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Biological oceanography

Seeing Swarms from Space, Zooplankton in Action

Basedow, Sünnje L., David McKee, Ina Lefering, Astthor Gislason, Malin Daase, Emilia Trudnowska, Einar Skarstad Egeland, Marvin Choquet, and Stig Falk-Petersen. “Remote sensing of zooplankton swarms.” Scientific reports 9, no. 1 (2019): 686.

Background- Observing Ocean Organisms from Space

Zooplankton play an extremely important role in both the oceanic food web and our food chain. Fish that we eat rely on zooplankton as a food source. While these creatures sure are small, they have a big impact on the entire marine food web. Zooplankton are a major food source for higher trophic level organisms. For example, the Northeast Arctic cod and Norwegian herring feed upon Calanus finmarchicus (figure 1), a type of zooplankton key to a recent study by Basedow and co-authors.  Though it can be challenging to study small organisms on a large scale, as Calanus zooplankton are about 3 mm in size, researchers recently identified Calanus all the way from space.

Figure 1. Image of a Calanus finmarchicus zooplankton, which are typically 3 mm. Image credit: Michael Bok https://www.flickr.com/photos/mikebok/4635279337.

How might one detect something so small from so far away, or rather remotely? Satellite remote sensing turned out to be a useful tool. Satellites orbiting the Earth observe the amount of light reflected back from the land and ocean. Originally this light was emitted by the sun in many different forms, what we call wavelengths. Each of the different colors we see is a different wavelength of visible light, and ultraviolet light is another form that we can’t see but unfortunately can give us sunburns. In order for light to make it to Earth, it has to transmit through the atmosphere, which has molecules in it that absorb much of the light or scatter the light, giving the sky a blue color or red as the sun sets. Once light reaches Earth’s surface, it again will interact with the land or water. For ocean remote sensing, when light reaches the ocean surface, the ocean absorbs red light in the surface waters, whereas blue light is able to travel deeper into the ocean. Organisms, like phytoplankton also absorb light to photosynthesize. In the case of this study, the researchers focus on this red Calanus zooplankton, which is red because of the color of its prey. Since these organisms appear red, it means they reflect red light. The small fraction of light that isn’t absorbed ultimately is reflected back out to space where extremely sensitive sensors on satellites detect this light. In this case, the researchers are interested in whether the red from massive amounts of Calanus in surface waters can be detected from the Visible Infrared Imaging Radiometer Suite (VIIRS) satellite.

With satellite remote sensing, it’s important to ground truth; to make sure that the red signals viewed from space are actually from the Calanus. It’s important to go out in the field to confirm that swarms of these creatures were actually present in the water. To ground truth, researchers collected data in the field on a cruise in May 2017 off the coast of Northern Norway. During the cruise they used nets with a very fine mesh size to collect zooplankton samples. Researchers also towed a laser optical plankton counter behind the ship so that it travelled with the ship but also moved up and down from the surface to ten meters deep. While moving, this device counts and measures the size of plankton and other particles between 0.1 and 35 mm that it collects in a sampling tunnel.

The researchers returned to two of their sampling areas and repeated them while towing a video plankton recorder. This collects color images of plankton in the water so the researchers could see whether the particles counted by the plankton counter might be Calanus zooplankton. Figure 2 below shows the location of the study using a weekly averaged image from the VIIRS satellite off of Northern Norway with their sample sites. Note, the three sites marked with stars, O for open ocean, C for coastal and P for in the zooplankton patch.

Results

During the cruise, large amounts of Calanus zooplankton were found in the surface waters concurrent with a red patch observed in the satellite imagery (figure 2). Can you imagine how many organisms 3 mm long are needed to be detected from space at scales of 100 kilometers wide in the coastal ocean? From the nets, researchers found about 4000 of the critters per cubic meter of water. The concentration of Calanus zooplankton that the researchers observed from the net tow was similar to the amount that they found using the sensors, the video plankton recorder and the laser optical plankton counter (over 1000 Calanus per cubic meter of seawater in the upper 10 meters of water).

Figure 2. Image from VIIRS satellite from April 27- May 3 2017 showing the study site, the red patch and the red colored Calanus finmarchicus zooplankton. Image from Basedow et al., 2019.

Of course, studying living organisms is not without its challenges. Researchers remind readers that ships’ thrusters can cause mixing of the water making it challenging to get accurate amounts of these organisms in the water without influencing the measurement. Zooplankton are also active swimmers and may avoid the sensors further impacting the results. Additionally, the water is always moving which makes it challenging to match these small scale studies done in the water with large scale observations from satellites. This can cause a mismatch between the observations, which researchers suggested occurred during one of the satellite image zooplankton patches that was not detected by their net tows because the water moved before they were able to collect samples.

Main Idea and Future Implications

Overall, this study provided an exciting observation of zooplankton all the way from space, opening up new applications of satellite remote sensing. Researchers suggest that future applications may include obtaining more quantitative amounts of zooplankton in surface waters from space. This study may also inspire future studies aimed at better understanding the behavior of these creatures by detecting when they choose to swarm the surface.

 

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