Video Lesson: How Ocean Acidification Impacts Marine Life

Coral Reefs Under Pressure

Coral reefs cover less than 0.1% of the ocean floor, yet support about 25% of all known marine fish species. Millions of people depend on coral reefs for food, income through tourism, and protection from storms. But coral reefs are in crisis, and ocean acidification is a major reason why.

Corals are animals, not plants. Each coral polyp contains single-celled algae called zooxanthellae. The algae photosynthesize and share sugars with their coral host; the coral provides protection and access to nutrients. This symbiosis allows corals to grow rapidly even in nutrient-poor tropical waters.

Coral polyps build skeletons from calcium carbonate. As the ocean acidifies and carbonate ion concentration drops, corals struggle—they can still build skeletons, but it requires more energy. At the same time, stress from warming water causes corals to eject their zooxanthellae—a condition called bleaching. Without their algae, corals turn white and lose their nutrition source.

Ocean acidification doesn't just reduce growth rates; it actually dissolves existing coral skeletons at rates depending on pH and saturation state. In highly acidified conditions, dissolution can exceed growth, meaning reefs slowly shrink rather than expand.

Shellfish and Calcification

Oysters, clams, mussels, and scallops depend entirely on building calcium carbonate shells. The global shellfish aquaculture industry produces billions of dollars in product annually. Oyster reefs provide habitat for hundreds of species and filter coastal water, improving water quality.

Shellfish face challenges at multiple life stages. Adult animals generally tolerate modest pH decreases, though they may grow more slowly. But larvae are vulnerable. Oyster larvae begin secreting their shells within hours of hatching. When larvae are exposed to acidified water—especially the undersaturated water brought to the surface by upwelling—shell development goes wrong.

The mechanism involves saturation state (Ω). When water is undersaturated (Ω < 1.0), calcium carbonate is thermodynamically unstable. A shell beginning to form is tiny, with huge surface-area-to-volume ratio, making it especially vulnerable to dissolving. The larva must invest enormous energy to maintain the shell's structure against dissolution. Mortality rates in hatcheries have spiked during upwelling events that bring waters with pH as low as 7.7 and aragonite Ω values below 1.0.

Watch: Acidification Impact Data

The video above presents real data from NOAA monitoring programs showing the biological impacts of acidification across different marine ecosystems. Key segments include:

• Microscopy footage of pteropod shells showing dissolution damage
• Time-lapse sequences of coral growth under different pH conditions
• Interviews with oyster farmers adapting to changing conditions
• Data visualizations mapping biological stress indicators
• Footage from NOAA research expeditions to affected ecosystems

What the Data Shows Us

Oceanographic monitoring over the past 30 years reveals clear trends. At the Hawaii Ocean Time Series station off Oahu, pH has declined by 0.016 units per decade. Saturation state for aragonite has declined from about 3.5 in the 1990s to below 3.2 today. Project this trend forward, and the surface ocean will reach aragonite undersaturation (Ω < 1.0) around 2050 if current CO₂ emission trends continue.

In the Southern Ocean around Antarctica, surface water is approaching aragonite undersaturation already. This region is home to pteropods—sea butterflies—that form a crucial part of whale and fish diets. As the water around these creatures becomes corrosive to their shells, the food web supporting Antarctic wildlife faces disruption.

The data also shows that this trend is driven by atmospheric CO₂. Monthly measurements of CO₂ concentration and pH track together perfectly over decades—when CO₂ rises, pH falls. The correlation is so tight that scientists can predict pH changes directly from atmospheric CO₂ measurements.

Back to Ocean Acidification Module

You've now completed both video lessons in the Ocean Acidification module. Return to the main module page to explore interactive simulations, data tools, and assessment activities that will help reinforce these concepts.