Conservation News: Ocean Acidification

What is Ocean Acidification?

Ocean acidification is the process of CO2 being absorbed by the ocean changing the chemistry of the seawater. The term is also used to describe the impacts that the increasing carbon dioxide concentrations are having on the ocean and marine organisms.

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Is it like Global Warming?

Yes and no. Ocean acidification and global warming are two different problems, but are caused by the same reason, human CO2 emissions.

The atmospheric concentration of CO2 has significantly increased since the Industrial Revolution. And like the atmosphere, the ocean has the capacity to hold enormous amounts of carbon. That same carbon dioxide gets diffused into water, reducing the environmental consequences of global warming and their associated socio economic impacts. While it stalls the negative effects of global warming, it has caused an increased rate of ocean acidification. The oceans cannot continue to absorb CO2 at the current rate without undergoing significant changes in chemistry, biology, and ecosystem structure as well as our society and economy.

Carbon Dioxide Levels

15_14_co2_left.gif (700×453)Carbon dioxide naturally exists in the atmosphere and in the ocean, a part of the natural environment. Though carbon dioxide levels have been gradually increasing, since the beginning of the Industrial Revolution, the release of CO2 from industrial and agricultural activities significantly increased the natural amount of CO2  in the atmosphere. Prior to industrialization, the concentration of CO2 in the atmosphere was 280 parts per million (ppm). As of February 2016, the number has reached 403.19 ppm.

Henry’s law states that “at a constant temperature, the amount of a given gas that dissolves in a given type and volume of liquid is directly proportional to the partial pressure of that gas in equilibrium with that liquid”. Because there has been a significant increase of CO2 in the atmosphere, there has been a proportionate amount being dissolved into our oceans. It is estimated that the seas absorbed 25% to 30% of manmade CO2 emissions, approximately 530 billion tons in the past 250 years.

Le Quer’ e et.al (2012) estimated that from 2002 to 2011, about a quarter of the CO2 released into the atmosphere was absorbed by the ocean. During this time, the average annual total release was 9.3 billion tons of carbon per year. The ocean absorbed an average of 2.5 billion tons annually.

Currently, the rate worldwide is now at one million tons per hour.

How Does Ocean Acidification Occur?

Given what we already know about Henry’s law (mentioned in the above section), the increased concentration of CO2 affects the chemical equilibrium of the involved molecules. Seawater itself contains a variety of acid-base ions (CO2, CO32-, HCO3) is assumed to be at equilibrium, which is governed by a number of parameters of equilibrium constants. This depends on the initial composition of the seawater. In North Atlantic surface waters, for example, the concentration of bicarbonate ion, carbonate ion, and unionized dissolved carbon dioxide is about 2 x 10-3 mol kg–1. About 90% is made up on bicarbonate ions, the proportion of carbonate ions is about 2×10-4 mol kg–1 and unionized carbonate dioxide is about 2×10-5 mol kg–1.

When the equilibrium is disrupted, the equation will shift towards a certain direction depending on how the stressors affect equilibrium. Le Chatelier’s principle states that “when any system at equilibrium is subjected to change in concentration, temperature, volume, or pressure, then the system readjusts itself to (partially) counteract the effect of the applied change and a new equilibrium is established”.

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When excess CO2 is added to seawater, it reacts in accordance with the acid-dissociation equilibrium. An acid-dissociation equilibrium is when an acid dissociates in an aqueous solution, releasing the hydrogen ion H+. The equation will shift to the products (H+ + HCO3) in order to accommodate the excess CO2 and reach equilibrium once more.

CO2 + H20 ⇄ H+ + HCO3

An increased concentration of H+ + HCO3affects a second equilibrium. Note that because of the initial concentrations, there will be a much larger proportional change in the concentration of H+ than there is in that of HCO3. Therefore, a second reaction will occur to use up the excess H+, causing an increase in HCO3or in other words, the concentration of bicarbonate ion levels.

H+ + CO32-  ⇄  HCO3

To summarize the reactions:

CO2 + CO32-  + H20 ⇄  2HCO3

Thus, the net effect when CO2 is added to seawater, causes the concentrations of H+ + HCO3 (hydrogen and bicarbonate) to increase and the concentration of  CO32- (carbonate) to decrease. This increase of hydrogen ion concentration causes the decrease in ocean pH levels. The decrease of carbonate ion concentration will mean that there are less carbonate ions to bond with calcium for the necessary calcium carbonate for shell growth. These chemical changes to the ocean water will have severe effects on our ecosystems.

Why Should We Be Concerned?

Scientific awareness is relatively recent with little extensive research completed as of yet. However, it is clear that ocean acidification has, and will continue to have, a profound chemical and biological impacts on our ecosystems. Furthermore, ocean acidification ends up affects our economy as well as human cultures and societies worldwide.

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pH Levels

Ocean acidification is measured using pH levels. The pH scale runs from 0 to 14, solutions with low numbers are considered acidic and those with higher numbers are basic. Seven is neutral. The character of acidic, basic, and neutral is defined by the concentration of hydrogen ions [H+](mol/L) in comparison to the concentration of hydroxide ions. A solution with a concentration of hydrogen ions higher than 10-7mol/L is acidic, and a solution with a lower concentration, meaning a higher concentration of hydroxide ions, is alkaline or basic. Using the formula, pH=-log[H+], a pH of 7 is neutral, a pH less than 7 is acidic, and a pH greater than 7 is basic.

