I’ve been listening to seagrass meadows to understand how their underwater soundscapes reflect biodiversity

The rugged west coast of Scotland looks glorious in the sunshine. The turquoise sea is calm but even in July, it’s a chilly 12°C. Armed with my recording kit, snorkelling equipment and my thick wetsuit, I’ve been listening to the marine creatures living in three different Scottish seagrass meadows.

For my marine ecology PhD, I’m investigating the biodiversity of Scottish seagrass meadows which, right now in midsummer, are in full bloom. Unlike seaweed, this marine plant has got flowers, seeds, pollen and underground roots.

Seagrass meadows are buzzing with activity. Sea snails scrape rocks as they eat algae, young fish feed on tiny zooplankton, crabs fight to defend their territories, plus birds, seals and otters hunt for food.

There’s a cacophony of sounds from all of this busy-ness and I’m researching how seagrass soundscapes – that’s the collections of sounds that can be heard in an environment – differ depending on the wildlife living there. Hearing a wider variety of sounds might mean there are more animals in the seagrass and potentially indicate a healthier, more biodiverse seagrass meadow.

Seagrass meadows have declined drastically due to sediment and nutrient runoff from agriculture, coastal development, destructive fishing practices and disease. The UK has lost more than 40% of its seagrass cover, possibly up to 90% compared to pre-industrial levels. Globally, 29% of seagrass has disappeared since the 18th century, and the rate of decline has accelerated, with about 7% lost every year since the 1990s.

Seagrass is an important breeding ground for fish, it improves water quality and acts as a carbon store. So its decline affects the marine creatures living within the habitat, animals further up the food chain and ocean health more broadly.

Recording soundscapes in seagrass is useful because it allows researchers like me to detect creatures that we can’t necessarily see, either because they’re camouflaged or hiding, or perhaps nocturnal. It also causes minimal disturbance compared to some other monitoring methods, and it could become cheap and efficient. In the future, you may be able to simply put down a recorder, pick it up, run some algorithms and get some information about the animals present.

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Listen to Isabel Key explaining the sounds she’s collected in an interview on The Conversation Weekly podcast.

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At each meadow I visit, I set up palm-sized underwater microphones on stands and leave them on the seabed for a week. On a daily basis, I snorkel down to put a video camera next to the microphone so I can match up the sound with the video. This helps me work out which sound is made by which animal.

Back in the office, I have been analysing my audio recordings using “acoustic indices”, which are measures of the complexity of the soundscape. That includes animal sounds but also waves, boat noise and chinking mooring chains.

Next, I assess phonic richness by listening to one-minute-long clips. Looking at the spectrogram – a visual depiction of these sounds – I can count how many different types of animal sounds are present. That’s time-consuming but gives a great insight.

So far, I have identified 14 different types of sound that I suspect belong to fish and crabs living in the seagrass, plus dolphin whistles and clicks which I can hear from further away as they swim past. I can look at the exact frequencies (or pitch) that sounds are hitting and the patterns they make, then more accurately assign that sound to an animal or human activity.

I’ve found some evidence for a characteristic seagrass soundscape with certain sounds occurring more commonly in seagrass than in sandy habitat. Fish make low-pitched grunting, burping or purring noises. Crabs make higher-pitched metallic sort of scraping sounds.

I often hear a popping noise which becomes more prominent during the day. As the seagrass photosynthesises, especially during the middle of the day when the sun is warm and bright, the plant produces bubbles of oxygen that collect on the surface of the seagrass blades and pop as they move into the water.

It’s hard to decipher which animal is making which sound, especially since our oceans are such noisy places. Acoustic pollution can be a serious problem for marine animals that rely on sound for their survival, either to find a mate, navigate, communicate with each other or hunt for food.

Interestingly though, seagrass could act as a buffer to some underwater noise pollution. As a 3D structure, seagrass acts as a physical barrier to wave energy - that’s one reason why it plays a crucial role in protecting coastal areas from erosion. It can also absorb sound waves and even protect fish from dolphins that use echolocation to navigate towards their prey. The clicks of dolphins don’t penetrate seagrass very well, so fish can be safer in this seagrass sound shelter than in the open sea.

In two meadows, I found more fish and crab sounds in the seagrass than in the sandy sites that I was comparing them against, just as expected. But at one location, I heard more sounds over the sand than in the seagrass despite there being less wildlife there. So biodiversity levels are not necessarily directly reflected by the soundscape.

This may be partly due to differences in how sound travels in different habitats. Sound is transmitted more easily over sand than through seagrass. This phenomenon could lead to misleading results where it’s harder to hear fish in denser seagrass, due to the seagrass itself absorbing the sound, even if it is home to more fish.

Researchers need to be cautious about interpreting soundscape data, and take into account how the habitat structure affects how sounds are heard. Acoustic monitoring could therefore be more useful for studying changes in animal life over time at one site rather than comparing between different areas.

Meadow monitoring

The hope is that this type of work can be used to train machine-learning algorithms and eventually build an easy-to-use tool for monitoring biodiversity in seagrass and other marine habitats. That requires a comprehensive library of sounds. This already exists for dolphins and other marine mammals but it’s not yet well-established for sounds of fish, crabs and other invertebrates like shrimp.

Capturing all the different sounds each species can make usually starts with recordings taken in an aquarium. Then, automated detection can attempt to match that up with sounds that researchers like me record in the field. This should enable scientists to identify early indicators of meadow decline or measure the success of seagrass restoration projects.

Perhaps one day, marine scientists around the world will set up sound recorders that can transmit audio clips from coastal seas to a central online database where soundscapes could be automatically analysed to assess ocean health. This could give us near-real time data on animal populations and movements, helping to inform marine protection measures and sustainable fishing practices. That’s an exciting prospect.

Isabel Key receives funding from the Natural Environment Research Council and NatureScot.