What Is Ocean Acidification?

Underwater Ellisella Gorgonian sea fan coral a carbon capture system
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Ocean acidification, or OA, is the process by which increases in dissolved carbon make seawater more acidic. While ocean acidification occurs naturally over geologic timescales, the oceans are currently acidifying at a faster rate than what the planet has ever experienced before. The unprecedented rate of ocean acidification is expected to have devastating consequences on marine life, particularly shellfish and coral reefs. Current efforts to combat ocean acidification are largely focused on slowing the pace of ocean acidification and bolstering the ecosystems capable of dampening ocean acidification's full effects.

What Causes Ocean Acidification?

Smoke from a powerplant in front of a sunset.
The burning of fossil fuels releases greenhouse gases into the atmosphere, including the excess carbon dioxide causing ocean acidification.

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Today, the primary cause of ocean acidification is the ongoing release of carbon dioxide into our atmosphere from the burning of fossil fuels. Additional culprits include coastal pollution and deep-sea methane seeps. Since the start of the industrial revolution about 200 years ago, when human activities began releasing large amounts of carbon dioxide into Earth's atmosphere, the ocean's surface has become about 30% more acidic.

The process of ocean acidification begins with dissolved carbon dioxide. Like us, many underwater animals undergo cellular respiration to generate energy, releasing carbon dioxide as a byproduct. However, much of the carbon dioxide dissolving into the oceans today comes from the excess of carbon dioxide in the atmosphere above from the burning of fossil fuels.

Once dissolved in seawater, carbon dioxide goes through a series of chemical changes. Dissolved carbon dioxide first combines with water to form carbonic acid. From there, carbonic acid can break apart to generate standalone hydrogen ions. These excess hydrogen ions attach to carbonate ions to form bicarbonate. Eventually, not enough carbonate ions remain to attach to each hydrogen ion that arrives in seawater via dissolved carbon dioxide. Instead, the standalone hydrogen ions accumulate and lower the pH, or increase the acidity, of the surrounding seawater.

In non-acidifying conditions, much of the ocean's carbonate ions are free to make connections with other ions in the ocean, like calcium ions to form calcium carbonate. For animals that need carbonate to form their calcium carbonate structures, like coral reefs and shell-building animals, the way in which ocean acidification steals carbonate ions to instead produce bicarbonate reduces the pool of carbonate available for essential infrastructure.

The Impact of Ocean Acidification

Below, we analyze specific marine organisms and how these species are impacted by ocean acidification.

Mollusks

about 100 blue mussels attached to a rock in the intertidal zone.
The blue mussel is one of a few shell-building marine animals with the ability to adapt to ocean acidification.

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The ocean's shell-building animals are most vulnerable to the effects of ocean acidification. Many ocean creatures, like snails, clams, oysters, and other mollusks, are equipped to pull dissolved calcium carbonate out of seawater to form protective shells through a process known as calcification. As human-generated carbon dioxide continues to dissolve into the ocean, the amount of calcium carbonate available for these shell-building animals dwindles. When the amount of dissolved calcium carbonate becomes particularly low, the situation becomes significantly worse for these shell-dependent creatures; their shells start to dissolve. Simply put, the ocean becomes so deprived of calcium carbonate that it is driven to take some back.

One of the most well-studied marine calcifiers is the pteropod, a swimming relative of the snail. In some parts of the ocean, pteropod populations can reach over 1,000 individuals in a single square meter. These animals live throughout the ocean where they have an important role in the ecosystem as a source of food for larger animals. However, pteropods have protective shells threatened by ocean acidification's dissolving effect. Aragonite, the form of calcium carbonate pteropods use to form their shells, is approximately 50% more soluble, or dissolvable, than other forms of calcium carbonate, making pteropods particularly susceptible to ocean acidification.

Some mollusks are equipped with means to hold on to their shells in the face of an acidifying ocean's dissolving pull. For example, clam-like animals known as brachiopods have been shown to compensate for the ocean's dissolving effect by creating thicker shells. Other shell-building animals, like the common periwinkle and the blue mussel, can adjust the type of calcium carbonate they use to form their shells to prefer a less soluble, more rigid form. For the many marine animals that cannot compensate, ocean acidification is expected to lead to thinner, weaker shells.

