Researchers are listening to the breath of streams
Streams need to breath, just as humans, to stay healthy. This allows oxygen to improve water quality and sustain fish life, and greenhouse gases out which affects global warming. Gas exchange is driven by turbulence and bubbles, which in turn generate the characteristic stream flow sound. Researchers from Umeå University have developed a new method that uses sound recordings to estimate how fast gases exchange between air and water. The study is published in Limnology and Oceanography: Methods.
Marcus Klaus is measuring underwater sound in a forest stream.
An important question for ecologists and geoscientists is how healthy an ecosystem is and how it interacts with the global climate. In streams, many organisms, not least fish, are dependent on oxygen intake from the atmosphere. At the same time, many streams emit greenhouse gases to the atmosphere, often a result of the decomposition of organic material. To better understand and quantify these processes, it is important to measure how fast gas can exchange between air and water.
“To measure gas exchange in streams has so far been associated with high work load and required advanced equipment. A good gas exchange measurement often took a whole working day for me and my colleagues. During these measurements, I started to listen to the stream and realized quickly that there must be a simpler and faster method”, says Marcus Klaus, Postdoc at the Department of Ecology and Environmental Science at Umeå University.
The so called gas exchange velocity depends on turbulence (i.e. how chaotic water molecules move), and air bubbles which can entrain under particularly turbulent conditions. Both turbulence and air bubbles generate vibrations, which can be noticed as the typical sound of flowing water.
“Every water movement and air bubble generates its own characteristic sound and at the same time contributes to gas exchange. The gas exchange velocity can be described as a function of the sounds’ frequency and pressure”, explains Marcus Klaus.
Together with his colleagues, he has now published the relationship between the streams sound characteristics and gas exchange velocities in a scientific Journal. The article describes the new method and how it has been developed by lab and field experiments in forest streams in Northern Sweden. The new method advances pioneering ideas, published by another research team already a decade ago, and could make it easier to collect a lot more data on gas exchange than what had been possible so far.
“On the one hand, the new method opens up many new opportunities, for example continuous gas exchange measurements and a better understanding of gas exchange processes. But on the other hand, we still have to deal with many challenges. Just think of the hum of the mosquito passing by our microphones”, says Marcus who is first author of the published article.
To develop the method further, the EU research network AQUACOSM has now granted Marcus transnational access to an experimental stream facility in Austria. Together with researchers from Austria, Sweden, France and Italy, he will continue listening in order to better understand what the stream is telling us about its health status and climate impact.
Klaus, M., Geibrink, E., Hotchkiss, Erin R., and Karlsson, J. (2019): Listening to air–water gas exchange in running waters. Limnology and Oceanography: Methods. doi: 10.1002/lom3.10321.
Marcus Klaus on the sound of air-water gas exchange rates