AP Environmental Science

## Learning Outcomes

- Explain how rising atmospheric carbon dioxide causes ocean acidification
- Describe why lower carbonate ion availability makes coral calcification harder
- Distinguish between ocean acidification, reduced calcification, and coral bleaching
- Understand how **aragonite saturation state** affects coral growth and reef stability
- Apply the causal chain from CO2 emissions to weaker coral reef framework
- Use this topic in AP Environmental Science multiple-choice and free-response answers

## AP Environmental Science Syllabus

For the AP Environmental Science, you are expected to understand how ocean acidification harms coral calcification, with a focus on the following syllabus points:

- Know that ocean acidification is caused by absorption of atmospheric **CO2** into seawater
- Explain the reactions that form **carbonic acid**, release **H+ ions**, and reduce **carbonate ions**
- Identify that corals are marine calcifiers that build **calcium carbonate (CaCO3)** skeletons
- Understand that lower carbonate ion concentration reduces coral calcification rates
- Distinguish reduced calcification from complete dissolution of coral skeletons
- Know that coral reefs are already threatened by both acidification and ocean warming
- Understand that seawater can become less basic without dropping below pH 7
- Recognise reef-scale consequences such as weaker reef framework, lower reef accretion, and reduced habitat complexity

## Test Your Knowledge

_Attempt these questions before reading this article. If you find some difficult or cannot remember the answers, look more closely at that area during your revision._

1. How does atmospheric **CO2** lower the availability of carbonate ions in seawater?
2. Why does reduced carbonate ion concentration make coral calcification more difficult?
3. What is the difference between reduced coral calcification and coral bleaching?
4. How does lower **aragonite saturation state** affect reef-building corals?
5. Why can ocean acidification damage reefs even if seawater remains basic overall?

## Introduction

Ocean acidification is one of the clearest links between the carbon cycle and marine habitat damage. For AP Environmental Science, you need more than a definition. You need the mechanism: more atmospheric **CO2** dissolves into seawater, seawater chemistry shifts, carbonate ions become less available, and corals struggle to build and maintain their calcium carbonate skeletons.

This matters because coral reefs are not just individual animals. They are reef-building systems that support biodiversity, fisheries, and coastal protection. When acidification slows coral calcification, the problem spreads from one coral polyp to the whole reef framework.

> **Key Term: ocean acidification**
> The ongoing decrease in seawater pH caused primarily by the absorption of atmospheric carbon dioxide.

### How ocean acidification changes seawater chemistry

The starting point is increased atmospheric carbon dioxide from fossil fuel combustion and land-use change. Oceans absorb a large share of that CO2, roughly **30%** according to standard APES material. That absorption slows atmospheric warming somewhat, but it creates a different problem in the ocean.

When carbon dioxide dissolves in seawater, it reacts with water:

$$
CO_2 + H_2O \rightleftharpoons H_2CO_3
$$

That forms carbonic acid. Carbonic acid then dissociates:

$$
H_2CO_3 \rightleftharpoons HCO_3^- + H^+
$$

The key part for the exam is the rise in **H+ ions**. More hydrogen ions mean lower pH. But students often stop there, and that is too shallow. The most damaging part for coral calcification is not simply “more acid.” It is what those extra hydrogen ions do next.

They react with carbonate ions:

$$
H^+ + CO_3^{2-} \rightleftharpoons HCO_3^-
$$

So extra hydrogen ions convert free carbonate ions into bicarbonate. The result is a fall in carbonate ion availability.

> **Key Term: carbonate ion**
> The ion **CO3^2-**, needed by corals and other marine calcifiers to build calcium carbonate structures.
>
> **Key Term: carbonic acid**
> A weak acid formed when dissolved carbon dioxide reacts with water.

You can combine the reactions into a simplified net effect:

$$
CO_2 + H_2O + CO_3^{2-} \rightleftharpoons 2HCO_3^-
$$

That is the chain you should be ready to write in words in an FRQ:
rising atmospheric CO2 -> more dissolved CO2 in seawater -> more H+ ions -> fewer carbonate ions -> reduced calcification.

