Ocean acidification represents one of the most pressing yet underappreciated consequences of rising carbon dioxide (CO2) emissions. As the oceans absorb roughly a quarter of anthropogenic CO2, a series of chemical reactions unfold that lower seawater pH and alter the carbonate chemistry that countless marine organisms depend upon. While much attention has focused on the impairment of calcification in shellfish and coral reefs, a quieter but equally profound effect is unfolding beneath the waves: the disruption of sound. Many marine animals, from the largest baleen whales to the smallest reef fish, rely on acoustic signals to mate, feed, navigate, and maintain social bonds. Changes in water chemistry can modify how sound travels through the ocean, potentially muffling or distorting communication at critical moments. This article explores the mechanisms by which ocean acidification influences marine acoustic communication, reviews the species most at risk, and highlights ongoing research aimed at understanding and mitigating these impacts.

Understanding Ocean Acidification

Ocean acidification is driven by the dissolution of atmospheric CO2 into seawater, where it reacts with water (H2O) to form carbonic acid (H2CO3). This weak acid quickly dissociates, releasing hydrogen ions (H+) and bicarbonate ions (HCO3-). The increase in hydrogen ions reduces the pH of seawater, making it more acidic. Since the start of the Industrial Revolution, the average surface ocean pH has dropped by about 0.1 units, representing a 30% increase in acidity. While that number may seem small, the pH scale is logarithmic, meaning each 0.1 unit change corresponds to a significant shift in hydrogen ion concentration.

Projections under business-as-usual emissions scenarios suggest that by the end of the century, surface ocean pH could drop by another 0.3–0.4 units. This rate of change is unprecedented in the geological record over the past 50 million years. The most immediate consequence for marine life is a reduction in the availability of carbonate ions essential for calcifying organisms like oysters, clams, and corals. However, the chemical changes also affect the physical properties of seawater, including its density, compressibility, and sound-absorbing characteristics. These shifts are now being studied for their potential to alter the acoustic environment on which many marine species depend.

The Importance of Sound in the Ocean

Light penetrates poorly in water, especially beyond a few hundred meters. In contrast, sound travels roughly five times faster in water than in air and can propagate over hundreds or even thousands of kilometers under the right conditions. This makes acoustic communication the primary sensory channel for many marine animals. Whales and dolphins produce complex songs, clicks, and whistles for echolocation and social interaction. Fish use sounds to attract mates, defend territories, and coordinate schooling behavior. Even invertebrates like snapping shrimp generate intense clicks to stun prey and communicate.

Sound also provides a sense of the environment: the breaking of waves, the crackle of coral reefs, and the distant rumble of a ship all convey information. Changes in how these sounds travel can mask critical signals, increase ambient noise levels, or alter the range over which communication is effective. Ocean acidification, by modifying the chemistry and physics of seawater, has the potential to act as an invisible disruptor of this acoustic world.

How Ocean Acidification Alters Acoustic Signals

Changes in Sound Absorption

One of the primary mechanisms through which acidification affects sound is by altering the absorption of low-frequency sound. In seawater, sound energy is absorbed through two main relaxation processes: one involving boric acid (B(OH)3) and the other involving magnesium carbonate (MgCO3) and related species. The boric acid relaxation is particularly sensitive to pH. As pH decreases, the concentration of borate ions decreases, reducing the absorption of sound in the 1–10 kHz range. This means that lower-frequency sounds will travel farther in more acidic waters.

While greater transmission distances might sound beneficial, they actually increase the background noise level. Shipping noise, seismic exploration, and other human activities produce low-frequency noise that will propagate further, potentially drowning out biological signals. For example, a whale call that might have been detectable over 100 kilometers in today's ocean could travel even further in a high-CO2 future, but so will the noise from passing vessels, creating a noisier environment overall. This is often referred to as the "ocean acidification quieting" paradox: less natural absorption means more ambient noise, not less.

Changes in Sound Speed and Propagation

Sound speed in seawater is primarily controlled by temperature, salinity, and pressure. However, pH indirectly influences sound speed by affecting the density and compressibility of the water. The effect is small—on the order of meters per second—but it can alter the sound speed profile and thus how sound waves refract. In many regions, the deep sound channel (SOFAR channel) is defined by a minimum in sound speed; changes in the vertical distribution of pH could shift this channel’s depth or intensity, affecting long-range communication by whales that use it.

Additionally, the chemical changes influence the formation of shallow water ducts that trap sound near the surface. In coastal waters where many fish and dolphins reside, small changes in sound propagation can have major consequences for effective communication ranges. Laboratory experiments and computer models are beginning to quantify these effects, though direct field validation remains challenging.

Impact on Signal Detection and Processing

Beyond physics, acidification may affect the sensory biology of marine animals. Elevated CO2 levels can disrupt the acid-base balance in fish and invertebrate tissues, potentially affecting auditory systems. Several studies on fish have shown that exposure to high CO2 alters behavioral responses to sound cues, such as predator avoidance and homing. For example, larval fish that normally use reef noise to locate suitable habitat show reduced attraction to those sounds when reared in acidified conditions. This suggests that the ability to detect or interpret acoustic signals is compromised at the neural level, compounding the physical changes in the environment.

