The invertebrates are removed from stones and organic debris, identified as belonging to a particular taxonomic group, and counted. Once counted, invertebrates can be compared to samples taken in the same stream but at different times, such as before and after a suspected pollutant has entered a stream. They also can be compared to samples taken from two or more streams at approximately the same time, such as from a stream suspected of receiving a pollutant and a nearby undisturbed reference stream. Use of aquatic dip net to collect benthic invertebrates in Coastal Plains streams. Flow is from right to left of photograph.
Flow is from left to right of photograph.
This index assumes that streams showing high EPT richness are less likely to be polluted than are streams showing relatively low EPT richness in the same region. Importance of Reference Streams Because streams differ by many natural factors besides nonpoint source pollution, it is critical to establish a baseline or reference condition upon which differences or changes in water quality resulting from pollution can be judged. Total invertebrate and EPT richness often may be higher in upland streams of the Blue Ridge or Southwest Appalachians than in lowland Piedmont or Coastal Plains streams. This demonstrates that stream invertebrates can vary geographically according to differences in natural watershed attributes, and measures such as the EPT index are useful in recording such natural variation. Thus, some measure of reference conditions that incorporates natural variation must be established if biomonitoring is useful in pinpointing changes resulting from nonpoint source impacts in streams.