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Zooplankton, mostly small animals at the bottom of the food chain, drift throughout the world's oceans. Autonomous underwater vehicles such as Zoogliders provide unique insights into the biological composition and living conditions of these fascinating and important, often overlooked organisms.
Zooglider - an autonomous underwater vehicle
Zooglider can operate autonomously in the ocean for up to two months in continuous wave motion from the surface to depths of up to 400 metres, without a motor. A pump inside the glider pumps oil from a bladder into a larger chamber or from the chamber into the bladder. The resulting change in internal density causes the glider to either sink or rise to the surface. Once on the surface, the robot slowly turns to the side and points one of its wings towards the sky to establish a satellite link. This allows us to exchange information, check that our engineer thinks the Zooglider is in good condition and ready for further exploration. Looking at a summary of the data the Zooglider has collected for us so far, we can decide whether to continue as planned or if we need to change our plans.
Background and Objective
Traditional methods of studying zooplankton are largely limited to invasive methods using nets or similar devices. What these methods have in common is that the zooplankton cannot be observed in their natural environment and are often harmed in the process. Acoustic and imaging methods, combined with autonomous underwater platforms, make it possible to observe these sensitive creatures non-invasively in their natural environment. Researchers from the Scripps Institution of Oceanography der University of California, San Diego, are working with scientists from the Thünen Institute on non-invasive methods to better understand the exciting world of zooplankton.
Zooplanktonic organisms respond rapidly to environmental changes. Typical adaptations include changes in population size, body size or diet. In addition, zooplankton play a very important role in all marine ecosystems, both as a major food source for many fish and whale species and as an important component of the ocean's carbon budget. Particularly with regard to changes caused by climate change or ocean acidification, it is very important to understand the dynamics of zooplankton communities in order to anticipate the effects on higher trophic organisms. In addition to these big questions, every underwater mission is accompanied by curiosity about the unknown, the immersion in a foreign world where there is still much to discover, from new species to unexpected behaviours and much more.
Approach
During the descent, Zooglider is in passive mode, with all active sensors switched off, except for the hydrophone, an underwater microphone that allows us to hear the 'chatter' of fish, whales and dolphins. As soon as we return to the surface, the Zoocam, a shadowgraph camera, the Zonar, an echosounder with two frequencies (200 and 1000 kHz), and a CTD (which measures ambient pressure, temperature, salinity and chlorophyll) are activated. The CTD data will allow us to accurately determine the physical conditions of the environment. The shadowgraph camera allows us to collect images of the often translucent, gelatinous zooplankton species even in the total darkness of the deep. The echosounder extends the field of view and detects organisms that can escape the camera.
Optical data (Shadowgraph)
Plankton (from the Greek planktos, drifting), consists of the phytoplankton (plant) and the zooplankton (animal). As most zooplankton species have a limited active swimming capacity, they can usually swim up and down to a certain extent, but cannot swim against a strong current or can only do so for a very limited distance. Zoocam takes advantage of this: Mounted at the front end of the Zooglider, it captures all particles that drift through the camera opening as the glider ascend.
An image is captured once or twice per second. Each image is first cleaned by automated image processing procedures. The next step is an automated segmentation - the extraction of regions of interest (ROI), i.e. the identification of pixels that need to be analysed in more detail. Artificial intelligence (AI) algorithms, which are continuously being developed, then identify all ROIs larger than 0.45 mm, while all smaller ROIs are only measured. This AI is currently capable of identifying over 52 taxonomic groups, with >95% accuracy for each group.
Acustics
In general, the acoustic signals from the low frequency echosounder (200 kHz) tend to be dominated by larger organisms, while the signals from the 1000 kHz echosounder tend to be dominated by smaller organisms. This difference allows a rough categorisation into areas dominated by smaller and larger organisms. More detailed analysis of the data, combined with concurrent Zoocam, feeding into acoustic models, allows a more precise classification or subdivision of the data. Vertical migration patterns are also routinely extracted from the acoustic data.
Passive acoustics
Passive acoustics can be used to record the soundscape. More detailed analysis of the data allows the different songs or sounds to be assigned to specific animals. This enables the presence of certain predators to be located from the perspective of zooplankton.
The optical and acoustic data can then be combined with the CTD data to describe differences in different environmental conditions. The letters CTD stand for Conductivity, Temperature and Depth.
Our Research Questions
- How can we better understand and map the world of zooplankton?
- How do changing environmental conditions affect the composition, distribution and fitness of zooplankton?
Publications
Gastauer S, Ohman MD (2024) Resolving abrupt frontal gradients in zooplankton community composition and marine snow fields with an autonomous Zooglider. Limnol Oceanogr: Online First, Jul 2024, DOI:10.1002/lno.12642
Gastauer S, Nickels CF, Ohman MD (2022) Body size- and season-dependent diel vertical migration of mesozooplankton resolved acoustically in the San Diego Trough. Limnol Oceanogr 67(2):300-313, DOI:10.1002/lno.11993
Links and Downloads
UC San Diego's public website shows real-time data from various Zooglider missions: "Zooglider - Public Mission Site"
Further information about our partner institute: Scripps Institution of Oceanography at UC San Diego
Thünen Contact
Dr. Sven Gastauer
Institute of Sea Fisheries
Herwigstraße 3127572 Bremerhaven
- Telephone
- +49 471 94460 450
- sven.gastauer@thuenen.de
