Periphyton and Biofilm
Algae, bacteria, fungi, protozoa, and the breakdown products of dying cells form layers on submerged surfaces, including bottom sediment, rocks, submerged leaves and branches, and macrophtes. The term periphyton refers to a layer consisting mainly of algae, but the entire assemblage of layers is often known as biofilm. Periphyton is an important food source in shallow, stony rivers with adequate light penetration. Heterotrophic organisms, including larger invertebrates such as snails and insects, scrape the biofilm from surfaces, while some larger animals, such as fish, also feed on biofilm. Biofilm can be important in absorbing or breaking down chemical contaminants as well. Seasonal changes in the abundance of periphyton reflect fluctuations in river discharge, as layers of algal cells build up in times of low or decreasing flow, and wash away during flood periods