Coral Reefs and Ocean AcidificationName:

Background:

Increases in carbon dioxide to the atmosphere from the burning of fossil fuels and deforestation threaten to change the chemistry of the seas. Evidence suggests that this increase in atmospheric carbon dioxide is lowering the pH of the oceans in a process called ocean acidification. Ocean “acidification” is a bit of a misnomer, as the ocean is actually basic (pH=8.1). It is expected to get less basic as more CO2 is added to the air and ocean. Carbon dioxide enters the water in two ways: it is taken in by ocean plants (called phytoplankton) and it enters as gas exchanges naturally between the air and water.

When carbon dioxide is exchanged and dissolved in water it lowers the pH as carbonic acid is formed. Ocean acidification appears to be a very real threat to the delicate balance of the oceans’ pH. If acidification continues at current rates, many marine species and ecosystems may not be able to tolerate the change. This lesson looks at the process, the factors involved in, and the effects of ocean acidification on marine organisms; namely those such as corals and other species who build and rely on carbonate exoskeletons for their existence.

How do mollusks, corals, and other shelled organisms make their exoskeleton?

The ocean has a complex way of processing carbon dioxide. This process is often referred to as the “ocean carbon cycle” or the “carbonate buffer system.” The major points are summarized below:

A. Carbon dioxide in the atmosphere mixes with seawater to produce carbonic acid.

B. Carbonic acid easily splits apart into hydrogen ions (H+) and bicarbonate (HCO3 - ) ions. (An increase in the concentration of hydrogen ions increases the acidity of the water, i.e. lowers the pH).

C. Carbonate ions (CO3 2-) combine with hydrogen ions to produce bicarbonate ions.

D. Carbonate ions and calcium ions are used by marine animals (such as corals, crustaceans, and zooplankton) to create hard shells and skeletons of calcium carbonate. This is called “calcification,” or more generically, “biomineralization.”

E. Calcium carbonate dissolves into calcium ions and carbonate ions. The rate of dissolution depends on pH, pressure and the amount of carbonate ions in seawater.

What happens when too much CO2 is added to the water?

For marine organisms, too much CO2 is actually harmful. When excess CO2 enters the water, carbonate exoskeletons are broken down by the carbonic acid that is formed.

You can see this process in the chemical reaction:

When CO2 is added to water it forms carbonic acid (H2CO3)

Like all acids, carbonic acid when dissolved in water releases hydrogen ions (H+)

It’s the hydrogen ions that define acids and give them their chemical properties.

The greater the concentration of hydrogen ions in water the lower the pH.

The carbonic acid reacts with the calcium carbonate resulting in calcium ions and bicarbonate ions which neutralizes the acid, but the calcium carbonate shell is broken down in the process.

Carbonate to Bicarbonate… a major concern with ocean acidification.

The “building blocks” for marine creatures to build calcium carbonate exoskeletons are calcium ions (Ca+2 ) and carbonate ions (CO3 -2) dissolved in the seawater. With acidification hydrogen ions (H+ ) from the carbonic acid bond to dissolved carbonate ions (CO3 -2) in the water forming bicarbonate ions (HCO3 -1). Bicarbonate ions are useless for shell building. As acidification increases, more carbonate ions become bicarbonate ions, making carbonate ions few and far between in the sea. This would make it more difficult for creatures such as corals to build calcium carbonate exoskeletons and the health of the coral reef ecosystem will decline.

Preactivity questions:

  1. What is the current pH of the ocean?
  1. What two anthropogenic activities are causing CO2 to enter the air at drastic rates?
  1. How is CO2 entering the ocean?
  1. What does CO2 form when it enters the ocean?

Materials:

• water

• pH test strips

• beaker or clear plastic cup

• crushed coral or seashells

• drinking straw

  1. Fill a small beaker halfway with water.
  1. Record the pH of the solution. _____
  1. Slowly!!! Exhale through the straw for about 60 seconds (Do not inhale! ) After 60 seconds and while your partner is still blowingadd a drop of carbonic acid to your cup. Place a new pH strip into the water.
  1. Record the pH of the solution here: ______
  1. What gas are you adding to the solution as you exhale? ______
  1. What does the addition of carbon dioxide gas do to the pH of water? ______
  1. Add a teaspoon or two of crushed coral or seashells to this solution and swirl it around slowly for several minutes.
  1. Now, record the change in pH here: ______

Analysis Questions:

  1. What is happening to the pH of the solution as the coral or seashells are added? Why?
  1. What material makes up the exoskeletons/shells? As you added an excess of CO2 to the water, show and explain the chemical reaction taking place in the oceans.

Data Analysis Activity

Based on the graph above, answer the questions that follow.

  1. What is the relationship between the CO2 levels in the atmosphere and the pH levels in the ocean?
  1. Describe the chemistry (chemical reaction) that explains the data in this graph. (Show completely)
  1. Although the average level of carbon dioxide is rising, the CO2 level has an increase followed by a decrease within each year. During what season is there a peak in CO2? Explain why.
  1. During what season is there the lowest level in CO2 within each year on the graph? Explain.