You can check this : "Methods for the Estimation of Zooplankton Abundance" RICCARDO DE BERNARDI (1984) https://downing.public.iastate.edu/tier%202/jadpdfs/1984%20IBP%2017%20Chapter%203%20Zooplankton.pdf
The water samples are collect from natural environments such as sea, pond, river etc. The samples are collect from both surface and middle of the water column. The collected water samples are centrifuge at 5000 rpm for 10 minutes. After centrifuge, a little amount of sample is pipette out on a microscopic slide and kept in microscope. The cells are pipette out on a microscopic slide and observed under 40x magnifications through the light microscope. Then the organism is transfer in to the culture medium by using micro pipette.