OCEAN ACIDIFICATION- ESSAY
THURSDAY, JANUARY 24TH, 2012
Covering more than 70 percent of the Earth’s surface, the ocean is one of planet Earth’s most distinguishing characteristics. Over recent years, human activities such as the burning of fossil fuels have increased the amount of carbon dioxide gas emitted to the atmosphere and the amount that dissolves into the ocean. Now, so much carbon dioxide has been absorbed by the ocean that the chemistry of seawater is changing, causing the ocean to become more acidic.

THE CARBON CYCLE

Carbon dioxide is a critical part of Earth’s atmosphere; it traps heat and prevents the Earth from being covered in ice. Normally, the Earth’s carbon cycle maintains a natural balance of carbon in the atmosphere, land, and ocean through the “breathing of the planet”. However, since the beginning of the industrial era, emissions of carbon dioxide have climbed, and now are exceeding the capacity of the carbon cycle to maintain equilibrium between the atmosphere and ocean. Excess carbon dioxide traps more heat in the atmosphere, which changes the Earth’s climate.

Not all of the excess carbon dioxide stays in the atmosphere. Scientists estimate that one-third of all the carbon dioxide produced by human activities has been absorbed by the ocean. The ocean’s removal of carbon dioxide from the atmosphere has undoubtedly helped curb the extent of climate change—but this benefit has come at a cost. The absorption of carbon dioxide is fundamentally changing the chemistry of the ocean by triggering reactions that make seawater more acidic, a phenomenon called ocean acidification. In fact, the ocean has become nearly 30 percent more acidic than it was at the beginning of the industrial era—a change larger and more rapid than seen in the fossil record going back at least 800,000 years.

THE CHEMISTRY OF OCEAN ACIDIFICAT ION

Atmospheric carbon dioxide is absorbed by the ocean, where it reacts with seawater to form carbonic acid (H2CO3). Almost immediately, carbonic acid dissociates to form bicarbonate ions (HCO3) and hydrogen ions (H+). As the concentration of hydrogen ions increases, the water becomes more acidic.

Some of the extra hydrogen ions react with carbonate ions (CO3 2- ) to form more bicarbonate. This makes carbonate ions less abundant—a problem for many marine species that absorb carbonate from seawater and use it to build calcium carbonate shells and skeletons in a process called calcification.
As carbonate becomes less abundant, these organisms, such as corals and clams, have more difficulty building and maintaining their shells and skeletons. Increased acidity can even cause some carbonate shells and skeletons to dissolve.

WHY CARBON DIOXIDE MAKES SEAWATER MORE ACIDIC

Scientists use pH—a measure of the concentration of hydrogen ions using a logarithmic scale—as an indicator of the acidity of a solution. As the hydrogen ion concentration increases, a solution becomes more acidic and its pH decreases. Because the scale is logarithmic, a decrease of one pH unit corresponds to a 10-fold increase in acidity.

EFFECTS ON SHELLFISH, CORALS, AND OTHER CALCIFIERS

Calcifies—organisms with shells or skeletons made from calcium carbonate—are among the most abundant forms of marine life. Ranging from tiny plankton species such as pteropods that form the basis of marine food chains, to the vast coral reefs that provide habitat for many ocean animals, calcifies are an essential part of many marine ecosystems.

As ocean acidification decreases the availability of carbonate ions, these organisms must work harder to produce shells. As a result, they have less energy left to find food, to reproduce, or to defend against disease or predators. As the ocean becomes more acidic, populations of some species could decline, and others may even go extinct.

COULD OCEAN ACIDIFICATION THREATEN EARTH’S LARGEST CORAL REEF?

Australia’s Great Barrier Reef is a massive coral ecosystem that is the largest biological structure in the world, visible even from space. The reef provides habitat for more than 1,500 species of fish and 400 species of coral—but recently, scientific studies have shown that the growth of the Great Barrier Reef’s coral colonies has decreased by 14 percent since 1990. If growth rates continue to decline to the point that damage and erosion outpace repair, the reef system may begin to shrink.

Some marine biologists think ocean acidification could be contributing to the decline in coral growth. Every coral reef begins with tiny coral polyps that use the carbonate ions naturally found in seawater to form a hard calcium carbonate skeleton. Over time, the skeletons of many coral polyps will build up the structure of the reef. But ocean acidification is reducing the availability of carbonate ions, making it harder for corals to grow or repair damage. Other stressors, such as rising ocean temperature and pollution, could also contribute to declines in coral reef ecosystems.

POTENTIAL IMPACTS ON ECOSYSTEMS

As important as it is to understand how ocean acidification will affect individual organisms, it’s even more important to understand how these changes will scale up to populations and marine ecosystems.

Marine ecosystems support wild fish stocks, shellfish farms, and provide other services to people through recreation, commerce, and even protection from storm surges. Because ecosystems represent a complicated network among organisms and their interactions with the environment, a disturbance to one part can have massive effects throughout the system.

Many species known to be vulnerable to ocean acidification play critical roles in ecosystems, such as the corals and sea grasses that form habitat for diverse communities of organisms. When ocean acidification triggers even a relatively subtle change in an organism’s performance, it can scale up to a change in the size of the population (e.g. through changes in survival, growth, or reproductive success), altering the composition of an ecological community, and potentially the entire marine ecosystem.

HOW WILL CHANGING ECOSYSTEMS IMPACT PEOPLE?

As the ocean’s ecosystems change, so too will the services they provide to society. For example, every year, millions of scuba divers and snorkelers visit coral reefs to enjoy their beauty and abundant sea life. Local businesses generate income by offering diving tours and recreational fishing trips, and hotels, restaurants, and other businesses based near the reef ecosystems also benefit from the influx of visitors. One estimate places the total global value of coral-reef based recreation and tourism at $9.6 billion. Ocean acidification threatens the survival of these beautiful and valuable ecosystems. In 2007, the wild fish and shellfish harvested by the U.S. fishing industry were valued at $3.7 billion. Ocean acidification could harm this industry by altering the growth and development of economically important species of fish, either directly or through effects on the ecosystems of which these species are a part. The bottom line could be a reduction in the yield of commercial fisheries, affecting livelihoods of fishermen and the availability of seafood in markets and restaurants.

SO WHAT CAN WE DO?

A better understanding of the potential effects of ocean acidification, as well as the ability to anticipate these changes, will be needed for fishery managers, industries, and communities to plan and adapt. Ocean acidification research is still in its infancy, but the United States government has taken steps to establish a national ocean acidification research program to support this emerging field.

Like climate change, ocean acidification is a global phenomenon with global consequences. As further ocean acidification seems unavoidable, adaptation to such change will be necessary.