August 12, 2020
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​​​​​​​It’s not just about marine mammals anymore: How ocean noise can harm marine ecosystems

Editor’s note: We’ve all read about how ocean noise can harm marine mammals. New research reveals that it can have profound impacts on lower trophic levels as well, with likely consequences for marine ecosystems. Catch up on the latest research with this month’s Skimmer.

A little background on sound in the ocean

  • As we get started discussing how ocean noise affects marine animals (and marine ecosystems), there are some critical points about sound in the ocean to keep in mind:

    • Sound is a “compression wave that causes particles of matter to vibrate as it transfers from one to the next.” How loud or soft a sound is perceived to be is a function of the amplitude or intensity of the wave[1] and the receiver’s hearing. Sound waves with higher amplitudes/intensities are perceived to be “louder” than sound waves with lower amplitudes/intensities.
    • Sound travels a lot faster and farther in seawater than through the air. Sound travels nearly five times faster in seawater than on land (1500 vs 340 m/s) and can travel hundreds to thousands of kilometers in seawater versus tens of kilometers on land. As a rule, lower frequency sounds travel farther than high frequency sounds because lower frequency sound waves lose energy (“dissipate”) more slowly. In addition, higher amplitude/more intense/louder sounds travel farther than lower amplitude/less intense/softer sounds because they have more energy to start with.
    • In the ocean, vision is limited to tens of meters at best, and sense of touch even less than that, while smell and taste are greatly dependent on local environmental conditions such as water currents. Consequently, many marine animals – including invertebrates – use sound as their principal way of sensing the environment. They use sound for finding food, finding mates, locating offspring and other community members, finding appropriate habitat, avoiding predators, navigating, and more.
    • Sound in the ocean has two components: a pressure component that acts in all directions at once and a particle motion component that has directionality. While mammals primarily sense the pressure component of sound, marine fish and invertebrates primarily sense the particle motion component of sound.
    • The “silent oceans” are anything but silent. The oceans have rich natural soundscapes from both abiotic and biotic sources (e.g., rain, waves, earthquakes, animal vocalizations). Sound does not travel well across the ocean-atmosphere boundary, however, giving outside observers the false impression of relative silence.
    • “Noise” is simply sound that is unwanted, unpleasant, and/or disruptive. If you think back to that list of things that marine animals use sound for (i.e., finding food, finding mates, locating offspring and other community members, finding appropriate habitat, avoiding predators, navigating, and more), manmade sounds have the potential to disrupt all of them. Given that manmade sound has intensified over a very short period of time in evolutionary terms (50-100 years) and most animals have not had time to adapt to these new sounds, it is assumed that most anthropogenic sound in the ocean is indeed “noise” to marine animals.