Zoomix

Understanding the mixing generated by zooplankton

Pregnant zooplankton

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📥  Measurement, Vobster, Zooplankton

It has been a few months since my last blog post here. Honestly I was super busy with prepping everything for the new field season going to conferences. And now here we are!

This year we wanted to start our measurements earlier than last year in Vobster Quay. There were two main questions we needed to answer first:

Is the lake stratified?

A lake is stratified when the temperature decreases with the lake depth and when the difference between the surface and the bottom temperature is at least 5C (this's our guideline). Vobster Quay is a particular site because even in winter there always is a difference between the surface and the bottom temperature.

However we needed to check if this difference was big enough for our study. So at the very end of April, despite the very bad weather with snow and hail at the same time, we went to Vobster.

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Snowing near the jetty

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Hail on our van

With our CTD probe (Conductivity Temperature Depth) we got this temperature profile.

CTD20160426_Cast 1

Here we have a first layer above 2m with a constant temperature (~15 C): this is the mixed layer. The temperature decreases with depth as expected, but not so rapidly as during summer. The bottom measures ~8C and so we can say that the lake is stratified!

In summer the stratification and so the temperature difference will be stronger with a more stable stratification but for now this condition is good for our measurements.

Are there any zooplankton in the lake?

That's the second big question. Zooplankton, in particular Daphnia, can survive to winter cold temperatures but population usually shrinks. Population peaks again in spring when the water temperature rises to roughly 10C. But organisms usually require days to reproduce and grow and Daphnia's life cycle may vary between 10 and 40 days.

So if we don't have Daphnia in the lake or the concentration is too small, we can't carry on with our measurements. We need a big zooplankton abundance to possibly generate currents and so turbulence and mixing in the water due to swimming organisms.

So let's check out our zoo samples:

Zooplankton in the vial with ethanol

Zooplankton in the vial with ethanol

Zooplankton on the filter

Zooplankton on the filter

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Zoom of the filter

Our samples contain mainly Copepods and Daphnia. Copepods are the orange bodies that are clearly visible on the filter and in the vial. If you click on the last image to zoom in, you will see some black dots surrounded by gelatinous-like and sandy bodies. Those are Daphnia!

Now to count and distinguish the different zoo-families we have to use the microscope. And that's how many daphnia we have in Vobster every 5m:

Daphnia concentration in Vobster

Daphnia concentration in Vobster

We have unfortunately a low abundance (for now) and Daphnia still need to develop. All organisms contain eggs in their abdomen which means we need to wait until female organisms release juvenile zooplankton.

Stay tuned for next updates!

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