Various methods for the estimation of populations of algae and other small freshwater organisms are described. A method of counting is described in detail. It is basically that of Utermöhl and uses an inverted microscope.
If the organisms are randomly distributed, a single count is sufficient to obtain an estimate of their abundance and confidence limits for this estimate, even if pipetting, dilution or concentration are involved.
The errors in the actual counting and in converting colony counts to cell numbers are considered and found to be small relative to the random sampling error.
Data are also given for a variant of Utermöhl's method using a normal microscope and for a method of using a haemocytometer for the larger plankton algae.