Macroinvertebrate monitoring involves identifying and counting macroinvertebrates. The purpose of macroinvertebrate monitoring is to quickly assess both water quality and habitat. The abundance and diversity of macroinvertebrates found is an indication of overall stream quality. Macroinvertebrates are organisms that lack a backbone and can be seen with the naked eye, including aquatic insects, crustaceans, worms, and mollusks. The organisms that are being sampled for are benthic macroinvertebrates meaning that they live in the substrate, or bottom of a waterbody. Macros live in various stream habitats and derive their oxygen from water. They are used as indicators of stream quality. These organisms are impacted by all the stresses that occur in a stream environment, both manmade and naturally occurring.

Aquatic macroinvertebrates are good indicators of stream quality because:

• They are affected by the physical, chemical and biological conditions of the stream.
• They are not very mobile. They can’t escape pollution and, therefore, will show
• effects of short- and long-term pollution events.
• They are relatively long lived – the life cycles of some sensitive macroinvertebrates
• range from one to several years.
• They are an important part of the food web, representing a broad range of trophic
• levels.
• They are abundant in most streams. Some 1st and 2nd order streams may lack fish,
• but they generally have macroinvertebrates.
• They are a food source for many recreationally and commercially important fish.
• They are relatively easy to collect, view, and identify with inexpensive materials.

Macroinvertebrates are present during all kinds of stream conditions from drought to floods. Macroinvertebrates are adaptable to extremes of water flow. Some may burrow when it is raining and flow increases. However, heavy rain in areas with a high percentage of impervious surface (most urban areas) can cause flash floods and carry macroinvertebrates downstream.

Populations of macroinvertebrates may differ in north and south Georgia. For example, since the Adopt-A-Stream macroinvertebrate index is based on dissolved oxygen, the “sensitive” organisms that require a lot of oxygen, such as the stonefly, may not be found in warm, slow-moving streams in south Georgia. That does not mean that the stream has bad water quality or habitat, just that streams in north and south Georgia support different populations of macros. If you are monitoring in south or coastal Georgia, it is important for you to conduct monitoring each season for several years. Doing this will help you recognize biological trends in your stream so that you can determine which changes are natural and which may be induced by human impact.

Populations of macroinvertebrates may vary from headwater streams to the river mouth. For more information, please review “The River Continuum Concept” in the Visual Stream Survey manual.

Seasonal cycles can also affect the number and kinds of macroinvertebrates collected. Organisms such as immature stoneflies and mayflies will gain weight and size primarily during the fall and winter. During the spring and summer they may reach maturity and begin to metamorphose into their adult (non-aquatic) stage. Therefore, the presence of
aquatic macroinvertebrates will tend to be more evident during winter and spring just before metamorphosis. After adults emerge, females lay eggs near or in the water. Soon after, the larvae and nymphs hatch and begin to grow, feeding on leaf litter, detritus and other organic matter that might be present. For more information on  acroinvertebrates and their life cycles, please turn to “Background on Aquatic Insects” in the Appendix. If conditions are unsafe for any reason, including high water or slippery rocks, DO NOTSAMPLE. 

Why Monitor for Macroinvertebrates?

The basic principle behind the study of macroinvertebrates is that some species are more sensitive to pollution than others.