INTERVIEW – In the BioSWOT-Med campaign, Sven Gastauer is in charge of Zooglider, a specialized glider to study mesozooplankton. He describes its functioning, the different instruments mounted on it, and the large range of data that can be gathered.
THE INSTRUMENTS OF OCEANOGRAPHERS – Sven Gastauer is a senior acoustic scientist at the Thünen Institute for Sea Fisheries, Germany, and holds a visiting scholar position at the Scripps Institution of Oceanography (University of California San Diego), USA. In the BioSWOT-Med campaign he is charge of Zooglider, a specialised glider, to study mesozooplankton.
What are your research interests besides BioSWOT-Med?
My main research interests are in the field of hydroacoustics, as a tool to better understand ecological and biological processes. This often requires coupling of hydroacoustics with auxiliary data sources, such as biological samples, optics and physical measurements. This requires me to constantly further my understanding of acoustic signals, which resulted in a strong interest in acoustic scattering models.
What is the zooplankton glider?
Zooglider is a specialised glider, aiming at furthering our understanding of mesozooplankton. Many zooplankton species are fragile or gelatinous animals, difficult to sample or observe with traditional methods, such as nets. Zooglider is designed to be as stealthy as possible under water, causing as little turbulence as possible in order to not disturb the marine organisms in their natural behaviour.
Zooglider is typically deployed by two or three people. Once released in the water it is autonomous. Zooglider does not have an engine or external moving parts. As a glider, it is equipped with a small oil bladder. An integrated pump will pump oil from or into this bladder, causing a small change in total density of the glider, resulting in a downwards or upwards force. The presence of wings creates lift, allowing the glider to swim upwards or downwards in the water column at an angle, and thus moving over the seafloor. A typical dive would be down to 400 m of depth and last approximately 3 hours. When Zooglider reaches the surface, it raises one of its wings into the air to establish a satellite link to shore. This is how it communicates us that everything is ok and it sends us a summary of what is has observed on its last dive.
How do you operate Zooglider?
Based on the latest position from Zooglider, we combine information from satellites about features we might want to sample with the later report we receive from Zooglider, to decide if we want it to continue its preprogramed path, or if we want to give it a new destination. This routing process is revised constantly during a mission. In the case of BioSWOT-Med, I will closely monitor the received information and negotiate the next waypoint with Prof. Mark Ohman from the Scripps Institution of Oceanography. Once we agree on our next destination, Dr. Jeff Sherman from the Scripps Institute Development Group will send the commands to Zooglider through a satellite link.
What data do you acquire with Zooglider?
During the descent, Zooglider is in a passive mode and only records ambient sounds of the sea with a hydrophone. During the ascent, Zooglider collects CTD information – Temperature, Salinity and chlorophyll fluorescence (as a proxy for phytoplankton concentration), allowing us to gain detailed insights about the physical environment in which Zooglider is operating.
But the ocean is not only physics and we also want to understand the drifting planktonic organisms near the base of the ocean food web. Therefore, we have equipped Zooglider with a specialised optical system we call the Zoocam. Zoocam is a shadowgraph imaging system, which as the name suggests, records the shadows of anything Zooglider crosses on its journeys. A red LED beam (bundled and parallelised to remove any spatial distortion) is projected across a sampling tunnel of about 250 mL, attached to the font of the glider. Anything that passes through this light beam will cast a shadow that is then recorded by the camera. This allows us to quantitatively record dense organisms, like small crustaceans, as well as translucent gelatinous organisms, like medusae, siphonophores or marine snow. With ambient light being so sparse at depths greater than a few meters, many organisms don’t rely on their optical senses alone but use sound as a way of communicating and sensing the environment.
Zooglider is also equipped it with a dual frequency active acoustic system we call Zonar. Zonar sends out acoustic waves at 200 and 1000 kHz and then waits for the signal to come back. A soundwave travels much faster underwater than in the air. On its path through the water column, any object or organism it encounters will send back a part of the acoustic energy to its source. In very general terms, the signal received at higher frequencies will be more dominated by smaller organisms and the signal at lower frequencies will be more dominated by larger organisms. This is a technique very similar to what dolphins use to find prey, friends and how they avoid potential obstacles. The latter inspired us to also build a seafloor detection algorithm into Zooglider, which allows it to sense the presence of the seafloor and automatically adjust its trajectory, should it get too close. Through a combination of acoustic scattering models, information received from zonar and the zoocam, we can separate the acoustic signal into organisms of different size classes or taxonomic groups. Coupled with observations of the physical environment and the zoocam images, this allows us to detect changes in density or composition of the mesozooplankton communities.
How do you use the hydrophone on a Zooglider to listen to marine mammal and fish calls?
Many marine organisms are very vocal; they communicate using sound. A hydrophone is an underwater microphone, which allows us to listen in into the chitchat of sound producing animals, such as marine mammals or fish. It is a well-known that whales have different songs or types of calls they use to communicate. We can for example distinguish different baleen whale species by the calls they produce and often we can even understand if they are performing social, feeding or mating calls. Not so well known is the fact that many fish are also rather vocal. Admittedly, the sound of a fish chorus is not quite as melodic or relaxing as the sound of a humpback whale might be, and more accurately described as grunting. Nonetheless these sounds can provide us with insights on the behaviour of fish. While marine mammals produce sounds using mechanisms similar to ours, fish mainly produce sounds through sonic muscles on or near their swimbladder, by rubbing together skeletal components or in a more passive fashion, through abrupt changes in swimming directivity or speed. On Zooglider we recently added an Acousonde, a kind of highly portable hydrophone. We have preprogramed the hydrophone such that is starts recording when the glider reaches a certain depth and stops when Zooglider begins the ascent. We use the descent for our listening session, because this is the time Zooglider is the quietest, with no other instrument noise. After the voyage we can then listen into all the conversations that Zooglider recorded on its mission.
It is possible to follow Zooglider in real time trough a public website.
Tosca Ballerini (firstname.lastname@example.org)