Course outline
Module 1
Module 1: What Is Ocean Acidification?
What Is Ocean Acidification?
Knowledge check
Q1. What is the primary chemical process that causes ocean acidification?
- A. The ocean absorbing excess carbon dioxide from the atmosphere, which forms carbonic acid ✓
- B. The ocean absorbing excess carbon dioxide from the atmosphere, which forms carbonic acid
- C. Industrial waste dumped directly into seawater, which lowers the pH through heavy metals
- D. Increased rainfall diluting seawater and making it more acidic
Why: When the ocean absorbs carbon dioxide from the atmosphere, it reacts with seawater to form carbonic acid, which releases hydrogen ions and lowers the ocean's pH over time.
Q2. What is the approximate pH of the surface ocean today compared to pre-industrial times?
- A. It has risen from about 7.0 to about 8.0, becoming more alkaline
- B. It has dropped from about 8.2 to about 8.1, a decrease of roughly 0.1 units ✓
- C. It has dropped from about 6.5 to about 4.0, a decrease of about 2.5 units
- D. It has remained unchanged at around 8.1 for the past two centuries
Why: Surface ocean pH has declined by roughly 0.1 units since the pre-industrial era, from about 8.2 to about 8.1. Because the pH scale is logarithmic, this represents roughly a 30 percent increase in hydrogen ion concentration.
Q3. Why is the ocean's pH scale described as logarithmic?
- A. Each one-unit change in pH represents a tenfold change in hydrogen ion concentration ✓
- B. Each one-unit change in pH represents a doubling of carbon dioxide levels
- C. The pH scale only measures values between 1 and 10 in a logarithmic pattern
- D. Logarithmic scales are required by international law for measuring seawater
Why: The pH scale is logarithmic, so a change of one whole pH unit corresponds to a tenfold change in hydrogen ion concentration. This means even small pH changes represent significant shifts in acidity.
Q4. Which group of marine organisms is most directly threatened by ocean acidification because of how they build their shells and skeletons?
- A. Sharks and rays, which rely on cartilage that dissolves in acidic water
- B. Calcifying organisms such as corals, oysters, and some plankton that build calcium carbonate structures ✓
- C. Whales and dolphins, which absorb acid through their skin
- D. Deep-sea fish, which depend on bioluminescent proteins that acid destroys
Why: Calcifying organisms build shells and skeletons from calcium carbonate. Lower pH reduces carbonate ion availability, making it harder for these organisms to form and maintain their structures.
Q5. Which of the following is a commonly used indicator that scientists monitor to track ocean acidification?
- A. The salinity level of coastal estuaries
- B. The average surface temperature of the Pacific Ocean
- C. The aragonite saturation state of seawater ✓
- D. The number of tidal cycles per month
Why: Aragonite saturation state measures how easily calcium carbonate minerals can form in seawater. Declining saturation, especially in polar and upwelling regions, is a key sign of ocean acidification.
Module 2
Module 2: The Chemistry Behind Rising Acidity
The Chemistry Behind Rising Acidity
Knowledge check
Q1. What is the primary chemical process that causes ocean acidification?
- A. Oceans absorbing excess atmospheric carbon dioxide, which reacts with seawater to form carbonic acid ✓
- B. Oceans absorbing excess atmospheric nitrogen, which reacts with seawater to form nitric acid
- C. Sulfur released from volcanoes dissolving into seawater to form sulfuric acid
- D. Freshwater runoff from rivers lowering the seawater pH through dilution
Why: The ocean absorbs roughly a quarter of the CO2 humans release into the atmosphere. When CO2 dissolves in seawater, it reacts with water to form carbonic acid, which releases hydrogen ions and lowers pH.
Q2. What happens to the pH of seawater as ocean acidification progresses?
- A. pH rises, making the ocean more alkaline
- B. pH falls, making the ocean more acidic ✓
- C. pH stays constant because seawater is strongly buffered
- D. pH fluctuates randomly with no consistent direction
Why: Acidification means the pH is decreasing. Since the pH scale is logarithmic, even small drops in pH represent large increases in hydrogen ion concentration.
Q3. Which ions are released when carbonic acid dissociates in seawater, causing acidity?
- A. Sodium and chloride ions
- B. Hydrogen and bicarbonate ions ✓
- C. Calcium and hydroxide ions
- D. Magnesium and sulfate ions
Why: Carbonic acid partially dissociates into hydrogen ions (H+) and bicarbonate ions (HCO3-). The increase in free hydrogen ions is what makes the water more acidic.
Q4. Why does the reaction between CO2 and seawater produce carbonate ions that marine organisms need?
