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.
Why is ocean acidification important to oyster farmers?
Surface ocean acidity has increased by 30% since pre-industrial times
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 acidificationa gradual acidification of seawatercould 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
- Click to enlarge the pH scale image on the right.
- 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.
- 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.
- 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.
- 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?
- Increased CO2 will cause the pH to become more acidic
- Increased CO2 will cause the pH to become less acidic and instead, become more basic (alkaline).
- 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:
- Pour 50 ml of BTB solution into the beaker. Make note of its color.
- 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.
- When a color change has occurred, stop exhaling CO2 into the straw. Compare your color change to the image below.
- Describe the results of your CO2/pH experiment.
- Which hypothesis is supported by the results? Explain why.
- Your experiment was a small scale experiment completed at a lab bench. Can the results of this experiment be extrapolated to understanding the effect of increased CO2 on pH in the oceans? Explain why or why not?
Comparing time series data of atmospheric CO2, sea surface CO2 and sea water pH can uncover trends and causal relationships
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.
- Describe the trend for each data set.
- Does comparing the trends from these three time series data sets provide evidence for a cause and effect relationship between increasing CO2 emissions and ocean acidification. Explain why or why not.
CO2, ocean acidification and the ocean's carbonate chemistry system
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 (CO32-)
Stop and Think
1. Throughout this module, you have learned that a change in one part of the carbon cycle can cause changes in other parts of the carbon cycle. Keeping this important theme in mind, explain how an increase in CO2 fossil fuel emissions can alter the ocean.
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.
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