Benthic macroinvertebrates (or “macros” for short) are animals that lack a backbone and are large enough to be seen by the unaided eye (size >0.2-0.5mm). As a result of their habitat, macroinvertebrates are often regarded as “benthos” which refers collectively to organisms which live on, in, or near the bottom.
Freshwater benthic macroinvertebrates include the immature and adult stages of many different types of invertebrates. A freshwater benthic community may consist of the immature stages of many flies, beetles (adults and immatures), mayflies, caddisflies, stoneflies, dragonflies, aquatic worms, snails, leeches, and numerous other organisms that inhabit the stream.
Macroinvertebrates, especially immature insects, are one of the best indicia for assessing water quality. Because they are not very mobile, these organisms must be adapted to their particular ecosystem and integrate the availability of food, type of stream bottom, predators, water temperature, water velocity, and the water quality itself into their survival. Many macroinvertebrates also spend many months to years in the same location. To understand how to interpret population changes in aquatic communities, one must first understand the distribution of these organisms under natural conditions.
Different insects have different methods of capturing food. These methods vary from the silken net made by some caddisflies to strain food out of the current to mayflies which scrape algae off the rocks into their mouths. The riffle or rapids areas are the most productive areas in a stream and, if unpolluted, yield the greatest diversity of macroinvertebrates within the system.
As the stream descends from the headwaters, the slope lessens. The water slows down, the temperature rises, the dissolved oxygen level may be lower, the carbon dioxide higher, and the bottom changes from gravel and sand to mud at the mouth of the stream. These changes in conditions mean a change in the kind of aquatic life that best survives. Many of the organisms found in this part of the stream are the same as those found in ponds. These include burrowing mayflies, dragonfly nymphs, damselfly nymphs, mussels, snails, leeches, backswimmers, mosquito larvae, and water striders. In the mud bottom areas, one would expect to find clams, midge larvae, fish fly larvae, alderfly larvae, water boatmen, and tubifex worms. This part of the stream, as long as it is not polluted, also has a high diversity of aquatic organisms.
If pollution occurs in the stream, some organisms such as mayflies, stoneflies, water pennies, freshwater clams, caddisflies, and hellgrammites may disappear entirely as they cannot tolerate degraded water. As the pollution is broken down and diluted downstream, some of these organisms may reappear. Meanwhile, at the original site of the pollution, more tolerant species such as blackfly larvae, planaria, crayfish, and aquatic sowbugs are found. The overall diversity of organisms will be reduced. If the water is heavily polluted, you may find only the most tolerant species such as air breathing snails, rat-tailed maggots, and leeches. Even though you may find large numbers of these creatures, the overall diversity will be quite low.
Be sure to follow the same collecting technique each time your group goes to the stream, as this will allow a more accurate comparison of the data collected over time. Mark your collection site so that you can sample from the same area each time. When collecting biological data, remember to take samples from three different riffle areas each time you visit your study site.
Use the same size kick net each time you sample. Make certain no macroinvertebrates can pass under or over the screen of the kick net. Students should start “kicking” the stream bottom beginning furthest from the screen and working toward it in order to capture all macroinvertebrates. Remind students to closely examine fist-sized or larger rocks for macros clinging to them. Be sure to return these rocks to the same location in the streambed. These are living creatures that need water to breathe. Do not keep them out of the water too long.
For students (and teachers), identifying the macroinvertebrates found during sampling can be overwhelming at first. Care should be taken as to how this task is handled. Groups will vary in their degree of readiness for this task.
In the classroom, introduce your students to dichotomous keys and practice using such a key to identify major groups of macroinvertebrates. You may wish to use preserved specimens or pictures for this purpose. Once the students are familiar with keying out macroinvertebrates to orders, you may wish to stop there and simply divide members of each order into visually apparent kinds. These visually apparent kinds will most likely be sub-orders or families. For example, once members of the order Ephemeroptera (mayflies) are separated from other macroinvertebrates, students could then identify them to family level; such as, 17 Flathead Mayflies, eight Small Minnow Mayflies, and two Common Burrower Mayflies. As long as you counted the numbers (relative abundance) of different kinds of mayflies and knew the Pollution Tolerance Values (PTV) for these families, you could still calculate the diversity index for the water quality of your stream. If you are utilizing the online WE Database, each listed macroinvertebrate includes its PTV. If you click the “Show More” button, macros are listed at sub-order and family level. The system will calculate the Stream Quality Rating for you.