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Remote Sensing

SURFO Special: From outer space to the microscope: How NASA’s satellites are helping us understand the ocean’s smallest life

Each summer, the University of Rhode Island Graduate School of Oceanography (GSO) hosts undergraduate students from all over the country to participate in oceanographic research. These Summer Undergraduate Research Fellows (SURFOs) have not only been working with GSO scientists, but they also have spent part of their time learning how to communicate this science to the public. Although their research experience was virtual this summer, they still did a fantastic job. Read on to find out what they have been up to, and why they everyone should be as excited as they are about their work.

 

Julia Lober is a rising senior at Tufts University in Medford, MA with a major in Environmental Geology and a minor in Computer Science. Her advisor this summer was Dr. Colleen Mouw.

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Close your eyes and imagine the ocean. What do you see? Maybe you picture crystal clear water and a sandy beach. Or murky, green water and a rocky shore. Or of a blue open ocean. No matter what you see, you’d be right. The ocean takes on many faces, depending on weather, waves, nutrients, temperature, and other factors.

Orbiting many miles above us, NOAA’s Visible Infrared Imaging Radiometer Suite (VIIRS) satellite sees, and records, all of this. The satellite has an instrument called a radiometer that measures the light emerging from the surface of the ocean – the “color” of the ocean. Scientists in the field of ocean color optics are working to figure out what this satellite information can tell us about the ocean itself.

This map shows average reflectance at 443 nm for July 2019, from the MODIS-Aqua sensor. Warmer colors indicate higher reflectance and cool colors show less reflectance (oceancolor.nasa.gov).

We tend to think of color as a single value; an object can be red or blue, not both. Actually, the color we see is an average of the object’s interaction with light throughout the visible spectrum (about 400 – 700 nm). Our brains translate this into a single color, but the satellite’s sensor allows them to be detected separately.

Right now, the VIIRS satellite takes measurements from 5 wavebands in the visible region of the electro-magnetic spectrum. The new Plankton, Aerosol, Cloud, and ocean Ecosystem (PACE) satellite, planned for launch in 2022, will bring us information from many more wavelengths. A lot of information about the ocean is hidden in the details – the small bumps and wiggles across the spectrum. With this additional detail, scientists are hoping to discover more about one of the ocean’s most important (and tiniest) inhabitants: phytoplankton.

At the base of the food chain, changes in phytoplankton populations affect all other marine life. Monitoring phytoplankton populations is key to managing fisheries and helping us understand the effects of large-scale changes like ocean acidification and ocean warming.

So how do we get from outer space to some of the smallest life in the ocean? When rays of sunlight reach the ocean, three things can happen: they can pass through undisturbed, they can be absorbed, or they can bounce off in a process known as scattering.

Much like your own signature, each sample of water has its own unique optical signature based on its interaction with different wavelengths of light. This signature can be separated based on the constituents in the water (Julia Lober, 2020).

The fate of the light depends largely on what it encounters in the ocean. Pure water absorbs longer wavelengths (reds, oranges, and yellows) and scatters shorter wavelengths (purples and blues). That is, red light disappears and we see only the reflected blues. But the ocean has more than just water; there is also detritus, dissolved substances, and, of course, phytoplankton. Each of these components interacts with light differently and complicates the overall “optical signature” of the water.

We can study phytoplankton composition by focusing on just phytoplankton absorption. Different plankton species have unique optical signatures driven by their shape, pigments, or taxonomy. By comparing characteristics of the overall phytoplankton optical signature and of the group-specific optical signatures, we can learn about what groups are present in the water.

What does this have to do with space? Satellites in orbit can give us the overall optical signature of the water. If we can separate out the individual components, we can get the optical signature of the phytoplankton. From there, we can identify the phytoplankton groups in the ocean. Pretty cool!

The variety of size and shapes of phytoplankton that might be in a single sample of water (ifcb-data.whoi.edu/timeline?dataset=NESLTER_broadscale).

In practice, each of these steps requires a lot of knowledge and assumptions. This summer, I worked with the Mouw Lab, virtually,  to improve our knowledge of these optical relationships specific to the continental shelf waters of the eastern US. Using data from a research cruise that looked at both phytoplankton composition and optical data, I identified special characteristics in the optical signatures that correspond to a particular phytoplankton group. Although I’d have loved to see the GSO campus in person, I had a great time getting to know the Mouw Lab team (and MatLab) this summer through video conferences!

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