Aquatic Macro-Invertebrates

Aquatic macro-invertebrates are an informative group of organisms that can be used to easily determine the health of a body of water. These creatures cannot easily escape pollution (as fish can) so are the group of organisms most affected by pollution entering the water. There are several methods of determining the health of the water by using these organisms.

One of the most common methods is to use a Pollution Tolerance Index (PTI), also called a Water Quality Assessment, that was developed by the Isaac Walton League and the Ohio Department of Natural Resources. This index places taxa (a group of similar organisms) into one of three categories: Pollution Intolerant, Wide Range of Tolerance, and Very Tolerant. Intolerant organisms cannot withstand pollution and have a higher index value than the other two. Organisms with a Wide Range of Tolerance can withstand some pollution and have a value between Pollution Intolerant and Very Tolerant. Very Tolerant organisns can withstand large amounts of pollution and have the lowest index value, large numbers of tolerant organisms indicate very poor water quality. Table 1 below shows the pollution tolerance of most of the organisms that are found here in the Mississippi Delta.

Table 1. Benthic macro-invertebrates found in the Mississippi Delta.

This link shows how the PTI is calculated. Notice that the PTI may be misleading. As previously stated, large numbers of Very Tolerant organisms indicate poor water quality, but will also increase the PTI value, thus leading to a false conclusion. It therefore makes sense to be able to calculate a pollution index with more accurate results. We can do this by using the Hilsenhoff Family Level Biotic Index. Click this link to find out more about the Hilsenhoff Family Level Biotic Index.