One of my favorite things to do is to float on my back in a warm sea with just my nose, mouth and eyes sticking out above the water line. It's easy to bob in this position in salty water; I can breathe, watch the clouds and listen to the ocean sounds below me. It's very relaxing.
Mostly what I hear when my ears are submerged is the sound of things moving across the seafloor. Sometimes I can even hear a large school of fish if they're zipping around nearby. If a motor boat goes by, it's louder than you might expect.
I've heard other sounds too. Hard coral brushing up against itself makes a clinking sound, and as underwater waves hit rocks or submerged cliff edges, it makes a soft sound. The noise of whale sharks filtering gallons of water as they graze the seas is distinctive, like a washing machine that breathes, while spinner dolphins squeak above and below the water's surface as the perform for each other.
The underwater world is not a quiet place at all, though sound certainly travels differently there.
But as with so much else on this planet, we're affecting the underwater soundscape. In places where glacial waters flow into the ocean, for example, that sound has increased to a background roar over time as meltwater increases due to climate change.
According to a 2015 study out of Alaska that looked at (or more correctly, listened to) glacierized fjords, scientists found that: "... average noise levels are louder than nearly all measured natural oceanic environments (significantly louder than sea ice and nonglacierized fjords). Icy Bay, Alaska, has an annual average sound pressure level of 120 dB (referenced to 1 μPa) with a broad peak between 1000 and 3000 Hz." The noise is due to the bubbles that form in the water column as the glacier ice melts. The researchers checked other fjords where glaciers were melting in Alaska and Antarctica and found similar results, supporting the conclusion that the noise was due to meltwater and not another factor.
Because marine creatures communicate with various sounds underwater — to find mates, locate offspring, find food and steer clear of predators — "These high noise levels likely alter the behavior of marine mammals," the researchers wrote.
The ways humans and human activities affect the surrounding environment go far beyond melting ice.
A 2015 study examined one particular marine soundscape. The Australian researchers wrote: "Because soundscapes are the sum of the acoustic signals produced by individual organisms and their interactions, they can be used as a proxy for the condition of whole ecosystems and their occupants." The soundscape survey found that ocean acidification, which is driven by higher carbon loads in the atmosphere, leads to quieter oceans — specifically a reduction in sound from a specific kind of shrimp clicking and clacking their claws. But why do quieter shrimp matter?
The researchers explain:
"As coastal marine soundscapes are dominated by biological sounds produced by snapping shrimp, the observed suppression of this component of soundscapes could have important and possibly pervasive ecological consequences for organisms that use soundscapes as a source of information. This trend towards silence could be of particular importance for those species whose larval stages use sound for orientation towards settlement habitats."
What is soundscape ecology?
It's not just in the ocean that sounds indicate ecosystem health; loud highways make birds sing louder and shorter songs as they attempt to cut through the din to find mates, and those birds that cannot raise their voices — certain species aren't able to get louder — have seen greater population declines.
A pioneer of soundscape ecology, Bernie Krause, has been recording soundscapes for 50 years via his project Wild Sanctuary. He's featured in the video above, which explains what soundscape ecology is and why it matters. In an interview with the BBC, Krause says he's heard dramatic changes in landscapes, places he's returned to over the decades: "Well over 50 percent of this archive comes from sites now either altogether silent, or so transformed by human endeavor they can no longer be heard in any of their original form," he says.
Scientists from many universities in the United States and around the world have been recording soundscapes, as the field has gained recognition over the past decades. They are looking at biophony (animal sounds), geophony (wind and water sounds) and anthrophony (human-made sounds). You can even take part in collecting data for their research through citizen science initiatives like Record the Earth from Purdue University.
As an adult, I've returned to places in Florida where I snorkeled as a kid, only to find bleached-out corals. It shouldn't be a surprise that one day, when I lay back to listen to the sounds of the ocean, it might be quieter too.
Maybe it won't be a surprise, but it will be a tragedy.