11-15 September 2017


Marine organisms evolved and live in an ocean having a relatively narrow range of chemical and physical characteristics. Changes in ocean chemistry may disrupt the functioning of ecosystems and the normal interactions among species and, in some cases, exceed the tolerance limits of organisms. Unless those organisms are mobile and able to migrate elsewhere to more favorable habitat, they likely will perish. Of particular interest today are changes in ocean chemistry that are linked to changes in the global carbon cycle and climate. Consider a few examples.

Chapter 3 of your textbook includes a discussion of the potential decline in the pH of ocean waters as increasing amounts of carbon dioxide are released to the atmosphere (as a byproduct of human activity) and absorbed by the ocean. Potentially, this ocean acidification is directly harmful to marine organisms that use carbonate ions to build their calcium carbonate shells or skeletons (e.g., coral reefs). The drop in pH is also likely to indirectly affect the relationships among various marine organisms.

Simon Rundle and his colleagues at the University of Plymouth in England studied the potential impact of ocean acidification on a defensive strategy used by the common periwinkle (Littorina littorea). Normally, when periwinkles encounter predators (e.g., the shore crab Carcinus maenas), they grow a thicker shell for protection. However, when Rundle and his colleagues lowered the pH of seawater in a laboratory simulation of this predator/prey interaction, they found that the periwinkle shells did not thicken thus making them more vulnerable to being eaten by shore crabs.

With global warming caused by the buildup of the greenhouse gas carbon dioxide, ocean water temperature is expected to rise with consequences for the supply of dissolved oxygen and marine life. As pointed out in Chapter 3 of your textbook, gases are more soluble in cold water than warm water. According to predictions made by global climate models, global warming will be accompanied by a decline in the concentration of dissolved oxygen in seawater. The decline in solubility is expected to primarily affect the dissolved oxygen supply of surface waters. The dissolved oxygen in deeper water will decrease primarily because of a weaker deep ocean circulation (that transports dissolved oxygen) and the ongoing action of decomposer organism that use dissolved oxygen in breaking down particles of organic matter that sink from surface waters to the ocean bottom.

In May 2008, a German/American research team reported on their analysis of records of dissolved oxygen levels at intermediate levels of the tropical Atlantic and equatorial Pacific. The researchers found that over the past 50 years, the zone where dissolved oxygen reaches a minimum had intensified and expanded vertically presumably because of warming of the seawater. While tolerance to oxygen deprivation varies among species of microorganisms, mobile marine organisms either avoid or die in these low-oxygen layers.

For more on oxygen-depletion zones in the ocean, refer to the second Essay in Chapter 10 of your textbook. The focus is on "dead zones" in nutrient-enriched coastal areas.

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Prepared by Joseph M. Moran, Ph.D. and Edward J. Hopkins, Ph.D., email
© Copyright, 2017, The American Meteorological Society.