- A. Carbonate ions are produced only when CO2 levels are low
- B. Some of the added hydrogen ions bind with carbonate ions, reducing the carbonate available to shell-building organisms ✓
- C. Carbonate ions are created by the photosynthesis of phytoplankton
- D. Carbonate ions increase because the ocean becomes more alkaline
Why: The extra hydrogen ions react with carbonate ions to form bicarbonate, lowering carbonate concentration. Organisms like corals, oysters, and plankton need carbonate to build calcium carbonate shells and skeletons.
Q5. What is the approximate drop in average surface ocean pH since the start of the Industrial Revolution?
- A. About 0.1 pH units, a change of roughly 26 percent in hydrogen ion concentration ✓
- B. About 3 pH units, a change of roughly 1000 percent
- C. About 1 pH unit, a tenfold increase in acidity
- D. No measurable change has occurred
Why: Average surface ocean pH has fallen by about 0.1 units, from roughly 8.2 to 8.1. Because the scale is logarithmic, this corresponds to about a 26 percent increase in hydrogen ion concentration.
Module 3
Module 3: Causes: Carbon Dioxide and the Oceans
Causes: Carbon Dioxide and the Oceans
Knowledge check
Q1. What is the primary way carbon dioxide from the atmosphere enters the ocean?
- A. Through rainfall washing CO2 out of the air
- B. Through diffusion across the air-sea surface, driven by a difference in partial pressure ✓
- C. Through deep volcanic vents releasing CO2 into seawater
- D. Through photosynthesis by floating plankton pulling CO2 down from the sky
Why: CO2 dissolves into seawater at the surface through gas exchange, moving from air into water when the partial pressure of CO2 in the atmosphere is higher than in the surface ocean. Rainfall and plankton photosynthesis play minor or indirect roles, and volcanic vents are a natural source, not the main pathway for atmospheric CO2 uptake.
Q2. When carbon dioxide dissolves in seawater, which compound does it form first?
- A. Calcium carbonate
- B. Carbonic acid ✓
- C. Sodium bicarbonate
- D. Hydrogen peroxide
Why: Dissolved CO2 reacts with water to form carbonic acid (H2CO3), which then dissociates into bicarbonate and carbonate ions along with hydrogen ions. The release of hydrogen ions is what drives the decrease in pH associated with acidification.
Q3. What happens to the concentration of hydrogen ions in seawater as ocean acidification progresses?
- A. It decreases, making the water more alkaline
- B. It stays exactly the same because CO2 is neutral
- C. It increases, lowering the pH of the water ✓
- D. It fluctuates randomly with no consistent trend
Why: As more CO2 is absorbed, additional hydrogen ions are released, increasing their concentration. Because pH is a logarithmic measure of hydrogen ion concentration, a rise in hydrogen ions means a drop in pH, which is what makes seawater more acidic.
Q4. Approximately what fraction of human-caused carbon dioxide emissions has the ocean absorbed since the Industrial Revolution?
- A. About 1 percent
- B. About 5 percent
- C. About 25 percent ✓
- D. About 90 percent
Why: The ocean has absorbed roughly a quarter of the CO2 emitted by human activities such as fossil fuel burning and land-use change. This uptake has slowed atmospheric warming but at the cost of changing ocean chemistry.
Q5. Why does the ocean's chemistry make it a major carbon sink compared to fresh water?
- A. Seawater contains dissolved salts that allow it to hold far more CO2 than pure water through chemical buffering ✓
- B. Seawater is colder than all freshwater, so gases dissolve only in salt water
- C. Seawater has no oxygen, which allows it to absorb carbon more readily
- D. Freshwater lakes are too shallow to absorb any carbon dioxide at all
Why: Seawater contains carbonate and bicarbonate ions that react with added CO2, buffering the chemistry and allowing the ocean to store far more carbon than an equal volume of pure water would. This buffering capacity is a key reason the ocean has absorbed so much human-emitted CO2.
Module 4
Module 4: Effects on Marine Life and Shellfish
Effects on Marine Life and Shellfish
Knowledge check
Q1. What is the primary chemical change that occurs in seawater during ocean acidification?
- A. An increase in the concentration of dissolved salt
- B. A decrease in pH due to increased dissolved carbon dioxide ✓
- C. A rise in water temperature that lowers oxygen levels
- D. An increase in dissolved nitrogen from runoff
Why: When the ocean absorbs carbon dioxide from the atmosphere, it forms carbonic acid, which releases hydrogen ions and lowers the pH of seawater, making it more acidic.
Q2. Which group of marine organisms is most directly affected by ocean acidification because they build shells from calcium carbonate?
- A. Sharks and rays
- B. Seabirds and sea turtles
- C. Shellfish such as oysters, clams, and mussels ✓
- D. Seaweeds and kelp
Why: Shellfish like oysters, clams, and mussels rely on calcium carbonate to build their shells. Lower pH reduces the availability of carbonate ions, making it harder for them to form and maintain shells.