Unfortunately, even these compensation strategies come at a cost to the animals that have them. To fight against the ocean's dissolving effect while grasping on to a limited supply of calcium carbonate building blocks, these animals must dedicate more energy to shell-building to survive. As more energy is used for defense, less remains for these animals to perform other essential tasks, like eating and reproducing. While a lot of uncertainty remains around the ultimate effect ocean acidification will have on the ocean's mollusks, it's clear the impacts will be devastating.

Crabs

While crabs also use calcium carbonate to build their shells, the effects of ocean acidification on crab gills may be most important to this animal. Crab gills serve a variety of functions for the animal including the excretion of carbon dioxide produced through breathing. As the surrounding seawater becomes full of excess carbon dioxide from the atmosphere, it becomes more difficult for crabs to add their carbon dioxide to the mix. Instead, crabs accumulate carbon dioxide in their hemolymph, the crab-version of blood, which instead changes the acidity within the crab. Crabs best suited to regulating their internal body chemistry are expected to fare best as the oceans become more acidic.

Coral Reefs

an underwater view of a coral reef with a school of fish swimming above.
Stony corals use calcium carbonate to create their skeletons.

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Stony corals, like the ones known to create magnificent reefs, also rely on calcium carbonate to build their skeleton. When a coral bleaches, it is the animal's stark white calcium carbonate skeleton that appears in the absence of the coral's vibrant colors. The three-dimensional stone-like structures built by corals create habitat for many marine animals. While coral reefs encompass less than 0.1% of the ocean floor, at least 25% of all known marine species use coral reefs for habitat. Coral reefs are also a vital source of food for marine animals and humans alike. Over 1 billion people are estimated to depend on coral reefs for food.

Given the importance of coral reefs, the effect of ocean acidification on these unique ecosystems is particularly relevant. So far, the outlook does not look good. Ocean acidification is already slowing down coral growth rates. When coupled with warming seawater, ocean acidification is thought to exacerbate the damaging effects of coral bleaching events, causing more corals to die from these events. Fortunately, there are ways in which corals may be able to adapt to ocean acidification. For example, certain coral symbionts — the tiny pieces of algae that live within corals — may be more resistant to ocean acidification's effects on corals. In terms of the coral itself, scientists have found potential for some coral species to adapt to their rapidly changing environments. Nonetheless, as the warming and acidification of the oceans continues, the diversity and abundance of corals will likely decline severely.

Fish

Fish may not produce shells, but they do have specialized ear bones that require calcium carbonate to form. Like tree rings, fish ear bones, or otoliths, accumulate bands of calcium carbonate that scientists can use to determine the age of a fish. Beyond their use to scientists, otoliths also have an important role in a fish's ability to detect sound and orient their bodies properly.

As with shells, otolith formation is expected to be impaired by ocean acidification. In experiments where future ocean acidification conditions are simulated, fish have been shown to have impaired hearing abilities, learning capacity, and altered sensory function due to the effects of ocean acidification on fish otoliths. Under ocean acidification conditions, fish also show increased boldness and different anti-predator responses compared to their behavior in the absence of ocean acidification. Scientists fear the behavioral changes in fish linked to ocean acidification are a sign of trouble for entire communities of marine life, with major implications for the future of seafood.

Seaweed

an underwater view of a kelp forest with light shining down from the surface.
Kelp forests may reduce the effects of ocean acidification in their immediate surroundings.

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Unlike animals, seaweeds may reap some benefits in an acidifying ocean. Like plants, seaweeds photosynthesize to generate sugars. Dissolved carbon dioxide, the driver of ocean acidification, is absorbed by seaweeds during photosynthesis. For this reason, an abundance of dissolved carbon dioxide may be good news for seaweeds, with the clear exception of seaweeds that explicitly use calcium carbonate for structural support. Yet even non-calcifying seaweeds have reduced growth rates under simulated future ocean acidification conditions.

Some research even suggests areas abundant in seaweed, like kelp forests, could help reduce the effects of ocean acidification in their immediate surroundings due to the seaweed's photosynthetic removal of carbon dioxide. Yet when ocean acidification is combined with other phenomena, like pollution and oxygen deprivation, the potential benefits of ocean acidification for seaweeds may be lost or even reversed.