Effects on Key Marine Species

Whales and Dolphins

Cetaceans are highly reliant on sound. Baleen whales produce low-frequency songs that travel hundreds of kilometers, enabling communication across vast ocean basins. Toothed whales like dolphins and porpoises use high-frequency clicks for echolocation and social sounds. Ocean acidification could affect these groups in several ways:

  • Increased ambient noise: Lower absorption of low frequencies means shipping and seismic noise will propagate further, potentially masking whale calls. This could reduce the effective range of communication, making it harder for individuals to find mates or maintain group cohesion.
  • Altered sound speed profiles: Shifts in the SOFAR channel could change the optimal frequencies or depths for long-range communication, forcing whales to adapt or suffer reduced signal reception.
  • Direct physiological effects: While less studied, high CO2 may affect the hearing sensitivity of marine mammals. Research on seals and manatees suggests potential impacts, but cetacean data are limited.

Fish

Fish use sound for a variety of purposes. Many species produce grunts, booms, or pops during spawning aggregations, which help synchronize reproduction. Others rely on the sounds of healthy coral reefs to navigate and settle as larvae. Acidification threatens these processes:

  • Impaired settlement: As mentioned, larval fish lose their attraction to reef sounds in high-CO2 water, leading to poor habitat selection and increased mortality.
  • Reduced predator detection: Fish that cannot hear the sounds of approaching predators are more vulnerable. Experiments with clownfish and damselfish show altered antipredator behavior under elevated CO2.
  • Disrupted schooling: Many fish use sound for coordination. If the acoustic environment changes, schools may fragment, increasing predation risk.

Invertebrates and Coral Reefs

Snapping shrimp are among the loudest animals in the ocean, producing clicks that create a constant crackle on healthy reefs. This noise is a key component of the reef soundscape, attracting fish and indicating habitat quality. Ocean acidification may reduce the number or activity of snapping shrimp, altering the acoustic signature of reefs. Additionally, the loss of structural complexity due to coral bleaching and acidification changes how sound is reflected and absorbed on reefs, further modifying the soundscape. Some studies have shown that degraded reefs sound quieter, which can deter larval settlement and accelerate ecosystem decline.

Research and Future Directions

Scientific understanding of ocean acidification’s effects on acoustics is still in its infancy, but several research fronts are advancing rapidly:

Laboratory Experiments

Controlled seawater mesocosms allow researchers to manipulate pH and CO2 levels while measuring sound absorption, sound speed, and animal behavior. For example, experiments at the Alfred Wegener Institute and the University of Otago have documented changes in the absorption coefficient and fish hearing thresholds under projected future pH levels. These studies provide crucial parameterizations for models.

Acoustic Modeling

Ocean models that couple chemical, physical, and acoustic components are being developed to predict how soundscapes might change under different emissions scenarios. The National Oceanic and Atmospheric Administration (NOAA) and the Scripps Institution of Oceanography are key players in this effort. Early results indicate that by 2100, the absorption of low-frequency sound could decrease by up to 40% in some regions, significantly altering the noise budget.

Field Observations

Long-term acoustic monitoring stations, such as those deployed by the NOAA Pacific Marine Environmental Laboratory, are collecting data on ocean sound levels alongside pH measurements. These time series will help correlate changes in ambient noise with acidification trends. However, disentangling the effects of acidification from other drivers like temperature and shipping is challenging.

Behavioral and Ecological Studies

Research on animal behavior under ocean acidification continues to grow. For instance, a 2020 study in Scientific Reports found that sea bass reared in high CO2 water had reduced auditory sensitivity to low-frequency sounds. Similar studies on coral reef fish and crustaceans are ongoing. Integrating these findings with acoustic propagation models will be essential for predicting ecosystem-level consequences.

Mitigation and Conservation Strategies

The root cause of ocean acidification is atmospheric CO2 concentration, so the most effective mitigation is reducing emissions. However, local actions can also help preserve acoustic communication in the face of inevitable changes:

  • Marine protected areas (MPAs): MPAs can reduce anthropogenic noise from shipping, drilling, and construction. By giving marine animals quieter environments, they may partially offset the increased propagation of noise from distant sources. For example, the Stellwagen Bank National Marine Sanctuary has implemented ship speed and routing measures to reduce noise.
  • Restoration of habitats: Healthy seagrass beds, mangroves, and coral reefs provide important acoustic habitats. Restoring these ecosystems helps maintain natural soundscapes that aid larval settlement and predator-prey interactions.
  • Noise management: Regulations on vessel noise, seismic surveys, and underwater construction can reduce the overall noise floor. The NOAA Noise Reference Station network provides data to inform such policies.
  • Public awareness and policy: Educating policymakers and the public about the dual threats of acidification and noise pollution can spur action. International bodies like the International Whaling Commission are beginning to consider acoustic impacts in their conservation efforts.

Conclusion

Ocean acidification is not only a chemical crisis; it is an acoustic one. By altering how sound is absorbed, transmitted, and perceived, the changing chemistry of the sea undermines the very senses that marine life relies on for survival. From the haunting songs of humpback whales to the synchronized clicks of snapping shrimp, the ocean’s symphony is being rewritten. While the full extent of these impacts is still being uncovered, the need for action is clear. Reducing CO2 emissions remains the most crucial step, but managing local noise and protecting critical habitats can offer some relief in the interim. Continued research, monitoring, and international cooperation are essential to safeguard the acoustic environment for future generations. The silence of the deep is not one we can afford to take for granted.