Using the Benthic Macroinvertebrate Biotic Index in the Classroom

Description: A unique lab that combines free materials and the power of the internet to empower students with real- world data, which then can be used to provide them a window into the current condition of a local water body.

Problem What benthic macroinvertebrates live in a given water body? What does this tell us about the health of the stream? How can the pollution level be quantified using benthic macroinvertebrates?

Background Information Benthic macroinvertebrates are those annelids, arthropods and mollusks that inhabit the bottom of a given water body. In the study of freshwater systems, or limnology, these animals are often sampled to provide a long term picture of a given water body’s health. Reasons for this lie in three simple facts: 1) being benthic, these creatures cannot swim to escape pollution; 2) species vary widely in their tolerance to pollution; and 3) being “r” strategists they can quickly rebound once pollutants are purged from the system. So while a chemical test may provide a glimpse of what is currently in a water body, a macroinvertebrate survey can provide insight into what is able to live within the water body, and consequently, what effects pollutants have had on the health of that water body. While some invertebrates can be caught directly with nets, this lab will use a leaf pack (“mac” trap) to provide a clearer view over a longer timeframe.

Materials Vegetable mesh bags (such as what citrus or onions come in) 1 gallon bucket 10 petri dishes Tweezers or blunt forceps Magnifying glass Apron Gloves Plastic tray Copious amounts of paper towels Lesson Summary & Procedure Students are first instructed on how to set a “mac” trap (a.k.a., leaf pack)– i.e., a mesh bag loaded with rocks, sticks, and leaves, then tied off with string and thrown into a water body to set for several weeks. In the ensuing weeks, students use class notes, book and web sources to create a set of flash cards they will use to identify their quarry. Just before the actual lab, run the students through a “dry lab” using the “Wet Your Waders” CD-ROM. (available from Fisher Scientific – item # S66995); a set of practice problems such as what can be found at http://people.virginia.edu/~sos-iwla/Stream-Study/Samples/ ; and/or an interactive site like http://fergusonfoundation.org/btw-students/macro-invertebrate-identification/.

After this the students will take up their mesh bags and break them open, sorting through the detritus for macroinvertebrates (“macs”) that have taken up residence there. Students must identify AND tally these. They may use their flashcards, online sources, and their notes to aid in identification. After counting the contents of each bag, they then go online to use a MBI calculator like that found at http://www.cod.edu/people/faculty/chenpe/RiverWatch/MBI_calculator.html, and use these numbers to place on a chart. They then pool their data with that of their classmates determine the relative health of their water body. The “macs” can be released back into their water body, or placed in an established classroom aquarium. While fish may often eat them, water scorpions (Family Nepidae) and dragonfly nymphs (Suborder Anisoptera) are known fish-eaters! To see a Water Stick (Genus Ranatra) ambush a hapless vertebrate is a teaching moment of the highest order. However, if you do keep them, be prepared to release dragonflies and damselflies later on!

Using the Data Being a field lab, there are many variables that can be tested. Dos the contents of the bag make a difference? Have some students use grass clippings, others fallen leaves, and others, pine needles, cones, or sweetgum balls. How does the species of one water body compare with that of another? Compare still versus running water, clear vs. turbid, sheltered vs. exposed – the possibilities are endless! Does the season make a difference? What about the length of time the trap is in the water? Upon collecting the data, you may want to save this data for next year’s classes to compare against.