[citation needed] To assign a biotic index value to a specific water site, the tester first collects macro invertebrates from portions of the sample area of the stream, river or lake, and separates them into groups of similar-looking organisms.
Aquatic macro invertebrates have some general characteristics that make them very useful to assess stream health: Macro invertebrates limited mobility and extended presence in the water means that they are exposed on a continuous basis to water quality in that stream or river.
Not all the macro invertebrates found in samples are listed on the biotic index scoring sheets.
Many are able to collect air from the atmosphere and hold a bubble alongside their body to use like a scuba diver uses a tank of oxygen.
The biotic index works by assigning different levels of tolerance to pollution to the different types of organisms.