Amelia Macapia (AM): Seagrass meadows make up some of the oldest organisms on Earth. Some meadows of Posidonia oceania, a type of seagrass species endemic to the Mediterranean, have been dated back to 200,000 years ago. Seagrass meadows can absorb 35 times more carbon than tropical rainforests. How are they doing that? When did we discover that?
Megan Ballard (MB): Seagrasses are highly productive ecosystems that absorb carbon dioxide through photosynthesis and then store organic carbon as biomass. Some of the organic carbon is stored in the plant tissue, but the largest organic carbon deposits in seagrass meadows are located in the sediment. The seagrass itself reduces the energy of ocean waves, slowing down the flow of water and causing sediments to trap and bury organic carbon, which keeps it from reentering the atmosphere. Once the carbon accumulates in the sediment, it can remain there for decades to centuries. Seagrass meadows cover less than 1% of the ocean surface but contribute an estimated 10% of the annual carbon sequestration in the ocean.
AM: Some of your research has been acoustically monitoring seagrass meadows or listening to the bubbles they produce as a byproduct of photosynthesis. When you are acoustically monitoring seagrass meadows, what are you hearing?
MB: When the bubbles form on the seagrass leaves, they detach and the bubbles resonate. You can imagine the bubbles ringing like bells when they release and causing the bubble to oscillate. The natural frequency of a bubble depends on its size and depth. There are so many bubbles being produced in a seagrass meadow all at once that we are actually listening to a chorus of bubbles. The sound of bubble production is the dominant source of ambient sound in seagrass meadows in the summer afternoons.
AM: Is the sound because there is seasonal and daily variance in how much sunlight the seagrass has?
MB: It definitely is, and the seasonal cycle of the seagrass also depends on water temperature. So it’s both sunlight and water temperature that are controlling when the seagrass has productive and dormant, meaning temporarily inactive, cycles. And then, of course, we see a diurnal daily cycle, driven by the sunlight that enables photosynthesis.
AM: You also have looked at Arctic environments. Glaciers are serving as indicators of a changing climate because of their sensitivity to temperature and precipitation, and many are nearing their melting points. The ice is incredibly loud as it is breaking up and cracking. How is the acoustic environment there changing? Is that environment just becoming increasingly noisy?
MB: From the standpoint of measuring underwater sound, we are especially interested in tidewater glaciers where the glacier terminates in a fjord. We consider both the sound of the calving from the glacier — the big chunks of ice that break off — and the sound of the melting glacier itself. Glaciers are formed by snow and there are little air pockets between the snowflakes that are pressurized from the overlying snow and ice. When the ice melts, these pressurized bubbles explode outward, making a loud noise like the sizzling of bacon. Tidewater glaciers can be some of the loudest underwater environments.
AM: How did you get started studying acoustics?
MB: I studied engineering in college. I took a couple of undergraduate acoustic courses that were part of my course of study in my degree program. Through these classes, I became interested in studying underwater acoustics and chose to study in the Graduate Program in Acoustics at Penn State.
I am really interested in using underwater sound to study the underwater environment, including things like seagrass ecology — for example, how can we understand what is changing in seagrass meadows from ambient sound measurements? We started looking at this data and one of the things that jumped out of our analysis was the evening chorus of spotted sea trout. Every night at sunset between the late spring and early fall, they produce a series of mating calls that last for several hours. We saw 40% more recordings with these vocalizations in 2023 compared to 2022.
There was a massive freeze event in 2021 in Texas — an estimated 3.8 million fish died on the Texas coast. We are seeing the recovery of this species of fish in our acoustic recordings. There is just so much you can learn about the underwater environment using recordings of underwater sound — and it’s the same with sea ice and the glacier ice and the other areas I have worked in.
AM: What caused the 2021 freeze event where all these fish died?
MB: We had a big ice storm that affected the entire state of Texas. This event was in the news; the power grid in Texas went down because the infrastructure here couldn’t handle the power demands.
AM: When you made that switch from engineering to bioacoustics were there any noises that particularly captured your interest?
MB: My PhD work was in geoacoustic inversion: using the ocean’s sound field to estimate seabed properties and sediment sound speed. This is important from a naval perspective when operating in the marine environment and using sound for navigation, ranging or target detection. The waveguide formed by the water layer on the continental shelf is thin; the water is on the order of 100 meters, and to propagate sound over tens of kilometers in range, the sound will reflect from the sea surface of the seafloor. The reflectivity of the seabed depends on its geoacoustic properties. So, understanding the properties of the bottom is important for understanding propagation, and for any system that would operate in an undersea environment.
From there, I have done work developing propagation modeling approaches, specifically focusing on environments for which horizontal refraction is an important effect. This is an alternative approach to conventional modeling approaches that assume azimuthal symmetry and calculate the pressure field in the range-depth plane. Although fully three-dimensional models are more computationally intense, out-of-plane effects are important in environments with strong asymmetry — for example, near the continental slope. In this case, a series of reflections from the sloped bathymetry causes wavefronts to change direction and turn back downslope.