Q3. What mineral form of calcium carbonate do many shellfish and corals use to build their shells and skeletons?
- A. Quartz
- B. Feldspar
- C. Aragonite or calcite ✓
- D. Gypsum
Why: Many marine organisms build their shells and skeletons from aragonite or calcite, two forms of calcium carbonate. Acidification makes these minerals harder to form and more prone to dissolving.
Q4. Why do young shellfish larvae often suffer more from ocean acidification than adult shellfish?
- A. They produce more carbon dioxide than adults
- B. Their shells are very small and thin, so they are more vulnerable to dissolution ✓
- C. They cannot swim and are carried to polluted areas
- D. They feed only on acidic plankton
Why: Larval shells are small, thin, and still developing, so they have less protective material. Under acidic conditions they can dissolve or fail to form properly, which can reduce survival of the population.
Q5. Which of the following is a likely economic consequence of ocean acidification for coastal communities?
- A. Increased tourism due to clearer water
- B. Reduced shellfish harvests, affecting fishing and aquaculture jobs ✓
- C. Lower costs for seafood because of faster growth
- D. Higher oxygen levels that boost commercial fishing
Why: As shellfish struggle to grow and survive, harvests decline. This can hurt fishing and aquaculture businesses and the livelihoods of coastal communities that depend on them.
Module 5
Module 5: Impacts on Coral Reefs and Ecosystems
Impacts on Coral Reefs and Ecosystems
Knowledge check
Q1. What is the primary chemical process behind ocean acidification?
- A. Oceans absorbing excess atmospheric carbon dioxide, which reacts with seawater to form carbonic acid ✓
- B. Oceans losing oxygen to the atmosphere, which raises water acidity
- C. Rivers depositing acidic minerals into coastal waters
- D. Sunlight breaking down salt molecules into acids
Why: The ocean absorbs roughly a quarter of human-emitted CO2. When CO2 dissolves in seawater it forms carbonic acid, which releases hydrogen ions and lowers the pH, making the ocean more acidic.
Q2. Why are coral reefs especially vulnerable to ocean acidification?
- A. Corals need warm water to survive, and acidic water is always warmer
- B. Corals build their skeletons from calcium carbonate, which dissolves more readily in acidic water ✓
- C. Corals feed on acidic plankton that disappear as pH drops
- D. Corals absorb carbon dioxide directly through their tissues and become poisoned
Why: Reef-building corals secrete calcium carbonate skeletons. As seawater becomes more acidic, it has less carbonate ion available, making it harder for corals to build new skeleton and easier for existing structures to dissolve.
Q3. What effect does reduced carbonate availability have on coral reef growth?
- A. Reefs grow faster because acidic water fertilizes coral
- B. Reefs grow more slowly and can become weaker and more prone to erosion ✓
- C. Reefs change color permanently but grow at the same rate
- D. Reefs move to deeper water to escape the acidity
Why: Lower carbonate ion concentrations slow calcification, so reefs accumulate skeleton more slowly. Combined with increased dissolution, reefs can lose structural integrity and become more vulnerable to storm damage and bioerosion.
Q4. Which of the following marine organisms besides corals is also affected by ocean acidification?
- A. Sharks, because their cartilage dissolves in acidic water
- B. Pteropods (sea butterflies), which build shells from aragonite ✓
- C. Whales, because acidity disrupts their echolocation
- D. Seagrass, which loses its ability to photosynthesize
Why: Pteropods and many other calcifying organisms such as mollusks, sea urchins, and crustaceans build shells or skeletons from calcium carbonate. Reduced carbonate saturation makes it harder for them to form and maintain these structures.
Q5. How do coral reefs support marine ecosystems, making their decline a broader concern?
- A. They produce most of the world's oxygen through photosynthesis
- B. They provide habitat and shelter for roughly a quarter of marine species despite covering a small share of the ocean floor ✓
- C. They regulate ocean salinity across entire ocean basins
- D. They serve as the main source of deep-sea nutrients
Why: Although coral reefs cover less than 1% of the ocean floor, they host an estimated 25% of known marine species. Their decline threatens biodiversity, fisheries, coastal protection, and the livelihoods of people who depend on them.
Module 6
Module 6: Consequences for Fisheries and Food Security
Consequences for Fisheries and Food Security
Knowledge check
Q1. What is the primary chemical change that occurs in seawater during ocean acidification?
- A. Seawater becomes more alkaline as carbonate ions increase
- B. Seawater's pH decreases as dissolved carbon dioxide increases hydrogen ion concentration ✓
- C. Seawater loses dissolved oxygen, causing its pH to rise
- D. Seawater temperature rises, which dissolves more salt and lowers pH
Why: When atmospheric CO2 dissolves in seawater it forms carbonic acid, which releases hydrogen ions and lowers pH. This makes the ocean more acidic, even though it remains slightly alkaline overall.
