Tangled web. Rising ocean acidity makes sound travel farther.

MBARI (original image courtesy David Fierstein)

Acidic Oceans Getting Noisy, Too

The ocean is becoming a noisier place. As seawater turns more acidic, due to absorption of carbon dioxide (CO2) building up in the atmosphere, it allows sound waves to travel farther, according to new research. That's potentially bad news for a host of marine animals, including whales and dolphins, that rely on sound for hunting and communication--and that are easily stressed by background noise from ship traffic and military sonar.

Scientists have known for more than 3 decades that lowering the pH level of seawater--making it more acidic--causes it to conduct sound more readily. Although the process isn't fully understood, it seems to be related to the way water and salt molecules vibrate when encountering sound waves.

A team led by ocean chemist Keith Hester of the Monterey Bay Aquarium Research Institute in Moss Landing, California, wanted to find out how rising atmospheric levels of CO2 are contributing to this phenomenon. They used projections by the U.N. Intergovernmental Panel on Climate Change, which has concluded that ocean pH levels will drop by 0.3 units by 2050--about four times faster than the rate that has occurred over the past 250 years. They combined those estimates with field and lab experiments to test sound conductivity, and they took into consideration estimated increases in ocean temperature and reductions in oxygen content, which also affect underwater acoustics.

The result, the team reports tomorrow in Geophysical Research Letters, is that underwater sounds in 2050 will travel up to 70% farther in some areas, such as the Atlantic Ocean, than they do today. This will be particularly true of the low-frequency moans and songs used by some marine mammals. "We were surprised to see the magnitude of the change was so great," Hester says.

Whales could be heavily impacted, says marine biogeographer John Guinotte of the Marine Conservation Biology Institute in Bellevue, Washington. Military sonar can disrupt whale behavior for more than 500 kilometers, he notes: "This is obviously not going to improve as the oceans become more acidic and less sound absorbent." Also likely to be affected are dolphins and species of fish that use sound to locate prey, avoid predators, and defend their territories. All of these functions could be disrupted by higher levels of background noise, Guinotte says.