Ocean Acidification

Ocean acidification is a term used to describe a reduction of the average pH in the ocean. pH stands for potential or power of hydrogen with the “p” meaning potential or power and the “H” standing for hydrogen. The pH scale is a common and worldwide scale that is used to rank how basic or acidic a solution may be based on the amount of hydrogen ion activity in the solution.

The pH scale usually measures a substance from 1 to 14. The scale, however, is logarithmic. Each whole pH value below 7 (the pH of pure water) is ten times more acidic than the higher value and each whole pH value above 7 is ten times less acidic than the one below it. For example, a pH of 3 is ten times more acidic than a pH of 4 and 100 times (10 times 10) more acidic than a pH value of 5. A pH near 7 is considered to be neutral.

As concentrations of atmospheric carbon dioxide (CO2) increase, a majority of this CO2 is being absorbed by the ocean, as over 70 percent of Plant Earth is blue and made up of water. This triggers a series of chemical reactions that ultimately will lead to lower pH in seawater.

Since the Industrial Revolution, the average pH of surface ocean waters has decreased by 0.11 units, from 8.21 to 8.10. This may not seem significant, but since the pH scale is logarithmic, the 0.1 drop in pH corresponds to a 30% increase in ocean acidity.

When carbon dioxide (CO2) is absorbed from the atmosphere into the ocean, it reacts with seawater to form carbonic acid, making the ocean more acidic. The excess CO2 humans emit from the burning of fossil fuels and other activities not only leads to warming temperatures, it produces an ocean that is more acidic.

According to NOAA, since the beginning of the Industrial Revolution, the pH of surface ocean waters has fallen by 0.1 pH units. Since the pH scale, like the Richter scale, is logarithmic, this change represents approximately a 30 percent increase in acidity (see our pH primer web page for more information). Future predictions indicate that the oceans will continue to absorb carbon dioxide, further increasing ocean acidity. Estimates of future carbon dioxide levels, based on business as usual emission scenarios, indicate that by the end of this century the surface waters of the ocean could have acidity levels nearly 150 percent higher, resulting in a pH that the oceans haven’t experienced for more than 20 million years.

Ocean acidification is expected to impact ocean species to varying degrees. Photosynthetic algae and seagrasses may benefit from higher CO2 conditions in the ocean, as they require CO2 to live just like plants on land. On the other hand, studies have shown that lower environmental calcium carbonate saturation can have a dramatic effect on some calcifying or exoskeleton species, including oysters, clams, sea urchins, shallow water corals, deep sea corals, and calcareous plankton.

Just like humans, marine organisms require optimal conditions inside their bodies to stay healthy. If the acidity of seawater is beyond the optimum range for that organism, its body must use more energy to maintain healthy body fluid chemistry. Organisms can often compensate when faced with increased acidity, but this comes at the expense of using energy to grow critical body parts like muscle or shell. 

For example, scientists have found that mussels, sea urchins, and crabs start to dissolve their protective shells to counter elevated acidity in their body fluids. Sea urchin and oyster larvae will not develop properly when acidity is increased. In another example, fish larvae lose their ability to smell and avoid predators. The vulnerability of larvae means that while organisms may be able to reproduce, their offspring may not reach adulthood.  

THE BIOLOGICAL IMPACTS

Ocean Acidification reduces the size of clams

A consequence of increasing CO2 in the ocean is that it becomes harder for carbonate to mineralize in the water. What exactly does this mean?

Many organisms, including crabs, clams, mussels, and oysters, use calcium carbonate (CaCO3), a common substance found in rocks, to build their shells or outer-skeleton. A lower pH will make it more difficult for these organisms to produce a viable shell in the same shape and size as in the past.

In addition, calcifying plankton, which are at the base of a healthy aquatic food web, will also find it difficult or unable to form shells as a result of ocean acidification. This could have serious effects on species further up the food chain, including commercially valuable fish such as striped bass, blue fish, flounder, and cod.

Even a small reduction in pH makes it just a little bit harder for calcifying organisms to perform normal body functions. Organisms that use calcium carbonate to form shells are more likely to be negatively impacted by ocean acidification.