BOSTON – The ocean was flat and the winter darkness over Cape Cod Bay (search) was unbroken by ship lights. But below the bay's surface, Christopher Clark found things weren't as serene as they seemed.
The bay is saturated with sound.
"It's just a great, big amphitheater," said Clark, a Cornell bioacoustics scientist who monitored the bay with underwater listening devices.
The sound carrying through the bay that evening was part of an ever louder man-made din that's filling thenal defense. But a broader concern for scientists is rising levels of ocean background noise, much of it generated by commercial shipping, and whether it interferes with the way the entire sea has operated for eons.
Based on volume of traffic alone, scientists know the North Atlantic (search) and North Pacific oceans, which are the busiest, are also the noisiest, Clark said. The area around Indonesia (search) is heavy too with shipping traffic.
Hearing is the primary sense for marine life, which uses sound for navigation and communication. Some scientists believe the spreading "acoustic smog" is essentially blinding marine life, affecting feeding, breeding and other crucial activities.
"Their world is just being collapsed," Clark said. "They rely so heavily on sound. They can't see anything."
Despite concerns, evidence is scant of the real effects of sound.
Even with new technology, ocean animals are hard to track, and drawing conclusions about how sound influences their behavior is difficult. No system exists to monitor ocean sounds worldwide, and the data that's collected is often taken from a small number of sites that measure only certain frequencies. Underwater sound also seems to affect different animals in completely different ways.
Businesses and the military are unlikely to make major changes before more is known.
Brandon Southall, an acoustics researcher at the National Oceanic & Atmospheric Administration (search), said better research is urgently needed.
"People are inherently tied to the ocean for food, for cures to diseases, for weather," he said. "We're figuring out things are more interconnected than we ever could have originally envisioned."
Sound, which is created when molecules collide, carries farther and five times faster in water than air because of water's density. Since molecules in water are spaced closer together, they lose less energy before colliding with other molecules and sound is transferred more quickly and efficiently.
Through the ages, marine animals have learned to take advantage of the ocean's natural sound stages. Whales, for instance, talk about basic things like where the best food or breeding is. They even seem to compete to produce the most intricate songs.
Researchers believe animals may use the ocean's natural "sound channels" to communicate over thousands of miles. The channel is created where dropping temperatures, which force sound waves downward, meet increasing water pressure, which forces sound waves upward. At a certain depth, the sound gets caught between the two opposing forces and bounds ahead with little resistance.
Researchers suspect that dumping a cacophony of new noise into this system isn't good. Southall said there's convincing evidence of a phenomenon called "masking," in which the increased ambient noise drowns out natural ocean communications.
Huge increases in commercial shipping have coincided with increased ocean noise. Between 1948 and 1998, the world shipping fleet has increased from 85 million tons worth of ship weight to 550 million tons, according to figures in a 2003 report "Ocean Noise and Marine Mammals," published by the National Academies. Scientists say the background noise in the ocean has increased roughly 15 decibels in that time.
Joel Reynolds, director of the Marine Mammal Program at the Natural Resources Defense Council, said there's evidence marine mammals are changing their sound patterns or rates, which could show their normal communication has been disrupted.
Kathy Metcalf, director of Maritime Affairs at the Chamber of Shipping in America, said she concedes that increasing ocean noise caused by ships will at some point interfere with marine life. Metcalf advocates pre-emptive steps, such as installing quieter propellers in new ships, which would reduce noise and likely benefit the industry by increasing the efficiency with which ships move through water.
But retrofitting current ships to reduce noise would be extremely expensive, and the benefit is uncertain, she said.
"If somebody is going to signal we need to start absorbing these costs when we're not even sure there's a negative impact, that's where we're digging in our heels," she said. "There's a huge issue surrounding the validity of the science on this issue."
Southall acknowledges the mountain of work ahead to come up with real answers about ocean noise. To illustrate the difficulties of applying the science to ocean life, he points to the beluga whale, which flees from ship sounds in the high Arctic, but moves toward certain vessels in Alaska.
He added that sound is perceived by ocean animals so differently than land animals that it's almost like a different sense, making it hard to apply what we know about the effects of certain decibel levels to ocean life.
Still, Southall said he's optimistic that the emerging interest in the topic will lead to breakthroughs.
Reynolds said regulating ocean sound doesn't mean ending all its benefits, whether it's better national defense or the robust trade that comes with heavy shipping.
"We have to treat it like any other form of pollution," Reynolds said. "We have to regulate it to protect other things we care about."
Clark said uncertainties can't be an excuse to do nothing, because the damage might be done by the time the effects of noisy oceans are known.
"It's like global warming," he said. "We're going to get one chance."