Watershed Facts / Benthic Macro-Invertebrates

Benthic Macroinvertebrates

Have you ever walked down to a river on a hot summer day, and wondered if the river is clean enough to swim in?  Most likely, you don’t carry a water testing kit around in your back pocket. However, there is another option - a natural water-testing method! In all rivers and streams, there are critters called benthic macroinvertebrates that are affected to differing degrees by water pollution. Some are extremely intolerant to pollution while others are not sensitive at all and can live in very dirty waters.

Perlidae Golden Stone
Golden stonefly from the family Perlidae.

So, what exactly what does the name "benthic macroinvertebrate" mean?  Well, benthic = bottom, macro = large enough to see with the naked eye, and invertebrate = without a backbone. So basically, benthic macroinvertebrates are visible critters that live on the bottom of the riverbed, and lack a backbone. They include aquatic insects, mussels, snails, crustaceans, leeches, worms, and more.

They are easy (and FUN!) to look for.  You can use the guide below to learn about each macro-invertebrate and how to identify them!  (click on group name)

Group 1 – very sensitive to pollution:

These critters require an environment high in dissolved oxygen (>5ppm), cold - cool waters, a neutral PH (6.5 – 7.5), and a low amount of nutrients (nitrogen and phosphorous from fertilizers, animal waste, etc.). 

Group 2 – somewhat sensitive to pollution:

These critters can tolerate some amount of pollution.  With some species in this group, a large number of individuals present will indicate better water quality, while with other species, a large number of individuals present will indicate poorer water quality.  Please read the individual description of each species to learn which species in higher numbers indicate better or worse water quality.

Group 3 – pollution tolerant/not sensitive to pollution:

These critters can live in polluted waters without problems.  They can survive in waters with low dissolved oxygen, high or low pH, and warmer temperatures.


Next time you’re out on the river look for these critters and you will get a better idea of how clean the river is!  Here are a few key points to remember:

  • Finding a high number of group one benthics is a very good sign
  • Group 2 benthics are still good, but for some, if you find them in high numbers it means the water is polluted.  However, other group 2 benthics in high numbers may indicate better water quality.  Please see individual descriptions.
  • If you find only a few group 3 benthics, the water quality should not be of too much concern.  When you find large quantities of these critters in one particular area however, this indicates more polluted water.
  • Always be respectful of these small animals when you go out to observe and collect them.  Do not remove them from their habitat or smoosh them!  They are a very important piece of the ecosystem and act as food for fish, birds, and other animals.


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