, shrimp, lobsters and tiny krill, which are very important to the whole marine food web).  Mollusks take in calcium carbonate, a molecule in ocean water, and excrete it over their bodies to form hard, protective shells.  Crustaceans also use calcium carbonate to fortify their exoskeletons.  But human activities have increased the acidity of the ocean by almost 30% making it more difficult it is for these creatures to create their natural armor. Even tiny zooplankton, the building blocks of marine food chains, need calcium carbonate and cannot grow properly in acidic seawater.

Coral reefs may be in even more trouble.  Corals secrete calcium carbonate skeletons over their bodies to protect them, just like mollusks and crustaceans do.  Ocean acidification can cause corals to grow more slowly, and it is estimated that at current rates of increasing acidity, corals will no longer be able to lay down skeletons by 2150.

 Profound Effects:

The animals directly harmed by acidification may be small, but the effects are far-flung.  Baleen whales like the endangered Atlantic right whale depend on krill and plankton as a food source.  We humans have built whole economies around shellfish, not to mention the hundreds of fish species we eat that depend on them for food, too.  And as coral reefs die, the diverse and unique ecosystems they support can collapse.

Ocean acidification pulls the rug out from under marine food chains and coastal economies.  The only way to stop it is to make serious cuts to carbon emissions worldwide.  If not, it won’t be long before species begin to disappear and many ocean systems collapse completely in their absence.