Ocean Acidification

Part A: CO₂ and Ocean pH - What's the Connection?

Oyster farmers sound the alarm about ocean acidification

Oyster farmers have been on the front lines of ocean acidification. In Washington and Oregon, oysters farms are in coastal Pacific waters where upwelling currents are bringing up cold, deep water with higher amounts of CO₂ and a more acidic pH. Watch and listen to two oyster farmers from Taylor Shellfish Farms in Washington state talk how about ocean acidification is impacting their young oysters.

Surface ocean acidity has increased by 30% since pre-industrial times

Recent estimates by scientists ( Le Quere et al., 2012) have calculated that approximately 26% of all CO₂ emitted from human-related activity was absorbed by the oceans during the decade 2002 - 2012. That's 2.5 billion gigatons of excess carbon that moved from the atmosphere into the ocean each year during that one decade. Some of this excess CO₂ ends up in deep ocean currents that eventually upwell along coastal areas bringing all that excess CO₂with them and to oyster farms.

Scientists have also observed that the oceans have become more acidic since the beginning of the Industrial Revolution. They project that this trend will continue in this century as indicated in the visualization pictured on the right. Scientists are concerned that ocean acidification—a gradual acidification of seawater—could have negative consequences for marine organisms, marine food webs, and entire ecosystems. To better understand what ocean acidification is and why it happens, you will:

• learn about pH chemistry in sea water
• conduct a short experiment to investigate the relationship between increased CO₂, acidity and pH
• analyze and compare times series data sets of atmospheric CO₂, sea surface CO₂ and ocean pH

Understanding pH Chemistry

You have probably used pH paper and a pH scale in past science classes to determine how acidic or basic (alkaline) a substance is. Whether a substance is an acid or a base depends on the concentration of hydrogen (H+) ions an atom or group of atoms that carries a positive or negative electric charge as a result of having lost or gained one or more electrons. compared to the concentration of hydroxide (OH-) ions in solution. A pH scale is used to measure the concentration of (H+) ions.

1. Click to enlarge the pH scale image on the right.
2. Take a few minutes to examine the relationship between the pH value (1-14) and the concentration of (H+) ions compared to the concentration of hydroxide (OH-) ions. Also pay attention to the logarithmic scale on the left which illustrates how much the concentration of H+ ions changes relative to the neutral pH of 7. Key points to know about the pH scale and acidity:
   • The lower the pH value, the more acidic the solution.
   • The higher the pH value, the more basic (alkaline) the solution.
3. The pH scale is based on a logarithmic scale (powers of ten). For example, pH 7 is ten times more acidic than pH 8. pH 6 is 100 times (10 X 10) more acidic than pH 8. This means that even a small change in pH can significantly change the concentration of H+ ions in seawater.
4. The term "acidity" does not mean the same thing as a solution being an "acid." For example, if a pH value of a substance changes from pH 10 to pH 9, the concentration of H+ ions increases making the substance more "acidic" but not making the solution an acid. To be an acid, the pH value has to be less than 7. This concept will become important in understanding ocean acidification.
5. NOTE: If you have no prior knowledge of pH, acids and bases, you may want to watch this video: Acids, Bases & pH - bozemanscience

Does a change in dissolved CO₂ cause a change in pH? What's the evidence?

So far, you have learned that the concentration of CO₂in the atmosphere is increasing and some of this extra CO₂is dissolving in the oceans. But, can an increase in dissolved CO2 change the pH of sea water? Try this experiment to find out! With your lab group or partner, choose one of the following hypotheses:

1. Increased CO₂ will cause the pH to become more acidic
2. Increased CO₂ will cause the pH to become less acidic and instead, become more basic (alkaline).
3. Increased CO₂ will have no effect on pH at all.

To test your hypothesis, you will use a chemical called bromothymol blue (BTB). This chemical is commonly used in many laboratory experiments to test for a change in pH. You will add CO₂ to plain tap water. The source of CO₂ you will use in this experiment will be the CO₂ that you exhale.

