Freshwater ecosystems are singing, and scientists are listening

Primary Article: Rountree, R A, Juanes F, Bolgan M (2020) Temperate freshwater soundscapes: A cacophony of undescribed biological sounds now threatened by anthropogenic noise. PLoS ONE 15(3): e022.1842. pone.0221842

Featured Image: Streams form part of the riverscape, and may each have their own unique collection of sounds. Source: Sarah Shainker

If you can, take a break from the endless Zoom meetings and scary news stories on your computer screen, open the door, and step outside! What’s going on?

Depending on the weather, you might feel the warmth of the sun on your skin, a cold breeze blowing through your hair, or a shower of falling raindrops. You might see clouds floating in the sky or notice springtime flowers beginning to bloom. Now, what do you hear?

In an idyllic setting, maybe you hear birds chirping, leaves rustling, a stream gurgling… maybe even some grunting squirrels. If your home is anything like mine, though, you probably can’t escape the equally-or-more noticeable sounds of road traffic, pop music emanating from speakers of backyard sunbathers, and the occasional obnoxious motorcycle engine.

Scientists study ecosystems, remote and urban alike, by observing them. We may assume that observation means looking for certain organisms or environmental characteristics. However, we can also use our ears to make observations and discoveries, like you just did on your outdoor adventure.

The evolution of organisms is shaped by their environment, including the sounds they hear. For example, different types of fish may be adapted to have the best hearing under different levels of ambient sound (Amozer & Ladich 2005), crayfish use sound to warn one another of potential predators , and some fish use sound to attract mates.

The science of natural symphonies

If sound is such a big deal to these organisms, how does more recently introduced human-produced sound influence their ecology and evolution? In recent years, scientific interest surrounding noise pollution has grown. It turns out that humans are quite loud, and the noise we make while working, playing, and traveling can cause a host of problems for both humans and animals.  In order to tangle out the effects of noise pollution, we need a baseline: what do ecosystems sound like without the influence of humans?

The field of soundscape ecology involves the study of the collection of sounds across a landscape (or waterscape). A soundscape is defined by Pijanowski et al (2011) as “the collection of biological, geophysical and anthropogenic sounds that emanate from a landscape and which vary over space and time reflecting important ecosystem processes and human activities”. These sounds are a significant component of ecological systems.

Freshwater ecosystems are among the anthropogenically impacted, but there have been few studies on the natural or human-produced sounds present in these ecosystems. An international team of scientists aims to improve the characterization of freshwater soundscapes in order to identify its individual contributors and better understand the roles of natural and anthropogenic noise. In a recently published study, they recorded 2,750 minutes of sounds in freshwater habitats along 5 major New England rivers.

Listening closely…

The team identified a variety of sites along the five main river stems, and grouped them into the following categories: “brook/creek”, “pond/lake”, and “river” (the latter of which included small tributaries and longer segments of the main river stem). At each site, they made visual observation of the species present, and used a hydrophone, audio interface, laptop, and special software to make audio recordings both during the day and at night.

Human-produced sounds (also called anthropony) and natural biological sounds (or biophony) were grouped into categories (e.g. “fish”, “insect-like”, “bird”, “airplane”, “running boat”, “traffic”) based on direct observations and knowledge gathered from prior studies. Some sounds were placed in an “unclassified” group because they couldn’t be definitively identified.

The researchers counted the number of sounds, measured their duration, peak frequency, and frequency bandwidth, calculated the number of sounds per minute (sound rate), and determined the percentage of recording time taken up by each category.

An example of sounds recorded and analyzed by scientists. Noise characteristics such as the sound frequency (pitch) and sound pressure level (loudness) can help scientists categorize sounds. For example, on this spectrogram of data taken from Mount Rainier National Park, insect sounds (in solid-lined boxes) have similar characteristics, as do the sounds from propellers (in ovals). Source: Wikipedia

What did the rivers have to say?

