The periphyton community is the slimy coating that adheres to rocks and other stable substrates that comprise the stream bed. The community is made up of variable proportions of algae, fungi and bacteria as well as organic matter entrained from stream flow. The appearance of the periphyton layer can vary greatly and provides a lot of basic information about conditions within the stream. Moreover, periphyton is a fundamental component of the stream ecosystem purifying waters by absorption of metals and nutrients, and providing a significant component of the food resource to the stream food web. Correspondingly, the periphyton community is highly responsive to degradation of water quality, shifts in invertebrate consumer communities and the occurrence of floods with sufficient energy to slough algal growths. High levels of periphyton cover can have detrimental effects on stream biodiversity, trout and recreational use of waterways. Biggs & Kilroy (2000) define these nuisance growths of periphyton and provide thresholds to be avoided in resource consents and regional plans.