After dark we take a 5 litre integrated sample from each tank. We do this at night to make sure of an equal distribution of zooplankton within the mesocosms - during the day they will hide in the macrophytes from their fish predators. The water is then filtered through a 20µm mesh and then the zooplankton caught are washed into a bottle before being preserved with ethanol. Identification and counts (the most fun part) happens at a later date.
Here is another stock photo of me looking cold. This time I am much happier, this was the first week of my PhD and after half my adult life being fascinated with plankton here came the moment that I got to do some night zooplankton sampling. I was really excited! I'm not an advocate of selfies but there's not much else to take a picture of in the dark and I wanted to send this to my family because finally I had made it (in my plankton obsession mind). Fast forward half an hour I was miserable - filtering 160 litres of water outside in the dark in December is no fun. If you are lucky there is no rain, if you are really lucky there is no wind either. The end of sampling (two hours when things go well) was marked with us both standing in a semi-hypothermic stupor over the heater in the shed both silently going through the pain of our fingers thawing out. I used to stand there thinking I can't wait until the summer when it will be warm.
Sometimes when algal biomass is really high its just not possible to get the sample through a 20μm gauze so we filter through a 50μm instead. This still captures most of the zooplankton but the mesh size is too big for some of the smallest rotifers. During the winter all of the tanks would go through a 20μm gauze without any problems, below shows you the situation at the end of August - mainly 50μm filters. It demonstrates nicely how different the biomass is between the winter and summer months.
All in the name of science. Plankton will make me do the strangest things.