The researchers captured a whole lot of noise! The recordings included 4,825 biological, 1,623 anthropogenic, and 834 unclassified sounds.

When counting the number of sounds that occurred, biophony won. It contributed 66% of the sounds during the day, and 67% at night.

However, when measuring their percent of recording time, anthropony won. It dominated over 92% of recording time during the day, and 88% at night.

The breakdown of these biological and anthropogenic sounds taught us a few important new things.

The study found that anthropony was a significant component of the soundscape. Anthropogenic sounds tended to last longer, be louder, overlap with, and occur more frequently than biophony. These patterns suggest that human-produced sounds may have unintended effects on freshwater organisms. It also suggests that in some cases, anthropogenic sounds may mask biological sounds, making it difficult for researchers and animals alike to detect them. Boats had especially severe auditory impacts; a boat in motion creates a short-term but loud sound, while idling boats are still loud and can last hours. Anthropogenic sounds may have far-reaching impacts as well, as road traffic sounds could be detected up to 270 m away.

When background levels of sound increased, due to either biological or anthropogenic noise, biophony decreased in frequency, number and percent time of sounds, and diversity. This observation suggests that freshwater organisms may either avoid or be drowned out by too much noise.

Biophony occurred more often at night than during the day; possibly because there is less anthropogenic noise at night. However, the specific window of time during which the most biophony occurred varied by site, suggesting that there are a variety of ecological auditory niches that organisms exploit in different habitats.

The researchers listened to a variety of habitats, and found some auditory patterns based on habitat type.Different types of anthropogenic sound predominated different habitats; for example, there were more boats in tidal reaches, but more road traffic in non-tidal reaches. Additionally, there were few bird sounds in lakes/ponds, insect-like sounds occurred frequently at night in brook/creek locations, and insects and non-air fish sounds made up most of the “noise time” in all habitats. Together, these patterns create a song that tells us where and when different organisms are active, and what anthropogenic activities occur in various habitats at different times of the day and night.

Big Mouth Billy Bass isn’t the only freshwater resident with something to say!
Source: Wikipedia

What’s next?

Interestingly, even though we know that biological communities change along the flow of a river, the main contributors to soundscapes remained relatively consistent. Identifying more specific sound categories may allow us to more clearly capture the differences in soundscapes among different habitats.

Previous studies have shown that the sounds of birds flying can influence fish behavior. It is conceivable that other terrestrial sounds could impact aquatic inhabitants as well. The authors recommend that future studies include sounds from both the terrestrial and aquatic environments for a more holistic soundscape.

Studies should also aim to identify unknown sounds; of which there are many! This can be done by combining field surveys and visual observations with auditory recordings.

As future researchers incorporate more sounds, more specificity, and more identifications into soundscape studies, we will more fully understand what the richly woven symphony of freshwater ecosystems is telling us, and how human sounds are changing the melody.


Amoser, S. & Ladich, F. Are hearing sensitivities of freshwater fish adapted to the ambient noise in their habitats? (2005) Journal of Experimental Biology 208: 3533-3542. doi: 10.1242/jeb.01809

Pijanowski, B. C. et al. Soundscape ecology: The Science of Sound in the Landscape (2011) Bioscience 61: 3, 203-216.

Rountree, R. A., Juanes F., Bolgan M. (2020) Temperate freshwater soundscapes: A cacophony of undescribed biological sounds now threatened by anthropogenic noise. PLoS ONE 15(3): e022.1842. pone.0221842

Reviewed by: Christina Marvin

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Sarah Shainker

Sarah Shainker

Sarah is a first year Phd student at the University of Alabama in Birmingham with interests in evolutionary ecology, conservation genetics, citizen science, and macroalgae. Before beginning grad school, she worked as an outdoor educator in the north Georgia mountains and as a coastal resource management volunteer for Peace Corps Philippines. Twitter: @SarahShainker

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