What are Benthic Macro Invertebrates?
by Katie Wolpert
In 2018 alone, over 600 of your children will conduct 66 chemical, physicalor biological samplings of West Virginia streams under our watch. They will analyze multiple indicators of stream health and water quality using various methods of analysis. The Biological analysis is largely carried out by assessing the range of benthic macro invertebrates present in the sample area.
But. What ARE benthic macro invertebrates anyhow? What kind of a word is that? And why are they important to our streams?
Our long-time course director Jackie Lambert says that in simple terms, “they are critters that are small, but big enough to see with the naked eye, who live under rocks and have no backbone.”
Benthic = bottom dwelling
Macro = small but visible with the eyes
Invertebrate = no backbone
They are often referred to as ‘benthics’ for short.
Many of these benthics are the nymph or larval stage of insects like mayflies, dragonflies, horseflies, stoneflies, caddisflies, and damselflies.
In the world of benthics, tolerance is a less noble characteristic than in the human world. And while “there are no ‘bad’ benthics,” Jackie says, “some are more tolerant of pollution than others.” Those are pretty obvious by their names: leaches or sewage worms, for example. But you can find those sewage worms in a nice healthy stream too alongside dozens of species of less-tolerant organisms.
We survey benthics in our stream studies because they are important indicators of water quality in a stream over time. Many of these nymphs spend several years of their life cycle in the water. “So if you are finding big stoneflies, for example, they can tell you the water quality has been good for a long time,” according to Jackie.
Now you may have skipped rocks across many a stream and don’t have any idea what a stone fly nymph looks like. They’re really good at staying hidden so you’ve got to know how to look for them. Flipping over a rock and looking for critters clinging to the bottom of it is a good low-impact way to get going.
We often use more thorough methods though for our stream studies so that we are able to capture a more complete picture of the stream sample area. One sampling method is with a “kick net” which is placed at the bottom of the study area and weighted to the bottom of the stream with rocks. Two people hold the net at an angle so that water is flowing onto it while others scrub rocks and kick up sediment. The small organisms are caught up in the water flow and then trapped by the net. We pull the net to the bank and take our sample.
A close look at these hidden creatures reveals features that would be appropriate in a science-fiction series with enormous pincer mouths (see Hellgrammite), spiny bodies and armoured-looking body segments. But if you take another moment to observe these secretive organisms, you are likely to notice a profound beauty in the intricate workings of their bodies, the delicate architecture of their casings, and their perfect adaptations to surviving under rocks in cold water surrounded by hungry predators.
(top photo: caddisfly larva construct intricate protective cases for themselves out of pebbles, leaves, sticks or any other material they find on the bottom of their stream)