A common mistake is to say that ocean acidification means the ocean becomes acidic like vinegar. That is not correct. Seawater usually remains **basic**, but it becomes **less basic**. A lower pH still counts as acidification.

> **Test Tip:** In APES questions, do not stop at “CO2 lowers ocean pH.” Add the next step: lower pH means fewer carbonate ions are available for shell and skeleton formation.

### Why coral calcification slows down

Corals are marine invertebrates that build hard external skeletons made mainly of **calcium carbonate**, often in the mineral form **aragonite**. To calcify effectively, corals need access to calcium ions and carbonate ions in surrounding seawater.

The skeleton-building idea can be simplified as:

$$
Ca^{2+} + CO_3^{2-} \rightarrow CaCO_3
$$

If carbonate ion concentration falls, this reaction becomes harder to sustain. Coral polyps can still live for some time, but building skeleton material becomes more difficult and slower. That means reduced growth, weaker reef construction, and less capacity to repair storm or biological damage.

> **Key Term: calcification**
> The process by which organisms such as corals build calcium carbonate shells or skeletons.
>
> **Key Term: calcium carbonate**
> The compound **CaCO3**, used by corals, shellfish, and many plankton to make hard body structures.

This is why the phrase “acidification harms coral calcification” is so exact. Acidification does not mainly attack coral by direct burning or poisoning. It shifts seawater chemistry so that the raw material for reef building becomes less available.

At reef scale, this matters because reefs depend on **net reef accretion**. In simple terms, a reef must build calcium carbonate faster than it is broken down. If coral calcification slows, but erosion and biological breakdown continue, reef structure weakens over time.

That is also why APES questions may mention food webs or coastal protection. Coral reefs are habitat-forming systems. Less calcification means less reef growth, which means fewer hiding places, nursery areas, and feeding surfaces for other organisms.

### Aragonite saturation state and reef framework stability

A more advanced but useful term is **aragonite saturation state**, often written as **Ωarag**. You do not need a long formula for APES, but you do need the idea: it is a measure of how favorable seawater is for forming and maintaining aragonite, the form of calcium carbonate many corals use.

When aragonite saturation is high, coral skeleton building is easier. When it falls, calcification becomes less favorable. NOAA guidance often describes **Ωarag above 3** as generally favorable, **below 3** as stressful for many calcifiers, and **below 1** as undersaturated enough that aragonite structures may begin dissolving.

> **Key Term: aragonite saturation state**
> A measure of how favorable seawater is for forming and maintaining aragonite calcium carbonate used by many corals.

This helps you distinguish two related but different ideas:

1. **Reduced calcification**  
   Corals are still building skeleton, but more slowly.

2. **Dissolution**  
   Existing calcium carbonate structures begin to break down faster than they are repaired.

Students often merge these into one process. On exams, keep them separate. Growth can slow well before skeletons start dissolving.

A related equation for dissolution under acidic conditions is:

$$
CaCO_3 + CO_2 + H_2O \rightarrow Ca^{2+} + 2HCO_3^-
$$

This shows how added CO2 and water can push solid calcium carbonate toward dissolved ions and bicarbonate instead of stable reef material.

The broader reef issue is not just whether one coral polyp survives. It is whether the entire **reef framework** remains strong enough to persist. If calcification drops and dissolution or **bioerosion** rises, the three-dimensional reef structure degrades.

> **Key Term: reef framework**
> The physical calcium carbonate structure built by corals that forms the base of a coral reef community.

A weaker reef framework means:

- lower structural complexity
- less habitat for fish and invertebrates
- lower protection from waves and storm surge
- reduced long-term reef growth

### Acidification is not the same as bleaching, but they interact

A major exam distinction is the difference between **ocean acidification** and **coral bleaching**.

