Humans make a lot of noise. No matter where we are on the planet, we inevitably make a din. And since the oceans cover 70% of the Earth's surface, there is no question that we affect the Big Blue as well. This phenomenon is well documented, and the scientific community is paying increasing attention to the ecological consequences of underwater noise generated by human activities such as shipping, oil exploration, sonar use, dredging, and the construction of renewable energy devices at sea. The arrival of these activities in previously unexploited areas of the ocean, such as the polar regions or coral reefs, is of particular concern.
According to the World Health Organization, man-made noise - also called "anthropogenic" - is one of the most dangerous forms of pollution and has become ubiquitous in terrestrial and aquatic ecosystems.
Underwater noise affects the anatomy, physiology, and behavior of marine fauna in several ways. Loud sounds can cause damage to their ears or other tissues, interfere with communication between animals by masking the sounds they make, and even alter their hormone levels, which can lead to stress reactions and lack of sleep. The louder the sound, the longer it lasts, and the closer the source, the more likely the animals are to suffer. Thus, human-created noise can cause not only small short-term behavioral adjustments but also significant behavioral or physiological changes that can lead to death.
The best-documented effects concern marine mammals, such as the beluga whales who live alongside ships in the St. Lawrence Estuary. But what about other marine species? This area still needs to be studied further; however, it would seem that the key factor for some organisms is the movement of particles rather than sound pressure.
Sound in Water: Beyond Acoustics
Sound is the propagation of a mechanical disturbance in a medium, such as air or water, as an acoustic wave. When traveling under the sea, sound momentarily changes the pressure in the path of the acoustic wave, in addition to vibrating the drops of water in its path. Not all living species in the path of the wave experience these two disturbances in the same way. Some organisms hear the noise resulting from the pressure changes (which is reflected in their eardrums). Others are disturbed by the vibration of the particles, and some experience both components of the sound.
Many scientific studies of the effect of noise on species other than cetaceans or pinnipeds (such as fish and invertebrates) have overlooked important issues, including the sensitivity of many of these species to particle motion rather than to sound pressure, as would be the case for marine mammals. To complicate matters further, noise and particle motion are not perceptible at the same frequencies, the same intensity, or the same distance from the source of the sound wave. Underwater noise thus has a broader impact on ecosystems than previously known and therefore requires closer management.
Currently, most limits and management on underwater noise exposure/levels relate almost exclusively to marine mammals. Criteria for fish and invertebrates are mostly inappropriate or inaccurate because they are based on sound pressure rather than particle motion. As a result, effects on fish and invertebrate populations are often unknown and remain unassessed.
Nevertheless, there is a growing understanding of how certain species such as cnidarians (sea anemones, jellyfish, and corals), crustaceans, and even plants can be affected by changes in their sound environment. Despite their lack of auditory structure, these marine organisms have specialized sensory organs that perceive gravity -essential for detecting components of their habitat, such as sand-but also pick up vibrations in the water. It is these organs that could be affected by noise exposure.
These effects are especially concerning because these organisms are sessile, meaning they do not move. In a recent study, a team of researchers from Spain and France explains that low-frequency sounds can produce alterations in the roots and rhizomes of some seagrass plants, thus hindering their ability to store energy or to live in symbiosis with other living species. Yet, not only does this crucial ecosystem provide protection against coastal erosion, but it is also home to a large portion of coastal biodiversity, in addition to slowing global warming by capturing some of our CO2.
The projected growth of the oceanic economy is expected to result in an expansion of noise-generating activities. Unlike other marine pollutants such as microplastics or persistent organic pollutants, underwater noise leaves no trace and disperses quickly into the environment. If effective, interventions to reduce noise pollution could quickly relieve the pressure on acoustically sensitive organisms or the movement of particles that this noise generates. New scientific advances and current research topics demonstrate the urgent need to better understand the ecological impact of anthropogenic noise on everything from whales to plants in order to better protect marine ecosystems.