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Biology

Herds of sea monkeys help scientists understand the role of diel vertical migration in ocean mixing

Article: Monica M. Wilhelmus and John O. Dabiri. Observations of large-scale fluid transport by laser-guided plankton aggregationsPhysics of Fluids, September 30, 2014 DOI: 10.1063/1.4895655

Background

Every day, as the sun sets, hundreds and thousands of individual zooplankton begin to swim up from deep waters to the surface. This daily migration is known as diel vertical migration and is an important phenomenon observed in the ocean. Plankton are an important food source for many organisms – as small organisms, typically a couple of millimeters long at most, you can imagine how many animals will be able to eat zooplankton. Thus, they’ve evolved diel vertical migration behavior to help protect themselves from the multitudes of hungry predators. They swim down to deep waters during the day and hide where there is little to no light, making it hard for predators to see them and journey to the surface at night to feed.

Figure 1. Open ocean zooplankton migrate from deep waters to the surface at night and return to depth during the day to avoid predators. It is hypothesized that this diel vertical migration potentially contributes to ocean mixing though it’s not known to what degree. Modified from Doney and Steinberg 2013 (doi:10.1038/ngeo1872)

Figure 1. Open ocean zooplankton migrate from deep waters to the surface at night and return to depth during the day to avoid predators. It is hypothesized that this diel vertical migration potentially contributes to ocean mixing though it’s not known to what degree. Modified from Doney and Steinberg 2013 (doi:10.1038/ngeo1872)

While an individual zooplankton is very small and does not swim particularly fast, many scientists think that a mass migration of hundreds of thousands of small individuals may contribute a large amount of mixing to the ocean. Unfortunately, this is very difficult to study, both in the ocean and in a laboratory setting. While we know diel vertical migration occurs and can be observed, there can be large variations in the number and species of zooplankton present at a time and place which can make standardizing measurements almost impossible. While there is more control in laboratory settings, scientists, until now, were unable to get zooplankton to do diel vertical migration behavior in a tank.

Approach

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Figure 2. A three-laser guidance system was used to guide zooplankton up and down a laboratory tank. A horizontal blue laser (2) is used to aggregate A. salina and a vertical green laser (3) is used to center the zooplankton in the middle of the tank. The blue laser is raised up the tank 1 cm/s to simulate diel vertical migration. A horizontal red laser (6) was used to help visualize silver coated glass spheres which can be used to track flow direction and speed.

Drs. Monica Wilhelmus and John Dabiri used a three laser guidance system to induce vertical migration behavior in a laboratory setting. They used large groups of the zooplankton Artemia salina (the same organism sold to children as Sea Monkeys) as their model species. A. salina is a commonly studied species of zooplankton since it is closely related to many of the zooplankton found in the ocean, are easy to breed and rear in a laboratory, and most importantly, exhibit phototactic behavior (they use light sources as a cue to move towards or away from a light source depending on the light’s wavelength). Researchers found that blue and green lights cause a positive phototactic response (the plankton swim towards it) so they used one horizontal laser to aggregate the A. salina. They used a green laser positioned vertically through the center of the tank to help evenly distribute the plankton throughout the tank (rather than aggregate near the light sources). The blue laser was then moved up the tank at a rate of 1 centimeter per second, simulating diel vertical migration. For a visual demonstration, check out their video supplemental.

Wilhelmus and Dabiri employed particle image velocimetry (PIV) to analyze the flow generated by vertically migrating zooplankton (see how it works in this nifty animation). They introduced very small, silver coated glass beads which can be visualized by a third laser (a horizontal red beam). A high powered camera takes 2 images of the moving particles and analyzes the movement of the individual particles between the two shots. Thus, scientists can visualize flow and velocity fields by following the path of the silver particles (Figure 3).

 

Figure 4. Examples of PIV analyses which show the resultant flow caused by zooplankton swimming. Velocity fields (superimposed colored arrows) are determined by following silver-coated glass spheres (small dots in background) with high speed cameras, which show flow velocity and direction.

Figure 3. (Top) Particle image velocimetry uses a high speed camera to follow the flow of small, silver coated glass particles. (Bottom) Examples of PIV analyses which show the resultant velocity fields (colored arrows superimposed on the pictures) caused by zooplankton swimming.

Results

Video and PIV analysis showed that as A. salina moves upward, they generate a strong downward flow. Surprisingly, the downward flow interacts with the flow generated from other zooplankton swimming nearby, creating a jet that increases with space and time. They found that these jets created Kelvin-Helmholtz instability (a form of turbulence which generates a distinctive wave-like pattern). This lead to the generation of small eddies near the zooplankton (Figure 4). These resultant eddies grew to be larger than the zooplankton itself! So even though an individual zooplankton is very small (compared to the size of the entire ocean), very large groups of zooplankton swimming together can influence and mix waters on a larger scale.

Figure 4. Particle image velocimetry analyses revealed that vertically swimming zooplankton aggregates form Kelvin-Helmoltz instabilities which result in eddies (blue oval) that mix water at scales much larger than the zooplankton itself.

Figure 4. Particle image velocimetry analyses revealed that vertically swimming zooplankton aggregates form Kelvin-Helmoltz instabilities which result in eddies (blue oval) that mix water at scales much larger than the zooplankton itself.

Implications

Drs. Wilhelmus and Dabiri have developed a way to quantitatively study biogenic mixing resulting from diel vertical migration. They found that zooplankton can potentially play a very large role in ocean mixing.

Mixing is an important process in the ocean since, for the most part, the ocean is very well stratified. Stratification, usually resulting from density differences caused by temperature or salinity differences, can impede the flow of water across density layers. This impediment to flow can alter the transport of dissolved constituents like nutrients. Thus, swimming zooplankton can alter nutrient distributions in the ocean by mixing across density layers. Thus, nutrients from deeper waters could be mixed up back into the surface of the ocean which can increase their productivity.

The primary drivers of vertical mixing are wind and tides which give off roughly a combined 2 trillion watts of mechanical energy with which they mix the ocean with. Considering the massive quantities of zooplankton that vertically migrate on a daily basis, Wilhelmus and Dabiri hypothesize that it is possible that zooplankton can contribute an equivalent amount of mixing as wind and tides do. Thus, zooplankton can potentially play a very large role in ocean dynamics, forcing scientists to reevaluate what we know about fluid transport and mixing in the ocean.

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