Last week we put down on the bottom of Vobster Quay the device shown in the figure above. The device is called ADCP and is an acoustic device that uses the same principles of a sonar or a fish-finder but it is actually much more advanced. It was firmly fastened to the aluminium frame and lowered with the three blue ropes from our boat.
But what does it really do?
The ADCP sends sound waves into water through the white circles on its head (see Figure at the beginning of the post). When the wave runs into a small particles like particulate matters or zooplankton (exactly like ours Daphnia), the wave is reflected back to the device. And using the information of the returned wave, the ADCP is able to measure the velocity of the water as well as give a description of the zooplankton swimming or floating in the lake.
Here I reported the first results we got last day after downloading the data from our jewel. The image above shows what happened between the night of 24th June and the beginning of 25th June. The coloured vertical bar shows the Amplitude, which is a characteristic parameter of the returned wave, and can be correlated to the zooplankton concentration in the lake.
Basically the higher the Amplitude, the higher the concentration.
In the image there is a constant layer of zooplankton floating between 12 and 18m during the day (black oval on the left). This is a common behaviour because zooplankton wants to hide and not be eaten by fishes. However at sunset, around 9.30PM, the zooplankton begins the migration to the surface to feed and since it is dark, fishes cannot see them.
Zooplankton stays on the surface until 3AM (black oval on the centre), when sun starts to rise and it returns back to the same layer (black oval on the right), repeating the same pattern every day.
I think it's pretty impressive being able to track their movement with time!