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Reef rhythms

By Jessica Perelman

In the wake of the largest and most widespread coral bleaching events in history, scientists are scrambling to better understand these vibrant, diverse marine ecosystems, and determine how to preserve the wealth of aquatic life they harbor. From what we currently know, corals are easily affected by increasing water temperatures, ocean acidification, and various other natural and human stressors. But what defines a ‘healthy’ coral reef environment? Is it the sheer amount of living coral and calcium carbonate structures covering the sea floor in an area defined as a reef? Or perhaps the abundance of animals and microorganisms living amongst the habitats they provide? Unfortunately, there is no straightforward measurement for reef health. To address this question requires gaining deeper insight into corals, their countless inhabitants, and the physical and chemical variability of their surrounding marine environment.

Due to heat stress caused by unusually warm ocean temperatures, global coral bleaching events (loss of symbiotic algae) have reached a record high in 2017 (Credit: Kelsey Roberts, USGS).

I recently had the opportunity to participate in the spring field season of a project that aims to unravel some of these reef dynamics. As a research technician in the Sensory Ecology and Bioacoustics Lab at the Woods Hole Oceanographic Institution, I traveled with a team of four equally enthusiastic researchers to the U.S. Virgin Islands. The project is cleverly titled Coral Chorus and places a large focus on the sounds associated with distinct coral reef sites within the Virgin Islands National Park off the southern coast of St. John. As I witnessed, this location is ideal for such a long-term study, with limited boat traffic or other anthropogenic noise interfering with the reefs under observation.

The project seeks to understand how sound plays a role in the recruitment of larval fish, corals, and other reef-associated animals, and how these young organisms use natural reef sounds as a cue to select habitats in which to settle. When examined altogether, the compilation of the natural sounds in such an ecosystem form what is known as a marine soundscape, which is often dominated by the ubiquitous crackling noise produced by snapping shrimp colonies, but also include fish calls and various other biotic and abiotic noises. By enabling us to isolate different components of the acoustic environment, and ultimately recognize the dominant sources of noise influencing specific ecosystems, soundscapes confer a great deal of information about the diversity of life in marine habitats.

An acoustic recorder (Sound Trap ST4300, Ocean Instruments NZ) deployed on a reef off of St. John measures environmental noise for months at a time (Credit: Rod Catanach).

The highly variable, complex, and unique soundscapes of different tropical reefs around St. John are the heart and soul of this project. To obtain this information, the team deploys acoustic recorders at each site that “listen” to the reefs for months at a time. This data is being used to understand how underwater noise may drive habitat selection of very young reef-dwelling organisms, and ultimately, local species biodiversity. In addition to audio recording and collecting larvae, water samples are taken from just above the reefs to assess differences in their microbial communities, and visual surveys of benthic coral cover and adult fish abundances are conducted as additional metrics of reef quality. There is never a dull moment in the field, and each trip has yielded new ideas and integrative experimental designs to further investigate the compound biological and physical processes affecting these reefs.

A light trap submerged overnight near mangroves on the edge of Lameshur Bay (St. John) attracts and collects fish larvae (Credit: Rod Catanach).

With many moving parts to this project, the principal investigators each provide expertise in a different specialization related to reef health: Dr. Aran Mooney (bioacoustics), Dr. Joel Llopiz (larval fish ecology), and Dr. Amy Apprill (marine microbial ecology). The combined efforts of these researchers, as well as the numerous students and volunteers involved, speak highly to the value of scientific collaboration. Evidently, Coral Chorus incorporates a broad range of parameters by which to address the status of reefs. This interdisciplinary approach to environmental research is one that should be greatly valued, for it requires scientists to keep an open mind about the many possible drivers of phenomena in the natural world.

These quiet Caribbean waters provide an ideal location to study natural marine soundscapes with minimal human interference (Lameshur Bay; Credit: Jessica Perelman).

Simply put, this project is unique, exciting and employs novel techniques to study coastal ocean systems. But it also serves a greater purpose in answering a key question: why is it so important to understand and preserve reefs? Coral reef ecosystems are hubs for some of the greatest biodiversity on the planet. Estimates of a quarter to almost a third of all marine life dwells within and around these underwater jungles, and because of this, they provide substantial benefits to humans including food, medicine, tourism and fishing revenue. Not only this, but hearty reef structures play a key role in shoreline preservation and protection from storms. Such ecosystem services, taken into serious consideration, could impact the conservation and management of the world’s many declining reefs. It is my hope that projects like Coral Chorus inspire future research, increase public awareness, and ultimately lead to policies that better govern the protection of these intricate and fragile ecosystems.

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Jessica Perelman is a recent graduate of the University of Southern California with a bachelor’s degree in biological sciences. She currently works at the Woods Hole Oceanographic Institution with plans to attend graduate school to pursue a career in marine science.