Lab 11.3

Lab 11.3 – The Carbon–pH Connection: Consequences for Ocean Organisms

Fundamental concept:  Link patterns in ocean CO₂ to ocean pH and its impacts on living organisms
Estimated time to complete: 30 minutes
Data skills preparation:
Materials needed: None

In previous activities, you have learned that CO₂ concentrations are increasing in the atmosphere (Lab 11.1) and that the ocean absorbs CO₂ from the atmosphere (Lab 11.2); in this activity we will learn what happens to the carbon dioxide that the ocean absorbs. Let’s start by exploring the relationship of pH and CO₂ in data from about one week at a specific location in the ocean. The graphs below show ocean CO₂ and pH data from the Pioneer MAB array in July 2024. Answer the questions that follow based on the data shown.

 

Quick Check Questions

Orientation Questions

1. What is the minimum and maximum of the data presented in the x axis?
2. What is the minimum and maximum of the data on the y axis on the bottom graph?
3. What is the minimum and maximum of the y axis of the top graph?
4. What is the correlation between pH and aqueous CO₂? (choose from positive correlation, negative correlation or no correlation)

When carbon dioxide enters into  the ocean, it influences shell-forming organisms in two ways. 

The first is through a change in pH. Carbon dioxide (CO₂) interacts with water (H₂O) to form carbonic acid (H₂CO₃). However, carbonic acid rapidly dissociates in water to form a bicarbonate ion (HCO₃⁻), releasing a hydrogen ion (H+) in the process. This increase in free hydrogen ions leads to a reduction in pH, leading to more acidic waters than can dissolve calcium carbonate (CaCO₃) shells.

Use the drag and drop to explore this equation visually.

The second is through 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 seawater with lower pH 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.

Now let’s look at long-term data from Mauna Loa that you first saw in the Keeling Curve in Lab 11.1. The graph below shows the same atmospheric CO₂ concentrations on the graph (shown in red) but also shows CO₂ concentration in the ocean (shown in green) and the pH of the ocean (shown in blue). Analyze this data, thinking about the relationship between CO₂ in the atmosphere and the ocean (Lab 11.2), and between CO₂ in the ocean and pH, then answer the questions that follow.

Quick Check Orientation Questions

 

Interpretation Questions

5. What is the correlation between atmospheric CO₂ and seawater pCO₂? (choose from positive correlation, negative correlation or no correlation)
6. What is the correlation between seawater pCO₂ and pH? (choose from positive correlation, negative correlation or no correlation)
7. What type of environmental change does the pH  graph indicate? 
   1. hydrogen ions are being produced, decreasing the pH
   2. hydrogen ions are being produced, increasing the pH
   3. hydrogen ions are being consumed, decreasing the pH
   4. hydrogen ions are being consumed, increasing the pH
8. Discuss what impacts these changing  conditions may have on the marine environment in this region. 

Reflection Question

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In Lab 11.1, you learned that photosynthesis by plants consumes CO₂ from the atmosphere. Similarly, in the ocean, photosynthetic plankton consume CO₂ in the surface ocean, where sunlight is readily available, which mitigates increased CO₂ concentrations in the atmosphere. 

In the deep ocean, away from the sunlight, microbial respiration consumes oxygen and produces CO₂. Therefore, CO₂ will be low at the surface, where it is being used up, and will be high at depth, where the constant rain of organic matter is broken down. 

Quick Check – Can you predict where photosynthesis will occur in the ocean?

Now that we’ve explored the chemical and biological processes linking O₂, CO₂, and pH, we can predict how their vertical profiles would compare. The following widget shows chemical data analyzed on board a research vessel at a long-term study site called Station Papa. Can you predict the vertical pattern of pH based on the observed patterns in CO₂ and O₂?

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Application Questions

9. Explain what process(es) lead(s) to the low CO₂ concentrations at the surface. 
10. Explain why and how this affects pH at the surface.
11. Explain what process(es) lead(s) to high CO₂ levels in deep water
12. Explain why and how this affects pH in deep water.