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ocean engineering

Mucus flux and other amazing discoveries with underwater cameras

Katija, K., Sherlock, R. E., Sherman, A. D., & Robison, B. H. (2017). New technology reveals the role of giant larvaceans in oceanic carbon cycling. Science Advances, 3(5), e1602374.

A scientific team from the Monterey Bay Aquarium Research Institute, lead by Dr. Kakani Katija, made headlines last week for measuring how much water giant larvaceans can filter in situ (fig. 1). These zooplankton construct huge mucus houses to capture food from the surrounding water. When a larvacean’s house gets clogged, it simply ditches the carbon rich filter. The ball of mucus then sinks to the bottom of the ocean.

Figure 1 – A larvacean as imaged by the DeepPIV system. The purple streaks are moving particles inside the organism. This image was taken at depth. (Image courtesy of MBARI)

The collective flux of larvacean houses to the sea floor is an important part of the biological pump. But quantifying how much carbon is actually being taken up is extremely difficult. Since the houses are so fragile, it is almost impossible to study them outside of their natural habit. To estimate how much water larvaceans are able to filter, Dr. Katija and her team developed a special imaging system they call DeepPIV.

PIV is an acronym that stands for Particle Image Velocimetry. It is a quantitative imaging technique that allows scientists to visualize how fluids flow by tracking small particles, called tracers, through a volume. In a lab setting, PIV systems use a highly tuned laser sheet, or plane of uniform illumination, to light up a 2D portion of fluid. The tracers then glow, allowing a computer to easily reconstruct their motion.

For DeepPIV, Dr. Katija had to develop some special equipment to make the whole shebang work underwater: an optical system to produce the special illumination needed for PIV, a fancy port for the camera to look through without distortion, and a housing to rigidly hold everything together. And, just to make matters difficult, she had to fit it all on a remotely operated underwater vehicle (fig. 2).

This feat of engineering is, to say the least, impressive. But really take a second and let what this group managed to do sink in. They made a sheet of laser light, used it to illuminate organisms that are mostly transparent, took video, and then examined how the tiny particles inside the samples moved.

Figure 2 – DeepPIV in action. The camera and laser fitting are on the bottom right. The big blob of stuff it is looking at on the left is a larvacean. (Image courtesy of MBARI)

Dr. Katija and her team’s work is at the cutting edge of what scientists are doing in the realm of underwater imaging. To truly appreciate how far these technologies have come, we have to go back to the early days of in situ photography in the 1970s. We could go back even further, to the 1890s, when Louis Boutan took some of the first images of hard-hat divers at work. But that might be a little much for a blog post.

In the early 70s, researchers at the Scripps Institution of Oceanography and Woods Hole Oceanographic Institute started putting film cameras in watertight housings to look under the waves. These instruments shot standard 35-mm film on special rolls up to 400 feet long (for reference, a regular roll of 35-mm format is about 4.5 feet long). Such systems were strapped to underwater vehicles and revealed fine-scale features of the ocean floor and helped locate the Titanic.

Figure 3 – An image from the early zooplankton imaging system developed by Ortner et al., 1979. This photo was taken on film with a special underwater camera.

It did not take long for biologists to get in on the act. A group of NOAA scientists adapted the technology to attach to the back of a plankton net in an effort to look at samples before being preserved in chemicals. The project produced amazing images of undamaged microorganisms (fig. 3).

These early in situ imaging systems were very successful. But they were also very limited; the space needed for film was substantial and wet processing for development was costly and time consuming. Both issues were mitigated by the digital revolution and the advent of electronic light sensors in the early 1990s. Almost as soon as these technologies became available, oceanographers figured out ways to put them underwater.

The past few decades since have seen a proliferation of highly specialized, in situ imaging systems. Besides producing fabulous images, these instruments have yielded fascinating insights into what happens deep below the surface. They have allowed scientists to see the ocean in new ways, answer questions that have confounded them, and ask new ones they did not they had.

Dr. Katija and her team’s most recent technological advance helped them discover that small larveceans can filter huge amounts of water. When larveceans are at their peak abundances, they could filter the entirety of Monterey Bay in just 13 days. Who knows what other questions Dr. Katija will be able to ask with her fancy new instrument? I, for one, am pretty excited to find out.

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