How do population changes of aquatic invertebrates indicate water quality?

In freshwater pollutant testing, various species of Daphnia are used.

(Daphnia magna and Daphnia pulex being most common). Specimens are added to the sample, and then the amount of time until death is one of the many ways they are used to identify the presence of and concentrations of various chemicals. These results are compared with results of similar tests in a lab environment.