Using underwater sound to estimate kelp density
Why use underwater sound?
As a previous lab note discussed, sound is everywhere in the ocean. Whales, fishes, waves crashing, and rain (among many, many others) all produce sound in the ocean, and marine animals use sounds for many different purposes (e.g., for communication or to find prey). Sound in sea water travels much faster than sound in air, which allows sound in sea water to propagate long distances.
However, the speed of sound in water is not homogenous - that is, the speed of sound in the ocean varies in response to different parameters of the ocean itself. For example, sea water temperature can change the speed of sound; sound moves quicker through warm water than it does through cold water. Other common variables that change the speed of sound in sea water include salinity (how salty the water is) and pressure (which in the ocean is a measure of depth).
These variables all interact in the ocean and affect the density of seawater. Density measures how much "stuff" (in our case, seawater) is within a specified volume. The more seawater there is in that volume, the more dense it is, and as the density increases so does the speed of sound. Scientists can use measures of salinity, temperature, pressure, and density to estimate the speed of sound in the ocean, but the opposite is also true: if we know the speed of sound, we can estimate the other oceanographic variables.
How does kelp fit in?
As we've mentioned in other posts, coastal kelp forests are incredibly productive ecosystems. Off of Southern California, the giant kelp, Macrocystis pyrifera, can grow as tall as 150 feet! So how do they stay up in the water column and reach the surface from deep water? Well, unlike trees here on land that use wood as their structural support, giant kelp use gas bladders (called pneumatocysts) at the base of each frond. These gas bladders cause the kelp to "float", providing buoyancy that lifts the fronds up toward the surface.
Now, let's say that we wanted to measure the density of seawater in an area of ocean that contained kelp. Because these kelps and their pneumatocysts are included in the volume of seawater where we want to measure the density, the density of this "kelp seawater" will be different than just seawater. The pneumatocysts in the kelp are filled with gas (generally oxygen, nitrogen, carbon dioxide, or a combination of these), and since gas is less dense than air, a volume of water that includes kelp will be less dense than plain seawater!
Putting it all together
We know that different oceanographic variables alter the speed of sound, and that we can estimate the speed of sound from these variables. We can also "back calculate" estimates for these variables if we know the speed of sound. So how do we put this all together to use underwater sound to estimate the density of kelp?
We plan to develop a technique where we use an underwater loudspeaker to broadcast a sound through the kelp forest and record it with a hydrophone recorder on the other side. We can calculate what the speed of sound through the seawater should be, and take experimental measurements of what the speed of sound through the kelp forest actually was. By comparing the theoretical sound speed that we calculated to the actual sound speed through the kelp forest, we'll get a "sound speed differential".
Once we have our speed of sound measurements, we'll dive in the kelp forest and count how many kelps were between the loudspeaker and the recorder. Since we know the distance between the speaker and recorder, we can calculate kelp density (as the number of kelps per square meter) along the sound path. We'll repeat these experiments many times through many different kelp forests that have different numbers of kelps to measure how the sound speed differential varies with the number of kelps in the sound path. And finally we can build a model that relates the sound speed differential to the density of kelps!
Why is this important?
Kelp forests provide habitat for hundreds of species of plants and animals and support the economies of coastal communities along temperate coasts, yet these iconic ecosystems are threatened by a myriad of factors (e.g., global climate change, ocean acidification, overfishing, and many others). Rapidly assessing the health of these ecosystems is critical to provide natural resource managers the best information possible to protect kelp forests. Satellite and aerial imagery is used to map kelp forest extents, but provides information at large spatial scales. Diver-based surveys provide incredibly detailed data, but require a lot of man-power and only cover small areas. We hope that by developing a rapid acoustics-based technique, we can "fill in the gap" and provide resource managers information about kelp forest health augmenting aerial and diver-based techniques to help protect these habitats and all the animals that live in them.
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