Periphyton is the slimy material attached to the surface of rocks but also other substrata (e.g. wood, sediment, etc.) in streams and in the littoral zone of lakes. It is composed primarily of algae, but can be dominated by bacteria and fungi if there is a source of dissolved organic matter and can include invertebrates (e.g. protozoans, insects, etc.). The periphyton mat is usually rather thin, but very slippery in fast-flowing streams with moderate enrichment. In that case, the community is usually dominated by diatoms, such as Navicula, Gomphonema, Diatoma, Cymbella, Cocconeis and Synedra. Most of these genera were not mentioned in the discussion on plankton because they are peculiar to the periphyton.