General Environmental Science I

Introduction:

Global Climate Change encompasses more than just changing temperature.  In addition to increases in temperature, we are also experiencing changes in precipitation, more frequent and intense storms, melting polar ice, sea-level rise, and the decreasing pH of the ocean.  The decreasing pH of the ocean is also called ocean acidification.

As we increase the amount of carbon dioxide (CO₂) in the atmosphere, mainly through the burning of fossil fuels, we also increase the amount of CO₂ in the ocean, because of gas exchange between the ocean and atmosphere.  As CO₂ in the ocean increases, the pH of the ocean decreases.  pH is a measure of free hydrogen ions (H⁺).  When CO₂ dissolves in water it forms carbonic acid (H₂CO₃).  This is less stable than bicarbonate (HCO₃^–) so the carbonic acid disassociates to form bicarbonate and a free hydrogen ion, resulting in a lower pH (H⁺ + CO₃^–).  This system is known as the bicarbonate buffering system.  A buffer is a weak acid and its conjugate base.  Buffering systems minimize the change of pH.  The bicarbonate buffering system is one of the systems our body uses to maintain a relatively constant pH.  Scientists only started biological studies of ocean acidification in 2003 because prior to that they believed the buffering capacity of the ocean would resist changes to pH.

In the mid-1800s the pH of the ocean was about 8.17.  The pH of the ocean as of 2010 was about 8.04.  This might not seem like a big change, but because the pH scale is logarithmic, a change of a full unit is actually a ten-fold change, so the decrease from 8.17 to 8.04 represents a 47 % increase in acidity.  In addition to increasing the acidity of the ocean, the additional CO₂ also decreases the availability of the carbonate ion (CO₃^2-) because more of it is bound to hydrogen ions to form bicarbonate.  Many organisms combine carbonate ions to calcium ions (Ca²⁺) to form calcium carbonate shells.  Less carbonate means that it is harder for these organisms to make their shells and grow.  These changes have the potential to not only slow the growth of organisms, but to impact their basic biology, eventually cause their shells to dissolve if the pH drops enough, and could lead to the extinction of many organisms.  These consequences have the potential to cascade through the ecosystem as many organisms would be without food or shelter if we lose the organisms that utilize calcium carbonate.

Because cold water can hold more gas than warm water, more CO₂ is able to dissolve in polar regions, making the pH there drop faster.

In this lab we will explore the changing pH of the ocean on the growth of sea urchin larvae.

Procedure:

Part 1:

We can’t do this without our lab!  We will watch a YouTube video instead.  Note:  Phenol Red is the same as phenolphthalein, but less concentrated.

Carbon Dioxide in Exhaled Air Using Phenol Red: https://youtu.be/KU6kcd8Ci64

You will need safety goggles and gloves for this portion!

When you exhale, you breathe out carbon dioxide (CO₂).  We are going to see how the CO₂ we exhale can decrease the pH of water, just like the CO₂ that we add to the environment can change the pH of the ocean.

1. Add about 100 mL of distilled water to a 250 mL beaker.
2. Add 2-3 drops of phenolphthalein and mix.
3. Add NaOH (sodium hydroxide) one drop at a time, mixing after each drop, until the solution turns a light pink. Sodium hydroxide is a base. When the solution is more basic it will be light pink.  As it becomes more acidic it will turn clear.
4. Place a straw in the solution.
5. Exhale into the straw until the solution changes color. You should be able to do this on one breath. If you need another breath, remove your mouth from the straw, take a breath, and exhale into the straw again.  Whatever you do, make sure you do not drink any of the solution.
6. As you add CO₂ through exhalation you are decreasing the pH of the solution so the pink to clear color change you observe indicates the solution has become more acidic.

Part 2:

Go to the following website:

Go through part one and read about ocean acidification and what types of organisms will be impacted. Take time to go through the model “exploring carbon levels and effects”

What is the current level of CO₂? (Use 2010 data)  What is the current ocean pH?

Under the most optimistic scenario what is the CO₂ level and ocean pH in 2090?  Under the most pessimistic scenario what is the CO₂ level and ocean pH in 2090?

What form of calcium carbonate is a sea urchins skeleton made of?  What is interesting about this form in regards to ocean acidification?

Part 3:

Complete the virtual lab exploring the impact of ocean acidification on sea urchin larvae.  Read and follow the instructions carefully.  You can’t move on to the next task until you have fully completed the previous task. I suggest keeping the hints on.  It will let you know exactly what you need to do to move on to the next task.

What are the two different pH treatments you are using?  Why?

How did you drop the pH of the second treatment?

Why do you add algae to the cultures?

After 2 days when you examine the pH of your treatments, did the pH change in either treatment?  If so, why?

How many replicates are there of each treatment?

Part 4: Data Collection

Measure each of the three sea urchin larvae from both of the treatments and record your individual measurements and your average.

Treatment_____________                                                                Treatment___________

1. 1.
2. 2.
3. 3.

Avg.                                                                                                     Avg.

How does your data compare to the complete data set?

After examining the data read the “so what?” section.

What are the consequences of shorter larval arms for the sea urchin larvae?

What could possibly happen to sea urchin populations as a result of this change?

What other species could be impacted by these changes to sea urchins?

Questions:

1. What is ocean acidification and where is it most intense?

2. What are some of the possible consequences of ocean acidification?

3. Why didn’t people think ocean acidification would be a problem?

4. What is causing ocean acidification? What can be done to minimize ocean acidification?