1. How does ocean acidification change ocean chemistry?
Lesson 2b of 4
Developed by Brian Erickson ©2017

2. Demo: Candles in indicator
How does this demo show ocean acidification?
(Use this question as an informal probe to see what registered and what needs to be reinforce from yesterday.)
Discuss student answers as full class. If needed, the following questions may prompt student recall: What happened at Whiskey Creek? Where is Whiskey Creek Shellfish Hatchery? What is ocean acidification? What did we see in the visualization we watched?

3. What do you notice about these molecules/ions?
Name
Chemical Formula
Carbonic Acid
H2CO3
Bicarbonate
HCO3-
Carbonate
CO32-

4. Ocean acidification involves the following steps
1. CO2 (g) ↔ CO2 (aq) 2. CO2 + H2O ↔ __________

5. Ocean acidification involves the following steps
1. CO2 (g) ↔ CO2 (aq) 2. CO2 + H2O ↔ H2CO3 3. H2CO3 ↔ ______ + _________ 4. H+ + CO32- ↔ HCO3-

6. Ocean acidification involves the following steps
1. CO2 (g) ↔ CO2 (aq) 2. CO2 + H2O ↔ H2CO3 3. H2CO3 ↔ H+ + HCO3- How does pH change as a result of adding CO2 to water? What happens to the acidity?

7. Ocean acidification involves the following steps
1. CO2 (g) ↔ CO2 (aq) 2. CO2 + H2O ↔ H2CO3 3. H2CO3 ↔ H+ + HCO3- 4. H+ + CO32- ↔ HCO3-

8. In summary, when CO2 enters the ocean…
Hydrogen [H+] ↑ or ↓ ?
“acidity” ↑ or ↓ ?
pH ↑ or ↓ ?
Carbonate [CO32-] ↑ or ↓ ?
For our purposes, memorizing the previous reaction’s isn’t necessary. We want to see the big picture. Let’s summarize some of the most important changes in ocean chemistry.
(Have students show their answer to each component that changes with their hands)
The key point: ocean acidification includes all of these changes (and more). When we define ocean acidification as a change in ocean chemistry due to anthropogenic CO2 emissions, this allows us to avoid talking about the details involved.
Does ocean acidification make carbonate more or less available for shell building?
Ocean acidification is a change in ocean chemistry due to anthropogenic CO2 emissions
This makes carbonate less available for shell building

9. Ω (“omega”) or “saturation state”
Are calcium carbonate shells/reefs/etc. expected to form or dissolve?
Ω < 1, solid shells not favored,
expected to dissolve
There’s one final idea we need to introduce, carbonate saturation state, or “omega”. It is related to the [CO32-]…
I think of it as a measure of whether solid carbonate shells are favored (meaning we expect them to form) or not favored (meaning we would expect them to dissolve). With ocean acidification, omega decreases, meaning shells are less favored.
Photos: Bednarsek et al. ?
Ω > 1, solid shells favored, expect formation

10. 1 week at Whiskey Creek (6/29 to 7/6/2009)
Look at the graphs in your notes… (walk students through reading this graph with the following questions):
What variable is shown along the x-axis? (Time/date)
What does each tick mark show? (6 hours)
What does each vertical (up and down) line show? (1 day)
What variables are shown along the y-axis? (pH and omega)
What is the range (highest and lowest values) for pH? ( )
What is the range for Ω? ( )
What patterns do you notice? (both pH and omega seem to go up and down regularly and at the same time)
At what time of day (morning, midday, afternoon, night) do pH and omega reach their maximum? (afternoon, 3-6p)
When are they at a minimum? (6am)

11. Work with your neighbor to answer questions 1-4 in your notes.
Be prepared to discuss
Discuss student answers to questions 1-3:
Daily changes are caused by photosynthesis (removes CO2 during the day, which allows pH and omega to increase/become less acidic)
Quite often. Every day the water is below 1.6 for some period of time, some days it never goes above 1.6!
Late afternoon, it is when pH and omega are highest (because photosynthesis has taken place all day long!)

12. pH is predicted to decrease from 8.1 to 7.8 by 2100
You saw that pH can naturally vary by 0.5 over the course of a week. Why do we even talk about ocean acidification, a pH unit change per year, when weekly variation is so much larger?
For oysters, at least larval oysters, it seems that omega is what matters (not pH).
Consider when Ω <1.6 to be a bad day for baby oysters. On bad days, it takes more energy to build a shell, which means there is less energy for things like growing and avoiding getting sick. These bad days happened occasionally in the past. With ocean acidification, bad days happen more often and are more extreme (Ω goes lower and lower).
While it is a crude analogy, think of your own life. Imagine you have a big test tomorrow stayed up all night. You finally go to bed around 5am, oversleep your 6:30am alarm, and finally rush out the door for school at 7:30a. You only slept 2 hours, you had a rushed morning

13. Ocean acidification will lead to…
Natural variation
With acidification
Bad days Ω
Time
Natural variation + ocean acidification
No change Ω
Bad days
This graph shows hypothetical data of omega over time. I find it helpful to think of it in terms of good days and bad days for an organism. You always have natural variation. Under natural conditions, there are occasional bad days (days when conditions fall below the dotted red line). Organisms have ways to deal with these; they’ve adapted under these conditions, they can repair after these bad days. That’s not to say bad days are never deadly under natural conditions, but just that we understand they are a natural part of life.
With ocean acidification, we see an overall shift in the baseline (average condition), a downward trend over time (blue line). This leads to two main changes: bad days happen more often (the frequency of bad days will increase) and the bad days are more severe (the omega and pH values will be lower).
(Teacher note- the graphs shown are a rough (digital) sketch, they are not real data. They are simply meant to show what would happen with an overall downward trend laid on top of naturally varying conditions)
Time
With ocean acidification, bad days will…
occur more often
be more severe

14. Video: Mark Wiegardt

15. Exit ticket
Complete the exit ticket in your notes