Phytoplankton are not randomly distributed across a water body, due to variations in flow and nutrients. So samples should be taken at various locations; e.g. at various points across a river and near and far from the shore of a lake.
The algal cells will decay rapidly and should be preserved immediately after collection. Various fixatives are used, but the commonest is Lugol’s iodine solution. A solution of 20 g potassium iodide in 200 ml distilled water is made and then saturated by adding 20 ml iodine. This is then acidified with 20 ml glacial acetic acid. The solution should be kept in a dark bottle to preserve the iodine. You should add 1–2 ml Lugol’s iodine to every 100 ml of the sample (remember to allow for this extra volume when calculating cell densities).