Learn: Stream Monitoring

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Benthic Macroinvertebrates

by Piper Wilson

Waterbenthic macroinvertebrate

macroinvertebrates, or benthos, are animals that lack a backbone and are visible to the naked eye, larger than half of a millimeter. These animals mostly live on rocks, logs, sediment, debris and aquatic plants during their life. The benthos include crustaceans including crayfish, mollusks such as clams and snails, aquatic worms and the immature forms of aquatic insects such as stonefly and mayfly nymphs.

These animals are widespread and can live on all surface types, even on manmade objects. They can be found in hot springs, small ponds and large lakes. Though most benthic species can be found throughout the year, the largest numbers transpire in the spring right before the reproductive period. During the colder months, species may burrow deep within the mud or linger inactively on rock surfaces.

Benthos cannot move far so they are less capable of escaping the effects of sediment and other pollutants that deteriorate water quality.

Meaning they are affected by local changes in water quality; thus, benthos can give us reliable information on stream and lake water quality. Their long life cycles allow studies to determine any detection of past pollution events, such as pesticide spills and illegal dumping. Some benthos are more able to tolerate higher amounts of pollution than other benthos. The wide range of responses, in the benthos, to stressors such as organic pollutants, sediments, and toxicants makes it possible to determine water quality.

The way benthic macroinvertebrate can be tested for water quality is by sampling and analyzing these stream water’s inhabitants. The majority of benthos is found in the riffles of streams. This riffle range can be between bedrock to cobbles to boulders. The flow of the fast flowing riffle allows for oxygen and food particles, is the most commonly sampled stream habit. Various nets are then used to capture the benthos. These nets are secured in the water against the riffle while benthos float down stream into the net. After a set amount of time, these nets are carried off stream and the macroinvertebrate are then counted and studied.

Benthic communities can be used to monitor stream quality conditions over a broad area. Ecologists that evaluate water quality, and who use the benthos repeatedly, consider certain characteristics of a benthic sample to be important indicators of stream, river or lake quality. The first characteristic is taxa richness, which means the greater the number of different types of animals indicates a better water quality. Another factor is pollution tolerance, which means that many types of benthos are sensitive to pollutants such as metals and organic wastes.

Some benthos are found more often, and in larger amounts, in waters that are generally clean, or unpolluted by organic wastes. Without too much organic matter, the waters usually have lots of oxygen for the benthos. This use as an "indicator" of water quality has been occurring for many years.

Extremely polluted waters, receiving high inputs of organic matter or nutrient enrichment, tend to have a low diversity of macroinvertebrates. Only those species capable of surviving and thriving under low dissolved oxygen or highly turbid conditions inhabit polluted waters.