For seaweeds that use calcium carbonate to create protective structures, ocean acidification's effects more closely match those of calcifying animals. Coccolithophores, a globally-abundant species of microscopic algae, use calcium carbonate to form protective plates known as coccoliths. During seasonal blooms, coccolithophores can reach high densities. These non-toxic blooms are quickly destroyed by viruses, which use the single-celled algae to generate more viruses. Left behind are the coccolithophores' calcium carbonate plates, which often sink to the bottom of the ocean. Through the life and death of the coccolithophore, carbon held in the algae's plates is transported to the deep ocean where it is removed from the carbon cycle, or sequestered. Ocean acidification has the potential to inflict serious damage on the world's coccolithophores, destroying a key component of the ocean food and a natural pathway for sequestering carbon on the seafloor.

How Can We Limit Ocean Acidification?

By eliminating the cause of today's rapid acidification of the ocean and supporting biological refuges that dampen the effects of ocean acidification, the potentially dire consequences of ocean acidification may be avoided.

Carbon Emissions

Over time, approximately 30% of the carbon dioxide released into Earth's atmosphere has ended up dissolving into the ocean. Today's oceans are still catching up to absorbing their portion of the carbon dioxide already in the atmosphere, although the pace of ocean absorption is increasing. Because of this delay, a certain amount of ocean acidification is likely inevitable, even if humans halt all emissions immediately, unless carbon dioxide is removed from the atmosphere directly. Nonetheless, reducing - or even reversing - carbon dioxide emissions remains the best way to limit ocean acidification.

Kelp

Kelp forests may be able to reduce the effects of ocean acidification locally through photosynthesis. However, a 2016 study found that over 30% of the ecoregions they observed had experienced kelp forest decline over the past 50 years. On the West Coast of North America, declines have largely been caused by imbalances in predator-prey dynamics that have allowed kelp-eating urchins to take over. Today, many initiatives are underway to bring kelp forests back to create more areas shielded from ocean acidification's full effect.

Methane Seeps

While naturally formed, methane seeps have the potential to exacerbate ocean acidification. Under current conditions, the methane stored in the deep ocean remains under sufficiently high pressure and cold temperatures to keep the methane secure. However, as ocean temperatures rise, the ocean's deep-sea stores of methane are at risk of being released. If marine microbes gain access to this methane, they will convert it to carbon dioxide, strengthening ocean acidification's effect.

Given the potential for methane to enhance ocean acidification, steps to reduce the release of other planet-warming greenhouse gases beyond just carbon dioxide will limit the impact of ocean acidification in the future. Similarly, solar radiation puts the planet and its oceans at risk of warming, therefore methods of reducing solar radiation may limit the effects of ocean acidification.

Pollution

In coastal environments, pollution magnifies the effects of ocean acidification on coral reefs. Pollution adds nutrients to normally nutrient-poor reef environments, giving algae a competitive advantage over corals. Pollution also disrupts a coral's microbiome, which makes the coral more susceptible to disease. While warming temperatures and ocean acidification are more damaging to corals than pollution, removing other coral reef stressors can improve the likelihood of these ecosystems adapting to survive. Other ocean pollutants, like oils and heavy metals, cause animals to increase their rates of respiration - an indicator for energy use. Given that calcifying animals must apply additional energy to build their shells faster than they dissolve, the energy needed to simultaneously combat ocean pollution makes it even harder for shell-building animals to keep up.

Overfishing

a parrotfish eating algae on a coral reef.
Parrot fish eat algae, helping to prevent it from taking over coral reefs.

Humberto Ramirez / Getty Images

For coral reefs in particular, overfishing is yet another stressor to their existence. When too many herbivorous fish are removed from coral reef ecosystems, coral-smothering algae can more easily take over a reef, killing corals. As with pollution, reducing or eliminating overfishing increases coral reef resilience to the effects of ocean acidification. In addition to coral reefs, other coastal ecosystems are more susceptible to ocean acidification when simultaneously impacted by overfishing. In rocky intertidal environments, overfishing can lead to an overabundance of sea urchins, which create barren areas where there once was calcifying algae. Overfishing also leads to the depletion of non-calcifying seaweed species, like kelp forests, damaging places where ocean acidification's effects are dampened by the photosynthetic uptake of dissolved carbon.

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