Bleaching usually happens when corals are stressed by **high water temperature**, causing them to expel their symbiotic algae. Those algae provide much of the coral’s energy. Without them, the coral turns pale and may starve if stressful conditions continue.

Acidification, by contrast, mainly affects the coral’s ability to build calcium carbonate skeleton.

So the difference is:

- **Bleaching:** mostly an energy/symbiosis problem, strongly linked to heat stress
- **Reduced calcification:** mostly a chemistry/materials problem, linked to carbonate ion loss
  > **Key Term: coral bleaching**
  > The loss of symbiotic algae from coral tissue, usually due to stress such as elevated water temperature.

But these stressors often act together. A coral facing heat stress may already be energy-limited. If it also faces lower carbonate availability, it has less capacity to grow, repair damage, and recover. That is why modern reef decline is often caused by multiple linked pressures rather than one isolated factor.

This interaction matters in APES because questions may ask for a chain of effects:
higher atmospheric CO2 -> warmer climate and warmer oceans -> acidification and heat stress -> weaker coral growth and more bleaching -> reef decline.

Do not say acidification is the main direct cause of bleaching. Heat is usually the main driver. Acidification is an additional stressor that worsens reef outcomes.

> **Test Tip:** If a question asks you to compare ocean warming and ocean acidification, write one sentence for each: warming mainly drives bleaching; acidification mainly reduces calcification.

### From individual corals to reef-wide consequences

The AP exam often pushes beyond organism-level effects to ecological system consequences. If coral calcification falls, the first direct result is slower skeleton formation. But the indirect effects are often more important.

Coral reefs are habitat engineers. Their hard structures create shelter, feeding areas, and nursery habitats for countless organisms. Reduced calcification can therefore lead to:

- lower biodiversity on reefs
- reduced fish habitat
- changes in food web structure
- reduced fisheries productivity
- weaker natural coastal barriers

This is one reason reef decline matters to humans as well as marine life. Many coastal populations rely on reefs for fisheries, tourism, and wave protection. A flatter, weaker reef absorbs less wave energy, so coastlines become more vulnerable to erosion and storm damage.

The effect is also cumulative over time. A single year of slower calcification may not look dramatic. But year after year, if reef-building slows while storms, warming, disease, and bioerosion continue, the reef can shift from net growth to net loss.

Cold oceans such as the Arctic and Southern Ocean are especially vulnerable because colder water naturally holds more dissolved CO2. That means they can approach undersaturated conditions more quickly, making life harder for marine calcifiers.

Finally, remember that corals are not the only affected organisms. Shellfish, sea urchins, calcareous plankton, and other marine calcifiers depend on the same carbonate chemistry. If these groups decline, the effects spread through marine food webss far beyond coral reefs.

## Key Point Checklist

_This article has covered the following key knowledge points:_

- Ocean acidification occurs when atmospheric **CO2** dissolves in seawater and lowers pH over time
- Dissolved CO2 forms **carbonic acid**, which releases **H+ ions**
- Extra hydrogen ions react with **carbonate ions**, reducing their availability
- Corals need carbonate ions and calcium ions to build **calcium carbonate** skeletons
- Lower carbonate availability slows **calcification** in reef-building corals
- Corals often build skeletons in the mineral form aragonite
- Lower **aragonite saturation state** makes coral skeleton formation less favorable
- Reduced calcification is not the same as dissolution, though severe acidification can promote dissolution
- Ocean acidification is different from **coral bleaching**, which is mainly driven by heat stress
- Acidification and warming can interact to worsen reef decline
- Reduced coral calcification weakens the **reef framework** and lowers habitat complexity
- Reef decline can reduce biodiversity, fisheries productivity, and coastal protection

## Key Terms and Concepts

- ocean acidification
- carbonate ion
- carbonic acid
- calcification
- calcium carbonate
- aragonite saturation state
- reef framework
- coral bleaching


Ocean Acidification - Acidification harms coral calcification