A new study from North Carolina State University reveals that the soundscapes of coral reef ecosystems can recover quickly from severe weather events such as hurricanes. The work also demonstrates that non-invasive monitoring is an important tool in shedding further light on these key ecosystems.
Soundscape ecology is a relatively new way for researchers to keep tabs on a variety of habitats without direct interference. In underwater habitats like coral reefs, soundscapes allow continual monitoring of an ecosystem that is difficult to access. By deploying underwater microphones, or hydrophones, researchers can get an acoustic picture of the types of animals in the ecosystem, as well as their behavior patterns.
Kayelyn Simmons, a Ph.D. student at NC State, used soundscapes and underwater mapping to monitor two different reef sites in the Florida Keys from February to December 2017. She deployed and collected eight hydrophones every three months between the two sites: a pristine reef located at Eastern Sambo, and a fishing site located at Western Dry Rocks.
Hurricane Irma struck the Florida Keys as a Category 4 storm in September 2017. Simmons was able to retrieve two of the hydrophones - one from each site - in December. Unfortunately, the hydrophone retrieved from Western Dry Rocks had been compromised by the storm, rendering its post-storm data unusable.
"Prior to the hurricane, we were able to determine what the 'normal' sound patterns were in each habitat, so we knew what the baselines were in terms of species and behavior," Simmons says. "You can tell which species are present based on where their sounds are on the frequency band. Similarly, the amount of noise from each species can give you an idea of their numbers. So the soundscape is a good way to measure abundance and diversity."
Each study site had the same species present. For example, snapping shrimp, with their high frequency "Rice Krispies in milk" popping noises, were active in the periods between dusk and dawn; while grunts, grouper and snapper, with sounds in the lower frequency bands, were mainly active during the day. The hydrophones also captured spawning activity during the full moon.
Simmons analyzed the sound captured by the surviving Eastern Sambo hydrophone and discovered that even though the reef suffered physical damage from the hurricane, the residents and their activity levels began returning to normal approximately 24 to 48 hours after the storm passed.
"The acoustic energy exposure for the reef was as loud as a small boat circling in one spot for two weeks," Simmons says. "So we didn't record any fish noises during the four-day period that Irma came through due to acoustic masking from the storm. However, the snapping shrimp were back to pre-storm sound levels within 24 hours. The fish noises on the lower frequency were back within 72. And on the next full moon we heard normal spawning behavior.
"Overall, the research shows that the coral reef soundscape was resilient and able to recover from the storm quickly."
The work appears in PLOS One and was supported by the Coral Reef Conservation Fund (grant NA18NOS4820113) and the National Oceanic and Atmospheric Administration's National Marine Sanctuary Program (grant NFFM7320-19-01243). David Eggleston and Del Bohnenstiehl, both professors of marine, earth and atmospheric sciences at NC State, co-authored the work.
Note to editors: An abstract follows.
"Hurricane impacts on a coral reef soundscape"
Authors: Kayelyn Simmons, Dave Eggleston, Del Bohnenstiehl, North Carolina State University
Published: Feb. 24 in PLOS One
Soundscape ecology is an emerging field in both terrestrial and aquatic ecosystems, and provides a powerful approach for assessing habitat quality and the ecological response of sound producing species to natural and anthropogenic perturbations. Little is known of how underwater soundscapes respond during and after severe episodic disturbances, such as hurricanes. This study addresses the impacts of Hurricane Irma on the coral reef soundscape at two spur-and groove fore-reef sites within the Florida Keys USA, using passive acoustic data collected before and during the storm at Western Dry Rocks (WDR) and before, during and after the storm at Eastern Sambo (ESB). As the storm passed, the cumulative acoustic exposure near the seabed at these sites was comparable to a small vessel operating continuously overhead for 1-2 weeks. Before the storm, sound pressure levels (SPLs) showed a distinct pattern of low frequency diel variation and increased high frequency sound during crepuscular periods. The low frequency band was partitioned in two groups representative of soniferous reef fish, whereas the high frequency band represented snapping shrimp sound production. Daily daytime patterns in low frequency sound production largely persisted in the weeks following the hurricane. Crepuscular sound production by snapping shrimp was maintained post-hurricane with only a small shift (~1.5dB) in the level of daytime vs nighttime sound production for this high frequency band. This study suggests that on short time scales, temporal patterns in the coral reef soundscape were relatively resilient to acoustic energy exposure during the storm, as well as changes in the benthic habitat and environmental conditions resulting from hurricane damage.