Missing Macroinvertebrate Discussion Questions

Background:

Aquatic macroinvertebrates (insects and other organisms that live in streams and ponds) display a wide range of adaptations to different aquatic conditions. Some types of macroinvertebrates are extremely tolerant of changes in temperature, flow, food, or even the presence of pollutants, while other types are so sensitive to these changes that they may die or move to other areas.

 

Discussion Questions:

1.  What kind of information does the water quality index provide that simple observations of diversity might miss?

Diversity tells you how many types of organisms are found in an area. The Water Quality Index includes some of the attributes of the macroinvertebrates found in a stream, such as their sensitivity or tolerance to pollutants or other adverse conditions. Therefore, it provides additional information. The two measurements are closely related, however, because polluted streams often have less diverse macroinvertebrate populations. 

 

2.  Why do some types of organisms seem to be more sensitive to pollutants than others?

This question doesn't have one simple answer, but it’s an interesting opportunity to discuss and speculate on the differences in these organisms.

More tolerant organisms may be those that evolved under more diverse conditions, and therefore are now able to handle a wider range of conditions. Animals that evolved under very unique or non-varying conditions may have very narrow ranges of tolerance to change. 

Another way to look at this question is to consider the adaptations these organisms have and the type of pollutants or stressors they experience. Mayflies, caddisflies, and stoneflies that are typically found in fast moving streams probably have a high metabolic rate and require a lot of oxygen. If your class has already looked at the chemical properties of a stream, you know that as the temperature increases in a stream, the oxygen concentration declines. Therefore, just increasing the average temperature in a stream may deprive these organisms of the oxygen they need. Also, with high metabolic rates, these organisms may be more sensitive to small concentrations of toxic pollutants in a stream. 

3.  What does the water quality index tell you about the conditions in a stream that a water chemistry sample collected at the same time doesn’t tell you?

A water sample only tells you about conditions in the stream at the very moment you are sampling. Water that was at that site yesterday is already downstream, and water that will be at that site tomorrow is upstream. Therefore, your water sample is like a “snapshot” of the stream.  Because the macroinvertebrates live in a stream for periods of up to several years (typically for months at least), they are exposed to many different conditions. Because of this, the types of macroinvertebrates found on a given day reflect the conditions in the stream for the past several months or more. Therefore, macroinvertebrates can tell you about past conditions.