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: time series, inferring relationships between parameters from paired time series
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 between pH and CO₂ using a week of data (July 2024) from the Pioneer MAB array. Answer the questions that follow the graph below.

 

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 of the bottom graph?
3. What is the minimum and maximum of the data on the y axis of the top graph?
4. What kind of correlation do these data suggest between pH and aqueous CO₂?

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 way that CO₂ influences shell-forming organisms 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 and increasing the pH. As a result, a decline in pH leads to a reduction in 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. Some organisms may still build shells, but at a greater energy expense, which can impact survival and reproduction. In seawater with lower pH existing shells can dissolve, especially in young or weaker organisms.

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 these 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 impact these changing conditions may have on the marine environment in this region.

Reflection Question

9. To understand the importance of even small changes in pH, let’s use an analogy in the human body. Just like the ocean, the human body has mechanisms in place to regulate the pH of your blood, blood has a pH range from 7.35 to 7.45.  If your breathing is insufficient, CO2 levels in your body can rise and cause blood pH to decrease. Even if it falls only 0.1 unit on the pH scale to 7.25, it is considered dangerous and causes health problems: a condition known as respiratory acidosis. Based on what you know about the pH scale, explain why a small change in pH can have a big impact and then relate this to the magnitude of change occurring due to ocean acidification.

Changes in CO₂ with Depth

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. Because of the lack of sunlight at these depths, there is no photosynthesis. 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 by microbes. Some of this organic matter will be buried in the deep ocean, making ocean sediments an important sink for CO₂.

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?

[open in new window]

 

Application Questions

10. Explain what biological process(es) impact CO₂ concentrations at the surface versus in the deep ocean.
11. Explain why and how these processes affect pH at these depths.