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In this De-Natured segment of Nature League, Brit breaks down a recent scientific journal article about the impact of invertebrate motion on the mixing of water columns.

Article citation:
Isabel A. Houghton, Jeffrey R. Koseff, Stephen G. Monismith & John O. Dabiri

Vertically migrating swimmers generate aggregation-scale eddies in a stratified column

Nature, 2018

Article link:
https://www.nature.com/articles/s41586-018-0044-z

Videos and images courtesy of Isabel Houghton. Schlieren videos and imaging obtained with the assistance and facilities of J.R. Strickler (UWM).

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On Nature League, we spend the third week of each month exploring a current trending article from the peer-reviewed literature.

Scientific information isn't just for scientists- it's for everyone! It just requires a bit of a break down. [CHEERY INTRO MUSIC].

For this month's De-Natured segment, we're going to look at an article released in April 2018 in the journal Nature. In this month's Lesson Plan, we talked about the different types of invertebrates here on Earth and some of the ways that they significantly affect the planet. I mean, there are just so many of them, and scientists are only beginning the uncover their profound impacts.

This paper, entitled, “Vertically migrating swimmers generate aggregation-scale eddies in a stratified column” investigated the very large effects that these very tiny organisms might have on mixing the Earth's oceans. So here's what's already known. There are several examples of organisms affecting the flow of materials around them.

For example, super dense groups of bacteria can generate convection patterns, and individual jellyfish can create significant water drift just by swimming. A lot of work on this so-called “biologically generated turbulence” has been done with marine zooplankton. Marine zooplankton are typically microscopic animals that either don't move or are too small to fight against the current.

This means that marine zooplankton are typically tiny drifters. However, some are bigger, and some are able to move more than others. Like everything else about life, exceptions abound!

Previous work has shown that centimeter-scale zooplankton can form dense bunches that stretch out vertically over tens of meters as they migrate hundreds of meters up and down during the day while they search for food. To put that in perspective, that would be like 1,000 people standing one on top of another moving as one blob up and down a distance of almost 17 kilometers during the day. So, it's pretty easy to see how these diurnal, or daytime, vertical movements of zooplankton might have some big effects on the water they move through.

And in oceans, all of that water can have biologically important bits inside: things like oxygen, carbon, and nutrients. And even though each individual zooplankton is small, the vertical height of zooplankton swarms is not, and scientists have documented large-scale effects on carbon and oxygen distribution from these vertical swarms. So, although large-scale effects have been seen, it turns out that directly observing the fluid dynamics surrounding the zooplankton swarms is hard to do, especially since scientists can't easily predict when and where the migration events might happen.

To address this phenomena, the research team set up experiments in the lab using the centimeter-scale swimmer Artemia salina. A. salina is a type of brine shrimp, and if you ever had Sea-Monkeys as a child, you've seen hybrid relatives of A. salina with your own two eyes. Sorry to spoil the sea monkey magic, but Sea Shrimp might have been a bit closer to the truth of your childhood aquarium...

The scientists set up two separate tank facilities for two separate goals:. The first tank was 1.2 meters tall and was used to measure irreversible mixing, and the second was 2 meters tall and used to try out several water flow visualization techniques. So let's figure out what the team was actually trying to measure.

Irreversible mixing, in this situation, is having a different density profile in the tank at the end of the experiment compared to the beginning set-up. The “irreversible” bit means that energetically, it is /very/ unlikely for the system to return to its starting condition. Alright, so, the researchers want to measure irreversible mixing due to the movement of the shrimp.

That means they had to get these guys moving. In order to get the shrimp to move, the researchers took advantage of the fact that A. salina is phototactic; that is, it moves in response to light. The 1.2 meter tank used focused LEDs with blue filters to create a vertical column of light.

The 2 meter tank used a blue laser to create a column of light that was half the diameter of the other tank. Using light to coordinate vertical migrations inside the tanks allowed the researchers to simulate how zooplankton migrate in a non-lab environment. To measure irreversible mixing, the researchers set-up two stable layers of different salinity, or salt concentration, inside one of the tanks.

