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Just like invertebrate communities, the algal community you find at a particular site is shaped by a combination of physical habitat and water quality factors. There are many algal taxa that are common in “natural” streams, and there are many taxa that can thrive in streams polluted by a wide range of contaminants. Natural, bush covered, stony bottom streams often support thin green or brown films made up of whole communities of unicellular diatoms, cyanobacteria (blue-green algae), and red algae. Communities dominated by these sorts of algae are therefore an indication of a normal, healthy stream.