Diel Vertical Migration
Vertical migration is a common behavior among zooplankton. Before we examine this behavior, we should understand some terminology.
Diurnal refers to events that occur during the day. We are diurnal animals (unless you happen to work nights).
Nocturnal refers to events that occur at night. Bats are nocturnally active animals.
Diel describes events that occur with a 24 hour periodicity. You could say that we have a diel cycle of activity and rest.
Diel Vertical Migration (DVM) describes a vertical migration pattern exhibited by zooplankton with a 24 h cycle. In general it describes migrations upward at dusk and downward around dawn.
Remember that planktonic animals cannot swim against a horizontal current; however, they can move up and down.
Example Vertical Migration
This image shows some data that we collected with a sonar system this past summer. Depth is shown on the Y-axis with time shown on the X-axis. Time is expressed in Year-Day format. Day 196 is the 196th day of the year. Fractions of a day are also used. Noon on day 196 would be 196.50.
The colors indicate the strengths of the echoes from particles in the water. Blues and greens are the lowest echoes and reds are higher. Grey and black are the strongest echoes.
The survey was conducted from Cape Cod Bay (shallow water on left) to Massachusetts Bay (deep water on right). On the way we passed over a pinnacle.
We can't say for certain that all of the echoes were due to plankton but in the areas where the arrows are located, we can see that two of the layers appear to have moved upwards towards the surface. This is an example of verticle migration and those layers were almost certainly zooplankton.
Diel Vertical Migration
Almost all holoplanktonic groups undertake DVM but DVM isn't always a migration from near the bottom to the surface. Each species will have a preferred day and night depth range and these will change with the developmental stage of the organism, sex and time of year.
One of the consequences of DVM is that the composition of the plankton at a particular depth will change from day to night. An oceanographer collecting samples at the surface in the day will likely encounter different species when he or she returns to sample at night.
Patterns of DVM
There are three general patterns of DVM: nocturnal migration, twilight migration and reverse migration. These diagrams show the general pattern described by each.
Nocturnal DVM
The most common is called Nocturnal DVM.
It describes a pattern where animals migrate upwards around dusk and remain at a shallower depth during the night.
Before dawn they begin to migrate downwards and remain at depth during the day.
The diagram shown here is a gross simplification. The actual pattern may be more of a sinusoidal curve and the depth of the animals during the day and night will vary slightly.
Twilight DVM
This pattern is less common. It involves a migration to the surface around dusk; however the animals don't remain at shallower depth during the entire night.
In the middle of the night some of the animals move deeper. This midnight descent is thought to be associated with a decline in activity among the zooplankton. Remember that they are denser than water and when their swimming activity diminishes, they will tend to sink. Later in the evening they move up to regain their shallow position and migrate down to spend the day at depth.
Reverse DVM
This pattern is the opposite of the nocturnal DVM. Animals that undertake reverse DVM remain up at a shallower depth during the day and migrate down deeper at night.
This strategy may be effective for animals that are predators on prey that undertake a nocturnal migration.
DVM ...
The distance travelled by individual zooplankton during their DVM varies among taxa but it is impressive.
Even small animals such as copepods can undertake migrations of several hundred meters. Remember that they do this twice. Larger taxa may migrate over 800 m.
Remember our comparison of the jet and the copepod. We compared their velocities in body lengths. These distances are particularly impressive in terms of body lengths. For a 1 mm long copepod to migrate 100 m is 1e2m/1e-3m or 10,000 body lengths!
The migration velocities are also impressive. Small animals such as copepods travel from 10-170 m/hr while larger animals such as euphausiids can move at 100-200m/hr.
Example Vertical Migration
Let's return to that echogram from Cape Cod Bay to Massachusetts Bay. We can calculate the migration velocity of one of the layers.
First we need to know how far they migrated. In this case it was about 38m. Second we need to know how long it took them. They completed the migration in about 68 min.
Therefore, their migration velocity was 38m/68min = 0.56m/min. That's about 33.5 m/hr. That puts their velocity in the range for smaller taxa so these were probably not large animals.
Deep Scattering Layer
The deep scattering layer was long a source of questions for oceanographers. With the advent of sonars, scientists began to see layers in the ocean. These layers appeared to be hard targets, almost a false bottom.
By day there was more than one layer at depth and up to five layers was not uncommon.
At first, it was thougth that these layers were physical but then it became apparent that their depth changed with a diel periodicity. At night they often migrated up towards a shallower depth where they merged.That suggested biology.
Sound Scattering Layers
Your textbook contains a good echogram of the deep scattering layer and its vertical migration. This image isn't of the deep scattering layer. It is an echogram from Wilkinson Basin in the Gulf of Maine.
The purpose of this image is to show you that there are a number of different layers in this region.
You can see several layers near the surface, a strong intermediate layer and a deep layer. Each layer is probably associated with a particular layer of zooplankton or fish.
SONAR 101
Since we've already looked at several echograms and we've been talking about sonars, a short primer on SONAR is in order.
SONAR stands for SOund Navigation And Ranging. It was originally developed to locate underwater objects such as submarines.
Why use SONAR? Well, sound travels much further, faster, and more efficiently in water than in air. The velocity of sound in water is about 1500 m/sec (about 5 times faster than in air). When sound strikes an underwater object, some portion of it will be reflected back.
The strength of the echo depends upon the size of the object in relation to the frequency of sound. Higher frequencies are required to detect smaller objects.
Two important parameters are the density contrast of the target in relation to the water and the velocity contrast of the target in relation to the water. The density contrast is the ratio of the density of the object to the density of the surrounding water. As this departs from 1.0, the echo strength will increase. That's why fish, which often have a swim bladder filled with gas, have a strong echo. The density of the gas is much lower than the water
SONAR 101 ...
This vehicle is called BIOMAPER II. It's a towed system that we used to map plankton and other particles in the water. In the yellow box you can see 5 upward-looking sonars. These are each of a different frequency. The biggest transducer is 43 kHz and the smallest is 1000 kHz.
One of the problems with sonars is that we can't easily determine what is producing the echoes that they detect. That's why we use multiple frequencies and other information. For example, on the front of BIOMAPER is a video camera system that photographs plankton to help us identify the content of different scattering layers.
Deep Scattering Layer
Returning now to the deep scattering layer. Remember that animals with a strong density contrast (and/or sound velocity contrast) will have strong echoes.
It turned out that the DSL was dominated by euphausiids, shrimp and small fishes called myctophids. Euphausiids and shrimp have a fairly hard exoskeleton made of chitin.That contributes to their strong echoes. Myctophids have a swim bladder that gives them a strong echo.
On this image of a myctophid, you can see a line of photophores along the belly. These give off a pale blue light that may serve to countershade them. When viewed from below, this pale light may merge with the dim downwelling light from above. Alternatively, flashes of light from the photophores may serve to confuse predators.
The deep scattering layer may also contain heteropods, pteropods and copepods.
What is the Cue for Migration
Changes in ambient light appear to be the most likely cue to synchronize migration. Animals follow isolumes - lines of constant light intensity. As the sun rises, the isolumes will deepen and to follow them, animals have to migrate down. At dusk, they become shallower.
To support this hypothesis, changes in animal distributions have been observed on sunny and cloudy days. Similarly, during eclipses, animals begin to migrate in the same directions that they would as the sun set.
In the next lecture we'll look why animals migrate and move on to plankton patchiness.