Q2. Which group of marine organisms is most directly threatened by reduced carbonate availability in acidifying waters?
- A. Sharks and other cartilaginous fish
- B. Marine mammals such as whales
- C. Calcifying organisms such as oysters, corals, and some plankton ✓
- D. Seabirds that feed on surface fish
Why: Calcifying organisms build shells or skeletons from calcium carbonate. Lower pH reduces carbonate ion availability, making it harder for them to form and maintain these structures.
Q3. Why is ocean acidification a concern for food security in coastal communities?
- A. It causes fish to grow too large to be caught by traditional fishing gear
- B. It increases the salt content of seafood, making it unsafe to eat
- C. It can reduce shellfish harvests that many coastal communities depend on for income and nutrition ✓
- D. It makes fish migrate away from all coastlines permanently
Why: Shellfish such as oysters, clams, and mussels are important sources of protein, jobs, and income for coastal communities. Declines in their survival and growth threaten both livelihoods and food supplies.
Q4. What is the main human activity responsible for the increase in ocean acidity over the past two centuries?
- A. Deep-sea mining
- B. Overfishing of large predatory fish
- C. Burning fossil fuels that releases carbon dioxide into the atmosphere ✓
- D. Discharge of untreated sewage into rivers
Why: Burning coal, oil, and gas releases large amounts of CO2. Roughly a quarter of this CO2 is absorbed by the ocean, which drives the chemical changes that acidify seawater.
Q5. Which fishery-related effect of acidification might affect fish populations beyond shellfish?
- A. Fish become immune to all parasites
- B. Changes in the food web, since acidification can alter plankton communities that many fish depend on ✓
- C. Fish stop producing eggs entirely
- D. Fish lose the ability to swim in any water
Why: Many fish rely on plankton and other small organisms as food. If acidification shifts plankton species or reduces their abundance, the effects can ripple up the food web and affect fish stocks that people harvest.
Module 7
Module 7: Solutions and Responses to Ocean Acidification
Solutions and Responses to Ocean Acidification
Knowledge check
Q1. Which of the following is the primary cause of ocean acidification?
- A. Increased ocean salinity from melting glaciers
- B. Absorption of excess atmospheric carbon dioxide by seawater ✓
- C. Pollution from agricultural runoff containing nitrogen
- D. Volcanic eruptions releasing sulfur dioxide into the ocean
Why: The ocean absorbs roughly a quarter of the carbon dioxide humans release into the atmosphere. When CO2 dissolves in seawater it forms carbonic acid, which lowers the ocean's pH and makes it more acidic.
Q2. Which group of marine organisms is most directly threatened by ocean acidification because it affects their ability to build shells and skeletons?
- A. Sharks and rays
- B. Sea turtles and seabirds
- C. Corals, mollusks, and calcifying plankton ✓
- D. Whales and dolphins
Why: Many marine organisms build calcium carbonate shells or skeletons. Increased acidity reduces the availability of carbonate ions needed for calcification, making it harder for corals, oysters, clams, and plankton such as pteropods to build and maintain their structures.
Q3. What does a decrease in ocean pH indicate?
- A. The ocean is becoming more alkaline
- B. The ocean is becoming more acidic ✓
- C. The ocean is losing dissolved oxygen
- D. The ocean is becoming saltier
Why: The pH scale measures how acidic or basic a solution is. A lower pH value means higher acidity. Since the pre-industrial era, surface ocean pH has dropped by about 0.1 units, which represents roughly a 30 percent increase in hydrogen ion concentration.
Q4. Which of the following is a commonly proposed solution to reduce ocean acidification at its source?
- A. Adding large quantities of lime to the ocean
- B. Reducing global emissions of carbon dioxide from fossil fuels ✓
- C. Increasing commercial fishing of shellfish
- D. Dumping iron filings into the deep sea
Why: Because ocean acidification is driven by atmospheric CO2 dissolving into seawater, the most fundamental response is to cut carbon emissions through energy transitions, efficiency, and reduced deforestation. Other ideas exist but are either local, experimental, or carry their own risks.
Q5. Which local action can help coastal ecosystems become more resilient to acidification?
- A. Draining coastal wetlands to reduce flooding
- B. Restoring seagrass beds and kelp forests that absorb CO2 ✓
- C. Increasing nutrient runoff to boost plankton growth
- D. Removing all coastal vegetation to improve water flow
Why: Seagrasses and kelp take up dissolved carbon dioxide through photosynthesis, which can raise local pH and create refuges for shellfish. Protecting and restoring these habitats is a practical local strategy that complements global emission cuts.