Lab 11 – Instructor Guide

Lab 11 – Ocean Acidification
Instructor Guide

This lab introduces ocean acidification, the decrease of ocean pH due to increased levels of carbon dioxide in the ocean. It starts by introducing the concept of pH and the pH scale and having students think about how decreasing pH affects marine organisms.  In a series of scaffolded activities, it guides students through discovering how carbon dioxide levels have been changing over time, what biological processes impact CO2 levels, how changes in CO2 levels lead to changes in pH and how upwelling zones give us a glimpse into the impacts of a future with lower pH.

Approximate time involved: approximately 30 -40 minutes for each of the four activities and approximately 2-2.5 hours to complete the entire lab.

Learning outcomes

• LO1. Identify long-term trends in atmospheric CO₂ and their relationship to ocean CO₂ through ocean-atmosphere gas exchange
• LO2. Link patterns in ocean CO₂ to ocean pH
• LO3. Identify the relationships between biological processes and ocean chemistry
• LO4. Describe how coastal upwelling impacts surface CO₂ and pH
• LO5. Analyze real oceanographic data visualized in a variety of formats, including time series data, correlation data, vertical profile data, and spatial data

Learning Outcome	Activity 1	Activity 2	Activity 3	Activity 4
Outcome 1	Introduced	Introduced and Guided Practice	Applied	Applied
Outcome 2	N/A	N/A	Introduced and Guided Practice	Applied
Outcome 3	Introduced	N/A	Guided Practice and Applied	Applied
Outcome 4	N/A	N/A	N/A	Introduced and Applied
Outcome 5	Introduced and Guided Practice	Guided practice and applied	Guided practice and applied	Guided practice and applied

Materials needed

None

Student Answer Form

Lab 11 OA Student Answer Form V3.1 draft

What students should know before this activity

• Data literacy: Read graph axes (Lab 2), interpret time series graphs, identify trends and correlations
• Basic familiarity with photosynthesis and cellular respiration
• Concept of global warming and the greenhouse effect
• What causes upwelling to occur and where it occurs

What instructors should know before this activity

TBD

Scientific Background

pH scale; it is reviewed on the landing page of the lab chapter (11.0)

Teaching Notes

Introduction and Lab 11.1

Here is a link to the keeling curve so you can have the most updated version: https://keelingcurve.ucsd.edu/

 

Lab 11.3

Helpful Resources:

• Video demo on effects of CO2 in water on pH: https://www.youtube.com/watch?v=E44FO1kOJt0
• WHOI online interactive: https://www.whoi.edu/multimedia/carbon-dioxide-shell-building-and-ocean-acidification/

Lab 11.4

Upwelling can have two impacts on ocean surface CO2 levels: 

1. An increase in CO2 levels when deep ocean water is brought to the surface. Remember that higher pressure = higher gas concentrations. Deep ocean CO2 levels are also higher due to a lack of photosynthesis and the breaking down of sinking organic matter through respiration. 
2. A decrease in CO2 levels due to an influx of deep-water nutrients and, as a result, higher rates of photosynthesis. 

Optional Pre-Lab Activities:

• You can do a demonstration of the effects of low pH on calcium carbonate by putting chalk in a cup with vinegar and in a cup of water and compare weights before and after.  You can also put seashells in vinegar and water for longer and observe the effects.  Check out these resources for some ideas:
  • NOAA. (2013). Ocean Acidification. National Oceanic and Atmospheric Administration. https://www.noaa.gov/education/resource-collections/ocean-coasts-education-resources/ocean-acidification
  • Amanda L. Kelley, Paul R. Hanson, Stephanie A. Kelley; Demonstrating the Effects of Ocean Acidification on Marine Organisms to Support Climate Change Understanding. The American Biology Teacher 1 April 2015; 77 (4): 258–263. doi: https://doi.org/10.1525/abt.2015.77.4.5

Pre/post-lab Assessment Questions:

1. TBD

Extensions:

The Ocean’s Carbonate Buffering System. 

The ocean carbonate buffering system is a natural mechanism that helps regulate the ocean’s pH, preventing it from becoming too acidic or too basic. It works through a series of chemical reactions involving carbon dioxide (CO₂), water (H₂O), carbonic acid (H₂CO₃), bicarbonate (HCO₃⁻), and carbonate (CO₃²⁻).

To maintain equilibrium, the additional hydrogen ions produced from the interaction between CO₂ and H₂O bind with carbonate ions, forming bicarbonate. As a result, a decline in pH reduces the availability of free carbonate ions, making it harder for organisms to form calcium carbonate shells. 

In summary, some organisms can’t make shells because there are fewer carbonate ions and organisms struggle to extract enough material to build strong shells.  In more acidic water existing shells can dissolve, especially in young or weaker organisms.  All of this results in increased energy cost, some organisms may still build shells, but at a greater energy expense, which can impact survival and reproduction.

Use the figure above to answer the following questions. The unit used in the y-axis of this graph is millimolar (mM 10−3).   The x-axis is the pH scale. For reference, the pH of pure water is 7 and the average pH of the ocean is 8.1.

Quick Check Questions:

• What type of correlation is displayed between dissolved carbon dioxide (CO2(aq)) and bicarbonate (HCO3–) and  between bicarbonate (HCO3–) and carbonate (CO32-)
  • Direct, inverse, no correlation
• When a reaction consumes hydrogen ions(lowers hydrogen ion activity)  the pH will:      
  • a. increase b. decrease  c. have no impact  d. not enough information is given in this graph
• Reactions that increase carbon dioxide __________ pH.
  • a. increase b. decrease  c. have no impact  d. not enough information is given in this graph

Interpretation Questions

1. Discuss the trends you recognize in the dissolved inorganic carbon graph and their relationship to the pH of the ocean.

 

Gas Solubility Exercise

Solubility refers to the amount of dissolved gas that the water can hold under a particular set of conditions, which are usually defined as 0℃ and 1 atmosphere of pressure. The solubility of a gas increases with increasing pressure, decreased temperature, and decreased salinity. 

Solubility by OpenStax is licensed CC BY 4.0. Original source

Use the above figure showing the solubility of several gases in water and its relationship with temperature to answer the questions below.

Orientation Questions: 

• How many different gasses are shown in the figure? Is CO2 one of these gasses?

Interpretation Questions:

• Briefly describe the relationship between the solubility of oxygen and the temperature of water.
• Is this relationship the same across all gasses? Choose one other gas to compare/contrast with oxygen in terms of this relationship.

 

Saturation refers to the amount of gas currently dissolved in the water, relative to the maximum possible content of that gas. If the water is undersaturated, more gas can dissolve. If the water is saturated or supersaturated, gas may be released. Therefore, when the ocean is undersaturated with CO2 it can act as a sink and. When it is supersaturated with CO2, it can act as a source.

Quick Check Question: 

When the ocean is supersaturated with CO2 for its current temperature and pressure:

• it will act as a source of CO2 and will release CO2 to the atmosphere 
• it will act as a sink for CO2 and will release CO2 to the atmosphere
• it will act as a source of CO2 and will take CO2 up from the atmosphere 
• it will act as a sink for CO2 and will take CO2 up from the atmosphere 

Associated Resources:

• TBD