Understanding the mixing generated by zooplankton

Zooplankton during the day

📥  Measurement, Vobster, Zooplankton

Daphnia. Source: wikipedia.org

Coepods. Source: http://www.britannica.com/animal/copepod

Following the previous post about how to collect the zoo samples, we measured the zooplankton concentration during the day to verify that Daphnia is actually hiding during the day from predators (fishes). To do that, we collected samples during June more precisely on 25/06 @ 2.40pm and 5.30pm and on 30/06 @ 2.30pm.

An example of small Cladocera. Source: wimpertier (Flickr)

Copepod nauplius or nauplii. It's a baby copepods. Source: wikipedia

After counting the beasts under the microscope, I grouped the zooplankton in four different groups:

  • Daphnia: I've already talked about this organism in my previous post. It has a prominent tail with swimming antennae (like arms). It is very small, less than the thickness of a penny. It is also known as “water flea” because its swimming style resembles the movements of fleas.
  • Copepods: they have an elongated body with long antennae whose size depends on the life stage. They are clearly distinguishable from the others.
  • Copepods nauplii: they are simply baby copepods, in the larvae stage.
  • Small Cladocera: Cladocera is the family Daphnia belongs to. I grouped the very tiny organisms of this set because they differ from Daphnia.

The images below show the result:

Daphnia (square in the plot) is always the most abundant in our samples and they always avoid the surface where the fishes swim and eat. The maximum concentration ranges between 9m and 15m depending on the light availability. In fact these organisms are very sensible and their position in the lake depends on the depth at which the light penetrates. In the early afternoon of 25th light arrived up to 13m and this justifies the maximum of Daphnia between 15m and 18m.

On the same day the abundance changed probably because the zooplankton migrated horizontally. However the trend is the same with the maximum between 9m and 12m due to change of light.

Last day shows instead an increase of baby copepods with slightly less organisms. But our Daphnia are always there!

Stay tuned for the concentration before and after sunset when Daphnia migrates to the surface.

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