Phytoplankton are free floating microscopic plants that constitute the basic component of the aquatic food chain. Diatoms are members of phytoplankton. Diatoms occur in almost all aquatic environments and are abundant in most. They are sensitive to changes in water quality and many taxa have well developed optima and tolerance. Due to their short cell cycles they are able to rapidly colonize new habitats causing changes in their community representing rapid response to environmental change. Diatoms response time is a useful intermediate between immediate and infrequent physico-chemical sampling where rapid changes can result in ecological significant fluctuations. Diatoms are limited in their movement between habitats thus either do well in the environment where they are found or they do not hence many species are more likely to be found in habitats with specific physical and chemical parameters. Diatoms are being increasingly advocated for as biological indicators as they respond strongly to environmental changes and integrate responses to a range of pollutants in water bodies over different time scales and can be used to compare relative water quality changes from site to site.