Materials you will need for your group:

• 200 ml beaker, flask, or similar size clear glass
• Drinking straw
• 50 ml of Bromothymol Blue (BTB) in solution
• A source of CO₂ — you!

Follow this procedure:

1. Pour 50 ml of BTB solution into the beaker. Make note of its color.
   Show me more about BTB as an acid/base indicator
   Hide
   Indicators are special dyes that "indicate" the presence of a specific chemical by turning color. When the Bromothymol Blue (BTB) has a neutral pH, the solution will be slightly green. When the BTB has a slightly acidic pH, the solution will appear yellow. When the solution is slightly basic (alkaline), the BTB solution will appear blue.
   
   

   

   ×

   Bromothymol Blue pH indicator dye in an acidic pH, neutral pH, and basic pH (alkaline) solution - left to right.

   
   
   
2. Exhale your CO₂ through the straw into the beaker. Make sure you don't "suck up" any of the BTB solution into your straw and mouth.
3. When a color change has occurred, stop exhaling CO₂ into the straw. Compare your color change to the image below.
   

   ×

   Bromothymol Blue pH indicator dye in an acidic, neutral, and basic (alkaline) solution - left to right.
   

Comparing time series data of atmospheric CO₂, sea surface CO₂ and sea water pH can uncover trends and causal relationships

The image on the right shows ocean researchers using a rosette device to collect seawater samples to measure pH and dissolved CO₂ in addition to other biological, chemical and physical parameters. By sampling in the same location for several years, researchers can generate times series data that can identify trends in a changing ocean chemistry.

Using the video of the interactive below, you can compare three different time series data sets collected from Station Mauna Loa and Station Aloha in the Pacific Ocean.

• Data Set 1 (in red) represents Keeling Curve atmospheric CO₂ (atm CO₂) data from Mauna Loa. You were introduced to the Keeling Curve in Lab 3. You also learned that the rise in CO₂ has been attributed to greenhouse gas emissions from the burning of fossil fuels.
• Data Set 2 (in blue) represents amounts of CO₂ dissolved in surface seawater. In Lab 5, you learned that CO₂ naturally dissolves in ocean sea surface as part of the ocean's physical pump.
• Data Set 3 (in green) represents pH measurements of surface seawater. A lower pH data indicates the ocean is becoming more acidic whereas a higher pH indicates the ocean water is becoming less acidic.

As you watch the interactive click forward, note that each data set shows considerable variability in yearly measurements but when this variability is averaged over time, definite trends can be observed. Make note of these trends and what these trends might tell you about causality between increased atmospheric CO₂ and ocean acidity. Pause or replay the video to view again.

CO₂, ocean acidification and the ocean's carbonate chemistry system

In Lab 6A, you learned that CO₂ can dissolve in seawater and become part of the ocean's carbonate chemistry system. As the oceans absorb increasing amounts of CO₂, the ocean's pH chemistry becomes more acidic. This change in acidity is causing changes in the ocean's carbonate system. Ocean acidification, or "OA" for short, is the term given to these chemical changes in the ocean as a result of increasing carbon dioxide emissions.

How is the chemistry of the ocean carbonate system changed by a more acidic pH? Find out by watching the short video below on ocean chemistry. As you watch the video, make note of what happens to the amounts of:

• H+ ions
• Bicarbonate ions (HCO₃^-)
• Carbonate ions (CO₃^2-)

Optional Extensions

Want to find out more about ocean acidification and ocean chemistry? Check out these resources:

• NOAA Ocean Acidification Program
• What is Ocean Acidification?
• A primer on pH
• Biological Impacts of Ocean Acidification
• Woods Hole Oceanographic Institute
• How Long Can the Ocean Slow Global Warming?
• How Long Can Oceans Continue To Absorb Earth's Excess Heat?
• Watch Acid Test - a 21-minute video on Ocean Acidification. Note: There is a misconception in this video that oceans will become an acid. Oceans will become more acidic as more CO₂ is absorbed, but oceans will not become an acid.
• Watch the NOVA video Lethal Seas.
• Watch If you are doing this lab online, Bromothymol Blue + CO₂.

Flash is no longer supported