Clean water is essential to life itself. Adverse changes to the water quality of one stream can impact all the bodies of water downstream – rivers, lakes, or even the ocean. When water quality degrades, changes to plant, invertebrate, and fish communities may occur and affect the entire food chain.
Through water quality monitoring, communities can assess the health of their streams and rivers over time. Once baseline data on the health of a stream is collected, subsequent monitoring can help identify when and where pollution incidents occur. Water quality can be assessed using chemical sampling or biological sampling.
Biological water quality monitoring involves collecting samples of aquatic macroinvertebrates. Aquatic macroinvertebrates live in water for at least part of their life cycle. They include insects, worms, snails, mussels, leeches, and crayfish. For the purpose of assessing water quality, sampling is focused on benthic macroinvertebrates, those organisms that live at the stream bottom.
Why is it useful to sample macroinvertebrate life in a stream when monitoring water quality? Because these organisms are easy to collect and identify, and tend to stay in one area unless environmental conditions change. Some macroinvertebrates are highly sensitive to pollution, while others tolerate it.
Aquatic Macroinvertebrates – Highly Sensitive to Pollution
Certain macroinvertebrates serve as bioindicators of good water quality. These organisms tend to require high dissolved oxygen levels. When present in large numbers, these macroinvertebrates suggest the stream is in good condition. If these organisms were once abundant, but subsequent sampling shows a decline in numbers, it may indicate that a pollution incident occurred. These organisms include:
- mayfles (nymphs)
- riffle beetles (adults)
- caddisflies (larvae)
- stoneflies (nymphs)
- water pennies
- gilled snails
- hellgrammites (dobsonfly larvae)
Aquatic Macroinvertebrates – Somewhat Tolerant of Pollution
This group of macroinvertebrates tends to tolerate some degradation of water quality. Their abundance and diversity indicates a stream is in fair to good condition. These macroinvertebrates include:
- alderflies (larvae)
- dragonflies and damselflies (nymphs)
- whirligig beetles (larvae)
- riffle beetles (larvae)
- fishflies (larvae)
Aquatic Macroinvertebrates – Pollution Tolerant
Certain macroinvertebrates thrive in poor quality water. An abundance of these organisms suggests environmental conditions in a stream have deteriorated. Some of these invertebrates use "snorkels" to access oxygen at the water's surface, and are less dependent on dissolved oxygen to breathe. These macroinvertebrates include:
- black flies (larvae)
- midge flies (larvae)
- lunged snails
- aquatic worms