Biotic Index

Why are we concerned?

• Aquatic macroinvertebrates are tiny animals without backbones. Their presence or absence can reflect a stream’s general condition.
• Certain macroinvertebrates respond differently to a stream's physical, chemical, and biological conditions.
• Aquatic macroinvertebrates are relatively immobile, so they can’t escape either short- or long-term pollution exposure.  This is important when assessing long-term pollution events within the stream.

 

Left to right: Stonefly larvae, Dragonfly larvae, and Isopod or sowbug Download Left to right: Stonefly larvae, Dragonfly larvae, and Isopod or sowbug

 

 

The background science

From the crayfish burrowing in the stream bed to the tiny aquatic insects skirting the water’s surface, streams and rivers swarm with life.  The inhabitants of this living place are affected by poor water quality, just like humans are affected by an unhealthy environment.

However, scientists have found that not all aquatic organisms react similarly to poor water quality.   Some species are pollutant-tolerant, while some are very pollutant-sensitive.  From this knowledge, a scale was developed to determine water quality based on the types of life found in the water.  For example, streams with primarily pollutant-tolerant organisms generally have poorer water quality than those streams with many pollutant-sensitive animals.  This is because poor-quality streams gradually lose pollutant-sensitive animals until only the pollutant-tolerant species are left.  A healthy stream will have many different organisms, both pollutant-tolerant and sensitive to pollution.

WAV volunteers look for the presence and absence of these specific macroinvertebrates.  Why these?  They are found throughout Wisconsin, their mobility is limited, they live all or part of their life in the water, and they depend on the water for the oxygen they need, as they do not breathe oxygen from the air.

Although relatively accurate in assessing stream conditions, the biotic index does have its limitations.  While it can indicate a problem, it cannot specify what that problem might be.  For example, manure, sewage, fertilizers, sediment, and organic materials negatively impact water quality.  To pinpoint these possible pollutant sources, monitoring for other parameters such as habitat, dissolved oxygen, and temperature needs to be done.  The biotic index helps identify long-term pollution problems since these organisms carry out a portion or all of their life cycles in streams.  Other parameters monitored in the WAV program (except habitat) only indicate the water quality conditions at the testing time.

 

 To learn more about the biotic index, read our "Biotic Index Method Factsheet Links to an external site.".

 

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