With higher levels of CO2 diffusing into the ocean surface, there has been a recent and rapid drop in the ocean surface pH levels. When carbon dioxide is absorbed from the atmosphere into the ocean and reacts with water, it creates carbonic acid. Carbonic acid, H2CO3, is then broken down to a hydrogen ion and bicarbonate ion. The additional hydrogen ions increase the overall concentration and decreases the pH level.

Ocean pH had always been slightly basic, averaging about 8.2 Now, after the industrial revolution, it is around 8.1. Because the pH scale is logarithmic, the change represents a 26% increase in acidity over roughly 250 years. Emissions could reduce surface pH by another 0.4 unit in this century alone and by as much as 0.7 unit beyond the year 2100.

Biological Effects

The overall drop in pH levels will disrupt the current ecosystem and food chains. It is possible that some organisms, like the photosynthetic algae and seagrasses that use CO2, will survive or even thrive under the more acidic conditions while others will struggle to adapt or go extinct. However, majority of marine species are adapted to live in the specific pH level. Lowered pH levels could also have negative effects on marine organisms’ egg and larval development and growth. Calcifying algae, corals, pteropods, and mollusks populations would most likely decline. When these shelled organisms are at risk, the entire food web may also be at risk. The decrease in food supply and energy will cause the predator populations to also decre6840310_orig.jpg (620×360)ase.

The increased acidity causes a reduction in carbonate ion concentrations, which in
turn, causes a reduction in calcium carbonate saturation. Instead, the increased concentration of CO
2 bonds with carbonate to create bicarbonate, which is unusable. Marine calcifying organisms rely on the oceans’ supply of calcium carbonate in order to build their shells. In order to create calcium carbonate, there needs to be a constant supply of carbonate. However, when hydrogen ions are released into the ocean from carbonic acid, the hydrogen ions bond with the carbonate to create bicarbonate, a molecule that cannot be used by the calcifying organisms. Take the pteropods as an example. This pteropod in the picture was placed in seawater with the estimated pH level and carbonate levels of the year 2100. As you can see, the shell thins out and begins to dissolve after 45 days. This applies to other shelled organisms like oysters and clams.

Furthermore, the lack of calcium carbonate hampers coral reef growth. Research has shown that increasing ocean acidification has significantly reduced the ability of reef-building corals to produce their skeletons. It could compromise the successful fertilization, larval settlement, and survivorship of coral species like the Elkhorn coral, making it much more difficult for the reefs to recover from disturbances. Other research indicates that by the end of this century, coral reefs may erode faster than they are being rebuilt. An entire ecosystem could collapse, leaving about an approximate of one million species without a home and food supply. Acidification would not only slow new growth, it will also corrode the already-existing structures, making the organisms more vulnerable to erosion.

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In fish and other gilled marine animals, the lower pH levels in the surrounding waters causes the their cells to balance with the seawater by taking in the carbonic acid. In order to rid of the excess, the fish will burn extra energy to excrete the acid out of its blood through its gills, kidneys, and intestines than on other activities. Change in pH levels in the body can also affect how the their brain processes information, leading to changes in behavior and navigation.

Economic and Cultural Loss

Beyond lost biodiversity, acidification will affect fisheries and aquaculture. Ocean acidification is an anthropogenic problem that will come back and influence human communities. Commercial and recreational fishing, tourism, and the protection of shorelines by coral reefs will decline. Not to mention the cultural and lifestyle changes, such as the way humans feed themselves, earn their livings, run their communities, and live their lives, that our society will have to make to adapt to changing marine ecosystems.

Cultural and lifestyle changes, though they may be significant, are difficult to quantify. To put a dollar value on the benefits that the ocean has made available to human society would mean taking into account and putting a value on what the current benefits are and what the potential damages from losing the benefits could result in. Coral reefs, for instance, bring tourism income, protect shorelines from erosion, and provide habitat for marine species that could be the main source of protein or income for local people.

Sarah Cooley, a postdoctoral researcher at Woods Hole Oceanographic Institution in 2010, estimated that the worldwide value of shoreline protection by coral reefs is $9 billion a year, if adding reef-supported fisheries could mean up to $30 billion a year. In 2006, direct income from coral reef tourism provided 15% of the GDP of the Caribbean island of Tobago; indirect income could total to 30%. Reef tourism, for many islands, is what sustains their economy. 217247_orig.gif (1039×785)

Furthermore, the cultural element of marine activities and ecosystems will change. Maritime societies’ cultures include healthy reefs and marine populations to be a part of their culture. To lose our current ecosystem destroys a part of their culture.

Other maritime economic activities such as shipping, marine construction, energy development, commercial fishing, recreational fishing and boating, aquaculture, and tourism will be affected by the impact of ocean acidification. In 2010, these maritime economic activities collectively contributed $258 billion in GDP to the United States’ national economy and supported 2.8 million jobs nationwide. Approximately 41% of our nation’s GDP was generated in the shoreline counties of the United States and territories.

Ocean acidification’s impacts on the seafood industry is much easier to predict. According to the Food and Agriculture Organization of the United Nations, first-scale value of ocean fisheries worldwide was more than $91 billion, aquaculture of marine organisms generated another $79 billion. Though it may be that marine organisms like mollusks will be immediately affected by the effects of ocean acidification, fish that depend on the mollusks and the predators that eat the fish will eventually decline as food shortage spreads. For humans, one out of every seven consume seafood as his or her primary source of protein.

For More Reading + References

 

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