Aquatic invertebrates have different pollution tolerances.
WAV citizen monitors use a biotic index to help assess water quality based on the macroinvertebrates present in their local streams. Streams are rated as having poor, fair, good or excellent water quality with the biotic index.
Aquatic macroinvertebrates have some general characteristics that make them very useful for us to study to assess stream health.
1. They are abundant and found in water bodies throughout the world.
(Did you ever turn over a medium sized rock in a stream and see how many things were hiding under there?)
2. They are not extremely mobile (as compared to other aquatic organisms such as fish).
3. They carry out part or all of their life cycle within the stream or river.
Because there are a lot of them in the water, they are fairly easy for us to sample. Their 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. In particular, many of these organisms breath oxygen that is in the water. (Go to the dissolved oxygen web page to learn what effects levels of oxygen in the water). Monitors may notice that not all the macroinvertebrates that are listed on their identification key are found on the biotic index scoring sheet. This is because some do not rely on oxygen within the water for survival. 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.
For those macroinvertebrates that do rely on dissolved oxygen, some can only live in water that has a lot of oxygen. Others can live in water that doesn't have much oxygen dissolved in it at all. Generally, we assume that the more pollution there is in water, the less oxygen. The biotic index works by assigning different levels of tolerance to pollution to the different kinds of macroinvertebrates. WAV's Citizen Monitoring biotic index has macroinvertebrates separated into four categories: tolerant, semi-tolerant, semi-sensitive, and sensitive to pollution. Learn more about the macroinvertebrates that are included in the biotic index and their pollution tolerances.
To assign a biotic index value to a sampled site, citizens first collect macroinvertebrates from the stream and separate them into groups of similar-looking organisms. Then they use an identification key to help determine which organisms they have in their sample. Then they circle those organisms on their recording form or Citizen Monitoring Biotic Index poster (18" X 24") and follow steps on the form/poster to determine a water quality score for the site. (The identification key, recording form and poster are all available free to Wisconsin residents. Use the order form to place your order.) The biotic index value for the stream site depends on how many types of organisms are present in a sample, and the tolerance category of those organisms.
Stream health ratings are assigned based on biotic index values and range from poor to good.
Precautionary note to monitors: There are a few macroinvertebrates that you may encounter that have a stinging bite (most do not). Check your identification key to find backswimmers, and giant and creeping water bugs, as these three, if handled in a way they do not like, may sting you.
How healthy is your stream?
|Biotic Index Value||3.6 and greater||2.6 to 3.5||2.1 to 2.5||1.0 to 2.0|
Data Sheet (710 K pdf)
Methods (1.24 MB pdf)