by Samantha Tracy
figures by Sean Wilson
A solo diver dips under the surface of crashing waves to reveal a plethora of attention-grabbing colors in astounding brightness. The image of a coral reef is exciting and overwhelming for the human brain, but another world comes to life when we close our eyes. A cascading orchestra of gentle bubbling, a faint crackling chorus of snapping shrimp, and the songs of shifting sand as a crab makes its way underneath the reef floor: these are the sounds of a healthy coral reef. This coral reef chorus contributes vital environmental information to our understanding of coral reef ecology and plays an increasingly important role in the scientific understanding of biodiversity loss.
What is a soundscape?
Soundscapes encompass all of the sounds heard in a particular location that form an immersive acoustic environment, and they vary based on time of day, location, and perception of sound. Natural sounds are part of our soundscapes, including everything from the faint chirping of birds to the breeze gently shaking tree branches. The perception of sound and the role it plays in establishing our understanding of our surroundings is not an experience unique to humans. Scientists have been recording soundscapes in various ecosystems for decades, and the comparisons between past and current recordings warn us of the perilous fate of ocean habitats through the loss of sounds.
The importance of natural sound
Sound is a driver of ecological function. Species in habitats throughout the world rely on audio cues to understand their environment. Whether they are capturing prey, avoiding predators, listening for weather changes, or communicating with other species members, species rely on sound as a critical component of normal function. The importance of natural sound is not immediately obvious; the loss of environmentally specific sounds seems inconsequential when compared with the global warming crisis, but soundscapes are directly tied to biodiversity, ecological health, and species well-being.
For instance, in the underwater world of coral reefs, more diverse sound patterns often means more thriving species and a healthier ecosystem. The ecological downfall of a community leads to a loss of sound and is often accompanied by additional anthropogenic changes that negatively impact coral reef ecosystems. Soundscapes are indicative of ecosystem health and have been used by marine species to determine where they should live. Therefore, the discovery that human interference, particularly global climate change, drives disturbances in natural soundscapes has become a topic of concern among scientists and led to research on the cause and consequence of soundscape disruption on coral ecosystems.
How are ocean soundscapes collected?
In the same way a radio show is recorded for production, soundscapes are recorded using microphone-like technology called hydrophones, designed to listen to underwater sound. Hydrophones are placed throughout the coral reef system with low noise amplifiers, and soundscapes are collected using passive acoustic monitoring. The devices are left in the coral reef system for hours or weeks before being collected and uploaded to comparative databases. Based on the type of sound, its frequency, and level, scientists are able to differentiate the noise patterns of different species in the soundscape. The data can be overlaid with previously collected soundscapes to identify changes that have occurred due to habitat alterations that are highly linked to global climate change. This information can be continuously collected and stored with slight modifications to the hydrophones to analyze different types of sounds at various noise levels and frequencies. Spatial and temporal mapping along with the collection of sound patterns paint an overall picture of soundscape patterns present throughout the coral reef system; however, global climate changes driven by anthropogenic carbon emissions, including coral bleaching and ocean warming, are threatening coral reefs and altering their unique soundscapes.
How has coral bleaching changed soundscapes?
The loss of habitat and coral bleaching has decreased the biodiversity present in coral reef habitats, and this is reflected in the soundscapes. Coral bleaching occurs when ever slight fluctuations in ocean water temperatures cause healthy corals to expel the symbiotic algae that reside in their tissues and provide them with food. The loss of the algae causes the coral to turn white. Although still alive, the coral is in a much more disease-sensitive state due to loss of its major food source. These bleaching events are often difficult for the coral reef to overcome as the oceanic waters continue to exponentially warm outside of the normal temperature fluctuation range. Coral bleaching has resulted in widespread loss of U.S. coral reefs, inducing habitat loss and subsequent biodiversity loss for hundreds of species residing in protective coral reef areas.
Differences in the soundscapes before and after a bleaching event are easily identifiable in the soundscape frequency patterns. A healthy coral reef typically has a sound similar to cracking pop rocks generated by the large populations of snapping shrimp that live in and around coral reefs. A dying or bleached coral reef is eerily silent. This silence makes it even more challenging for biodiversity recovery to occur because other animals in the ecosystem see the silence as a warning sign of ill health and are hesitant to return to coral areas with no sounds of life. Timothy Gordon, a marine biologist at the University of Exeter wrote, “Degraded reefs receive lower rates of [fish] settlement, compromising recovery potential.”
Increasing greenhouse gas emissions and detrimental effects of climate change disproportionately impact marine habitats, as the ocean is regarded as the global carbon sink. Global climate change causes coral bleaching and leads to the disruption of coral reef soundscapes with an overall loss of biological sounds. Coral bleaching has the potential to be mitigated, but without resolutions to the global climate crisis, coral reef systems may be irreversibly bleached and their unique sounds forever lost.
What can be done?
In recent years, scientists have been experimenting with artificial soundscape installations to attract greater biodiversity and encourage coral reef recovery. The installation of speakers equipped with audio of healthy coral reef systems encourages the habitation of these areas by juvenile fish and increases their residence time within the unhealthy coral reef area. The presence of greater biodiversity increases the rate of recovery for a bleached coral reef system due to nutrient recycling and decreased coral stress.
Still, this methodology only applies a bandage on the immediate issue of coral bleaching. To protect biodiversity hubs in oceanic waters, issues regarding carbon emission and global climate change must be addressed. Active management of coral reefs and assisted introduction of biodiversity using artificial sounds provides a potential solution to combat the silencing of reef soundscapes, but without stricter emission limits and global level change our coral reef soundscapes may become silent victims of climate change.
Samantha Tracy is a first year Masters of Science student in Environmental Health at the Harvard T.H. Chan School of Public Health
Sean Wilson is a sixth-year graduate student in the Department of Molecular and Cellular Biology at Harvard University