Marine Mix

Marine Mix

Emerging Explorer Kakani Katija's research suggests that tiny sea creatures could affect our oceans and our weather patterns.


6 - 12+


Earth Science, Oceanography, Engineering, Conservation

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Kakani Katija’s research may change the way we think about the ocean, its creatures, and the world’s weather patterns.

Katija, a National Geographic Emerging Explorer, is researching ocean mixing, the process of warm, sun-filled surface water mixing with cold, nutrient-rich water near the bottom of the ocean. Winds, ocean currents, and tides are responsible for most ocean mixing. However, new research from a variety of oceanographers suggests the movement of animals may have a greater impact on ocean mixing than previously thought. The process is called biogenic ocean mixing.

This was not, however, the original focus of Katija’s work.

“I would say my original interests were on animal swimming or jellyfish propulsion,” says Katija, who is not an oceanographer, but a bioengineer from the Woods Hole Oceanographic Institution in Woods Hole, Massachusetts. “In that investigation, some of this work . . . suggested that swimming animals can impart a lot of [ocean] mixing.”

Ocean mixing is necessary for marine organisms, Katija says.

“If you want to get oxygen, that comes from the surface or the atmosphere,” she says. “If you want nutrients, a lot of those nutrients settle to the bottom of the ocean. Somehow you need to spread oxygen as well as these nutrients through the ocean fast enough to sustain life. That’s where ocean mixing is very important.”

It’s not just sea creatures that are affected by this natural process.

Ocean mixing is important for climate, because the ocean is largely involved in circulating heat on our planet,” Katija says.

Ocean Mixing

Research suggests that small animals could affect mixing just as much—or more—than their larger counterparts.

“We are pretty big so when we swim through water it feels like water,” Katija says. “But . . . if we were something smaller, like a worm, it will feel more like swimming through honey. This difference is called fluid viscosity. Basically, it’s because of this stickiness in the fluid that if you are small you have the ability to transport or move fluid around you. That allows for smaller animals to transport and mix more fluid relative to their size than larger animals.”

Katija has found a group of tiny ocean animals, only 1 to 2 millimeters (.04 to .08 inches) in length, that may be major contributors to ocean mixing.

“These organisms, copepods, there are a lot of them, and they have this migrating behavior in these large numbers,” Katija says. “So we suspect that they can have a vital role.”

Of course, Katija needs a method to determine how much different organisms contribute to ocean mixing.

“To understand how well an animal mixes a fluid, you need two different quantities,” she says. “The big thing is energy. How much energy is required to make that process, or how much energy is imparted to the fluid to mix it? Then you have to measure mixing, and you do that by having dye in the water. Over time, when the dye patches mix, the dye becomes more diffused. Using the dye, we are able to measure the mixing, like how much mixing actually occurs. Then using the [camera laser] apparatus that is called SCUVA, it actually allows us to quantify the energy—specifically the kinetic energy—in the water that is due to the animal swimming.”

Katija’s research has taken her all over the world, from Palau’s Jellyfish Lake to Mijet Island, Croatia, and Friday Harbor, Washington. Katija says her upcoming work will keep her in Woods Hole.

“The next question I’m going to test within the next three years in a laboratory is whether or not populations of migrating copepods can mix substantially,” she says.


Katija believes that her discoveries regarding ocean mixing could be used to help marine animal conservation efforts.

“If it is indeed the case that swimming animals have an important impact on mixing the ocean, I think you have to seriously look at what we are doing to fishery stocks,” she says. “If we fish out all of these large migrators, that affects smaller organisms as well. How does this affect mixing? That may lead people to then question fishing practices.”

The information could also lead scientists to rethink what they know about climatology.

“In terms of the academic world, right now the way we simulate climate models or ocean models, we do it based on our understanding of physical mixing processes that don’t include the biology,” Katija says. “So somehow, some way, we are trying to work on figuring out a good method to incorporate biology into those models.”

Another aspect of Katija’s work is looking at how the designs and shapes of ocean organisms affect how they move through the water. She is particularly interested in salps, gelatinous sea creatures the size of a dime, and marine invertebrates known as siphonophores. Both salps and siphonophores have individuals that can join together to create a unit.

“There’s really interesting questions from that,” Katija says. “Why would you want to be together as opposed to swimming on your own? Are there advantages, and if there are, how can we take advantage of that ourselves when we are designing systems?”

This knowledge could change the design of items on land. Katija notes that it has happened before.

“If you look up a boxfish, they have a very boxy profile,” she says. “And Mercedes used that design.”

Fast Fact

Jellies, not Fish
Jellyfish are not fish! They are closely related to sea anemones.

Fast Fact

Shape Up
The transport of water around an animal as it moves through the ocean has more to do with the shape of the animal rather than how it swims.

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Stuart Thornton
Jeannie Evers, Emdash Editing, Emdash Editing
Kara West
National Geographic Society
Last Updated

October 19, 2023

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