Periphyton algae (Periphyton sp.) floating on the water surface. Periphyton is a complex matrix of algae, cyanobacteria (blue-green algae), heterotrophic microbes, and detritus that occurs in most aquatic ecosystems. It is a food source for invertebrates, tadpoles and some fish species. It is also an indicator of water quality. Photographed in the Everglades National Park, Florida, USA.