I’ve also done work with direct measurements of geoacoustic properties. We do in situ (in the environment) coring using acoustic probes mounted to the penetrating tip of a sediment corer. We developed this system because the changes in the pressure and temperature of the sediment — as well as the coring process which disturbs the sediments — changes the sediment properties. If we can make these measurements in situ, we can get a better understanding of the environment. We have compared these to ex situ measurements of sound speed, and that’s giving us new insights about how sediments are affected by the coring process.
AM: So are you developing those technologies to make the in situ measurements?
MB: Yes.
AM: That sounds so challenging, especially because water and electronics are such a horrible mix.
MB: Yes, it makes it a lot more challenging and expensive to make measurements underwater. When we do things in shallow water — like the seagrass meadows, which are only a few meters deep — we still put electronics underwater, but it is not too difficult. However, when you go to the continental shelf, there is additional pressure from the overlying water and pressure housings must be engineered to account for the increased pressure. In the deep water of the abyssal plain — over 5,000 meters deep — our pressure housings are typically glass spears. The level of difficulty, risk and expense increases as you go deeper.
AM: In shallow water, you have also done work linking the sounds of coral reefs to fish larval settlement, or attracting them back to damaged ecosystems by playing recordings of healthy reefs. What sorts of sounds do you hear when you listen to reef ecosystems?
MB: I did that work with a graduate student, Andria Salas, who was more on the ecology side. I helped with the propagation modeling, but she made all the measurements for that project and was the one who interpreted the data on how acoustic cues help the larvae find the reef. The acoustic field is complicated and a modal interference pattern is formed by the upward and downward going ray paths, which are reflected from the sea surface and sea floor. So, there are regions with high intensity sound surrounded by low intensity sound. One goal of this project was to understand the effect of the reef’s sound level as a function of range and depth on the larvae’s ability to navigate to the reef. We concentrated on reef sounds Andria noted in her recordings, including toadfish calls and snapping shrimp. She used the larvae hearing thresholds to determine which sounds they would actually be able to detect. I helped her to model the range- and depth-dependent sound field which is influenced by the properties of the seabed, the water column sound speed profile, the range-dependent bathymetry and the extent of the reef. This work addressed the question: How does the heterogeneity of the sound field affect how the larvae will be able to settle near reefs?
AM: As we are talking about the interactions of sound between the ocean’s surface and bottom, one of the things that comes to mind is the Sound Fixing and Ranging Channel, or SOFAR, where sounds can travel incredibly fast and for much longer distances than in air without losing energy. How do you work with these channels in your acoustic research?
MB: In deep water, the SOFAR channel supports long-range propagation of sound. There have been some historic experiments with loud sources; one famous experiment broadcasted sound from Perth, Australia, to receivers off the east coast of the United States. We do not usually make sounds in the ocean that are that loud anymore because we are much more conscious about the marine mammals and affecting their hearing and behavior.
The SOFAR channel enables long-range propagation by forming a region of low sound speed, with the sound speed minimum at the axis of the SOFAR channel. Above the channel axis, sound speed increases because of warming near the surface; below the axis, sound speed increases because of increasing pressure with depth. Sound waves are refracted toward regions of low sound speed such that rays launched at shallow angles relative to the horizontal plane are continually refracted back toward the sound channel axis as they propagate away from the source. Since these sound waves do not interact with the sea surface or seafloor, they can propagate to long range with relatively low transmission loss.
One area of study is related to variations at channel boundaries, which allow sound to leak out of the SOFAR channel. At the upper boundary of the channel, these fluctuations are caused by internal waves that propagate at the density gradient between the warmer upper layer of water and the cooler water below.
In the Arctic, the SOFAR channel is not present, because of the different water masses and temperatures present. In the Eastern Arctic there is an upward refracting profile since there is no surface heating to create the upper boundary of the SOFAR channel; in the Western Arctic, water warmed on the Chukchi Shelf in the summer is advected into the Beaufort and creates the upper boundary of a sound channel centered only about 100 meters below the surface. The Beaufort sound channel is narrower than the SOFAR channel, and it channels higher frequency sound. The upper boundary of the sound channel is growing warmer as the Pacific water that forms that layer is warming.
AM: The human ear cannot pick up noise in water well at all. Most people do not recognize how loud and spectacular the noise of the ocean actually is. How do you change how people think about that environment?
MB: From the perspective of comparing the underwater environment to terrestrial environments, we don’t have electromagnetic waves underwater, so we can't see well visually. Sound tells us a lot about the environment, and animals use sound to navigate, forage and communicate. It is well known that dolphins and many species of whales use echolocation to find their prey. There are many species of whales that are documented communicating hundreds of kilometers in the SOFAR channel. Then there are the things we talked about today, like the fish and seagrass and melting ice. There is so much information about the underwater environment that we can obtain from ambient sound recordings.
