Acoustic monitoring of freshwater ecosystems: Costa Rica study reveals diverse underwater soundscapes
The study site – Cantanara Swamp at La Selva Biological Station in Costa Rica. Image: Ben Gottesman.
Freshwater ecosystems across the world are disproportionately threatened by human activity, causing ongoing losses of aquatic biodiversity globally. Many freshwater conservationists highlight the need for more comprehensive ecological monitoring and assessment programmes to better understand ecosystem changes, and to strengthen conservation initiatives accordingly.
An innovative new study seeks to address this need using an unusual approach: acoustic monitoring of freshwater soundscapes. A soundscape is the aural equivalent of a visual landscape: all the sounds we hear in a space (whether through our ears, or through recording devices), emitted by both human and non-human sources. Soundscapes are never static, often varying with the time of day, weather conditions and human activity.
Researchers from Purdue University, USA and Nanjing University of Science and Technology, China recorded the aquatic soundscape of a Neotropical freshwater swamp in Costa Rica for 23 days in 2015. They used special underwater microphones – known as hydrophones – to delve into the acoustic world of the freshwater wetland. They wanted to better understand how soundscape recording might enhance existing freshwater ecosystem monitoring and assessment initiatives.
The promise of soundscape ecology
Soundscape ecology is a growing inter-discipline, popularised by the sound recordist Bernie Krause and the ecologist Bryan Pijanowski (Director of the Center for Global Soundscapes who led this study), amongst others. By recording soundscapes, it may be possible to gain a picture of the biological community present in an area, and to assess animal activity patterns. Such approaches have numerous benefits to ecologists: being non-invasive, offering high temporal resolution, facilitating long-term sampling strategies in remote areas, and providing an archive of detailed digital data for long-term analyses.
However, soundscape recording approaches have – as yet – been largely confined to terrestrial (e.g. bats and birds) and marine ecosystems (e.g. whales and dolphins). The research team behind the new study – published in Freshwater Biology – suggest that soundscape recording can provide a valuable tool in difficult-to-access aquatic environments where visibility is limited, and sound is a principle means of animal communication.
Lead author Ben Gottesman, a PhD Candidate at Purdue University’s Center for Global Soundscapes, explains, “The sounds of freshwater habitats are still mostly a mystery, especially in the tropics. Rainforests are noisy places, filled with sounds of birds, bugs and amphibians. But what about underwater? Is there a similar sort of cacophony in the forested wetlands?”
Such sonic explorations aren’t solely driven by curiosity, but could also yield valuable environmental data, Gottesman suggests, “If you are like my skeptical grandpa, you may be wondering why we would care about aquatic soundscapes? The reason is because these sounds could unlock an efficient and effective way to monitor freshwater biodiversity, which is rapidly declining and can be difficult and costly to measure.”
The paper is part of a special issue of Freshwater Biology exploring the potential of acoustic methods for freshwater ecology. Other papers cover topics including acoustic monitoring of piranhas in Peru, lesser water boatmen in a Mediterranean pond, and toads in Northern France; and the possibility of ‘acoustic refuges’ in fish habitats.
Exploring the underwater sounds of a Costa Rica wetland
The research team submerged a hydrophone connected to an automated acoustic recorder in the Cantanara Swamp in Costa Rica – known for its richness of amphibian, bird, mammal and invertebrate species. Their equipment recorded 10 minutes of underwater sound each hour, and 1 minute each 15 minutes. This sampling strategy created 2,121 sound recording ‘snapshots’ from the swamp over the 23 day fieldwork period.
Ben Gottesman outlines the aims of the study, “Our goals were to assess the diversity of sounds within this wetland and also the temporal dynamics. We found rich diversity in sound types – eighteen in all.
“We were surprised by the daily soundscape dynamics. Whereas the day-time and night-time soundscapes were filled with the clicks, stridulations and buzzes of aquatic insects, dusk and dawn were notably quiet. We posited several explanations for these crepuscular lulls, but still do not have a definitive answer.”
Spectrograms and oscillograms (below the spectrograms) of eight sound types from the study. Time (seconds) is on the x-axis and Frequency (kHz) is on the y-axis of spectrograms. See all eighteen sound types here.
Classifying underwater ‘sound types’ from the wetland soundscape
The eighteen sound types – creatively termed ‘Cyclops’, ‘Geiger’, and ‘Highchair’ amongst others – were classified based on their audible differences, and the visual patterns they generated in a spectogram. Spectograms are a visual representation of the spectrum of frequencies of an audio signal over time. The frequencies of the recorded sound types spanned a huge audio spectrum: from below 100 Hz (sub harmonic frequencies below human hearing) to above 22 kHz (higher than the sounds made by most mosquitoes).
Whilst the sound types have yet to be identified to their animal sources, the study shows that there was a rich diversity of underwater sounds emitted in the Cantarana Swamp, forming a soundscape with distinct daily dynamics.
“We are still in the early stages of classifying freshwater sounds and identifying the ecological factors that influence soundscape diversity and dynamics. Both of these are basic steps necessary for acoustic monitoring to become useful as a freshwater habitat assessment tool,” says Gottesman. “This study illustrates that healthy tropical wetlands like this one can contain rich soundscapes, and are therefore promising sites for acoustic monitoring.”
Acoustic monitoring: a valuable new method for freshwater science and conservation?
The study authors suggest that there are a number of necessary steps for freshwater acoustic monitoring and assessment techniques to become more widely used. These include ‘ground-truthing’ soundscape data with in situ field samples of biological and environmental conditions, and creating ‘field guides’ of the sounds emitted by individual species.
The authors state that for soundscape methods to be effective in freshwater environments, their measurements must have a strong positive correlation with at least one measure of biodiversity, such as species richness or abundance. Clearly, this is an emerging approach with significant potential for the monitoring and assessment of freshwater ecosystems globally.
“I hope that this work will inspire other freshwater ecologists working in the tropics to drop in a hydrophone. There is so much left to learn,” says Gottesman.
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