Densities of species or cell types (a less precise classification) can be determined by counting through a microscope. If densities are low you can concentrate the cells by centrifuging or simply
and then removing a measured subsample to a measuring cylinder; narrower cylinders are better so that the final sample is more concentrated. At 20
◦
C cells will settle at a rate of two hours per 1 cm depth of water in the cylinder and a sedimentation time of 48 h is usually recommended for a standard 100-ml cylinder. After centrifuging or sedimentation you siphon off the top 90% of water, shake the remainder and take a further subsample for counting. Cells can be counted, but it may be better to count colonies or filaments of some species. The two simple methods are use of the haemocytometer and use of a counting chamber. The former is more usually used to count red blood cells. It holds a precise volume of liquid in which you can count the numbers of each species. Phytoplankton keys are widely available, but identification of algal cells is a skilled job. Several counts should be made for each sample and averaged to get a density. A Lund counting chamber comprises an ordinary microscope slide with two long, thin pieces of glass or another rigid material glued to the longer