16.2 Ocean acidification and its impacts on calcifying organisms

Ocean Acidification and Its Effects

Ocean acidification is the gradual drop in ocean pH caused by seawater absorbing excess atmospheric $CO_2$. This process directly threatens organisms that build shells and skeletons from calcium carbonate, and its effects cascade through entire marine ecosystems, economies, and coastal communities.

Ocean Acidification and Atmospheric $CO_2$

The ocean acts as a massive carbon sink, absorbing roughly 25–30% of the $CO_2$ humans release into the atmosphere. That absorption is what drives acidification.

Here's the chemistry of what happens when $CO_2$ dissolves in seawater:

1. Atmospheric $CO_2$ dissolves into the ocean surface.
2. It reacts with water to form carbonic acid: $CO_2 + H_2O \rightarrow H_2CO_3$
3. Carbonic acid then dissociates into a hydrogen ion and a bicarbonate ion: $H_2CO_3 \rightarrow H^+ + HCO_3^-$
4. The extra $H^+$ ions lower the water's pH, making it more acidic.

Atmospheric $CO_2$ has been climbing due to fossil fuel combustion (coal, oil, natural gas), deforestation, and cement production. Since the Industrial Revolution, ocean surface pH has dropped by about 0.1 units. That sounds small, but pH is a logarithmic scale, so 0.1 units translates to roughly a 30% increase in hydrogen ion concentration. For context, the ocean's current average surface pH is around 8.1, down from a pre-industrial value of about 8.2. It's still technically basic (not acidic like lemon juice or vinegar), but the direction and speed of the shift are what matter for marine life.

Calcification in Marine Organisms

Calcification is the biological process by which organisms construct hard structures out of calcium carbonate ($CaCO_3$). These structures include shells (oysters, mussels), exoskeletons (crabs, lobsters), and the stony frameworks of coral reefs.

The basic reaction organisms use to build these structures is:

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

They pull dissolved calcium ions ($Ca^{2+}$) and carbonate ions ($CO_3^{2-}$) from the surrounding seawater. The problem is that ocean acidification directly reduces the supply of $CO_3^{2-}$. Here's why: the extra $H^+$ ions from acidification react with carbonate ions to form bicarbonate ($HCO_3^-$) instead. So the more acidic the water becomes, the fewer carbonate ions are available for shell-building.

The consequences for calcifying organisms are significant:

• Shells and skeletons grow thinner, weaker, or deformed because there isn't enough $CO_3^{2-}$ to build with.
• Existing $CaCO_3$ structures can actually begin to dissolve if the water becomes undersaturated with carbonate ions.
• Weakened structures make organisms more vulnerable to predation (from crabs, fish, and other predators), physical damage from waves and storms, and compounding stressors like rising temperatures and pollution.

Vulnerable Species

Not all marine organisms are equally affected. The most vulnerable are those that depend heavily on $CaCO_3$ structures.

Corals secrete $CaCO_3$ exoskeletons that form the physical foundation of reef ecosystems. Reduced calcification rates slow coral growth and weaken reef structures. Reefs like the Great Barrier Reef and Caribbean reef systems are already showing measurable declines linked to acidification combined with warming.

Mollusks such as oysters, mussels, and pteropods (tiny "sea butterflies") rely on $CaCO_3$ shells for protection. Pteropods are particularly concerning because they're a key food source for salmon, herring, and other commercially important fish. Studies have shown that pteropod shells in the Southern Ocean are already dissolving in more acidic waters.

Calcifying plankton, including coccolithophores and foraminifera, form tiny $CaCO_3$ plates or shells called tests. These organisms sit near the base of marine food webs, so declines in their populations ripple upward through the entire ecosystem. They also play a major role in the ocean's carbon cycle: when they die, their calcium carbonate structures sink to the seafloor and become part of marine sediments, effectively sequestering carbon.

Consequences of Ocean Acidification

Ecological consequences:

1. Reduced biodiversity as sensitive calcifying species decline, altering community structure and disrupting food web dynamics.
2. Coral reef degradation, which eliminates habitat for thousands of fish and invertebrate species and reduces the natural coastal protection that reefs provide against storms and erosion.
3. Disrupted biogeochemical cycles as shifts in plankton communities change how carbon and nutrients move through the ocean.

Economic consequences:

1. Fisheries and aquaculture losses from declines in commercially valuable shellfish (oysters, mussels, clams). The U.S. Pacific Northwest oyster industry has already experienced hatchery failures linked to acidified water, costing millions of dollars.
2. Tourism and recreation declines as degraded coral reefs and diminished marine life reduce the appeal of coastal destinations, cutting revenue for communities that depend on ocean tourism.
3. Coastal infrastructure costs as weakened coral reefs provide less buffering against storms and sea-level rise, forcing higher spending on artificial coastal protection and restoration.