Once the water column was layered, the shrimp were introduced and allowed to adjust to the environment. To start the experiment, the researchers used a green LED light at the bottom of the tank to get the shrimp to gather there. Then it was time to migrate!

They turned off the green LED at the bottom, and turned on the blue LED light at the top. The animals began to migrate upward with speeds reaching one centimeter per second…. Not exactly speedy for us, but definitely a good pace when one centimeter is your entire body length!

Think about traveling the distance of your height once every second. It's not exactly lightning speed, but it's certainly faster than leisure walking. After 10 minutes passed, the top blue light was turned off, and the bottom green light was turned on so the shrimp would go back to the bottom of the tank.

After ten minutes at the bottom, the lights switched again. The cycle was repeated 6 times for a total experiment of 120 minutes, and the researchers got density profiles of the tanks each time. In general, the researchers were looking to find out how much mixing these little guys could do, and also how quickly.

So what did they find? Overall, the team definitively found that irreversible mixing had occurred. More impressively, the shrimp were able to mix the water three orders of magnitude more efficiently than salt diffusing through water on its own.

That's about a thousand times faster! Visualizing the flow of materials inside of the water helped the scientists figure out the actual process of the irreversible mixing they observed. They found a movement signature that might explain how the mixing happens.

As the shrimp move upward, they actually push the surrounding water downward...and when there's a swarm of the shrimp, the collective downward jet of water is significant. The scientists also noticed whirlpool-like motions in the water right outside of the collective migration, and that eddy motion contributed to the mixing as well. The scientists concluded that the kind of mixing and downward jets of water observed in their lab experiments could create large-scale vertical motion and mixing of water in the ocean.

Moreover, the fact that many ocean invertebrates engage in this kind of vertical migration means that their movement could be a major player in the motion of the ocean. This paper was published in the journal Nature, which is one of the most prestigious science journals around. Here are some reasons why I think this study wound up in this edition of Nature.

First of all, scientists in previous research have estimated that the migrations of ocean zooplankton might affect ocean mixing just as much as wind and tides, and that is a major deal. Ocean mixing affects all /kinds/ of important things, including the spread of heat, carbon dioxide, nutrients, and even the circulation of the ocean. Which, by the way, is a big deal for life on Earth.

In general, I think the main reason this study made the cut in terms of interest and importance is because it's of great relevance to climate change. If zooplankton have this big of an effect on the movement of the ocean, researchers who create climate models can't leave them the little guys out of their calculations. This is especially important when it comes to modeling the storage of carbon, because the ocean is one of the biggest carbon sinks on Earth.

As our climate continues to change, potentially in detrimental ways, being able to model these changes is of great interest to scientists and the public alike. In terms of critiques, the biggest issue with lab experiments is that they are exactly that: lab experiments. These results weren't obtained from studying zooplankton in the ocean, so it's hard to know if they apply to the real situation.

The first thing of note is that the species of brine shrimp they used, A. salina, doesn't actually live in the ocean. However, A. salina is similar to the marine zooplankton of interest, and the scientists made sure that the density of animals they used in the set-up was comparable to the abundance usually seen in the ocean. There's also the issue of time and scale.

Did the experimental set-up really mimic what would be observed in the ocean? The duration of the experiment /did/ match up with the estimated time it takes for an oceanic daytime vertical migration. However, a water column in the ocean would be disrupted by multiple individuals over the 2 hour period, whereas in the lab experiment the column of water was affected by the same individuals multiple times.

Overall, it's hard to say if the results seen in this study will perfectly apply to ocean migrations of zooplankton. However, the scientists mention that they already have plans to use acoustic equipment to measure these mixture signatures in the ocean, meaning we might see in situ, or on site, results at a later date. So here's the thing.

On a personal note, I sort of just...love...the fact that these tiny organisms can have such a massive impact. We know that invertebrates make up an insane amount of life on Earth, but there's still so much we don't know about how they affect our world. This study is one example of how scientists are figuring it out one experiment at a time.

Thanks for watching this episode of De-Natured here on Nature League. Nature League is a Complexly production, and if you want to learn more about the effects of ocean organisms, you can watch this episode from our sister channel, SciShow, where Hank. Hank Green interviews a scientist about the impacts of deep sea creatures on our environment.