Macro invertebrates can be very descriptive of the overall water quality of a waterway, though they cannot pinpoint specific chemical parameters. Benthic (bottom dwelling) organisms: reflect the long-term condition of the stream from a variety of sources, have a high species diversity and are more sensitive to changes in the ecosystem than fish, and live at the bottom of the waterway, where pollution associated problems are often magnified.
Biotic surveys can be conducted in a number of ways. It is best to use a variety of capture methods as the insects have a wide variety of habitats. Many of the macro invertebrates are actually insect larva that bear little resemblance to the adult that many are familiar with. Some prefer to burrow into the sandy bottom of a stream, like midges. Others cling to the bottoms and sides of rocks. Some organisms prefer to live in quiet pools while others live in riffles. Riffles are the areas of water that are relatively fast-moving and flow over and around many rocks.
Macro invertebrate populations will respond readily to pollution of many kinds. It is important to remember this when doing a biotic index. If there are multiple pollution types in a stream, each has its own effect. The macros also respond differently to different types of pollution. Agricultural runoff with result in one kind of population change, acid rain another and AMD still another.
One way that macro invertebrates can be classified is by using their functional ecology, or food gathering techniques. The basic groups include: scrapers, shredders, collectors and predators. Scrapers usually have very flat, sleek bodies that allow them the easily cling to rocks and scrape off algae for food. Some species of mayflies and stoneflies are scrapers. Shredders are organisms that devour the large particulate matter in the streams, such as dead leaves in the fall. Collectors are usually either filtering collectors or gathering collectors. Filtering collectors usually cling to the rocks, either through suction-cup like appendages or protective structures (cases) that they build and attach to the rock with 'glue'. Nets or fine hairs are then released into the water to collect whatever food particles are passing by. Gathering collectors usually burrow into the sandy or muddy bottoms of the stream to find food. Collectors always search for fine particulate organic matter. Filtering collectors include caddis flies and black flies while an example of a gathering collector is a midge. The last group, predators, is self-explanatory in their feeding ecology. These insects pray on other macro invertebrates. Dragonfly nymphs are often found in area streams as dominating predators. Expect the number of predators to be the lowest portion of the population, if the predator population was too large, the entire macro invertebrate community would collapse.
For additional information on macroinvertebrates please go to Macroinvertebrate Guide.
The four most common means of collecting benthic organisms are: rock washes, kick nets, sediment drags, and leaf packs. It is a good idea to use a combination of these methods to obtain an accurate portrait of the macro invertebrate population. One important thing to remember before doing any data collection is to start downstream first! This prevents contamination of the next sample site by any dirt, sediment or organisms that are stirred up from the first sampling. It is also a good idea to repeat each sampling method 2 to 3 times at each site to ensure an adequate sample is obtained.
Rock washing is by far the simplest method to explain, but can be difficult to do correctly. Be sure to have some basins available to wash the rocks in as well as sample bags if the samples will be taken to a lab for analysis. Try to choose 3 - 4 dinner plate sized rocks from a riffle area. Fill the bottom of the basin with stream water and thoroughly scrub each rock, carefully setting them aside when done. Then proceed to find and identify your critters or transfer the water and critters to bags to return to the lab. If you are taking the samples to a lab, be sure to have everything clearly labeled and identified. DO NOT MIX SAMPLES!
Kick nets are relatively easy to do, but are best done with 2 people. One person holds a D-frame net (so named because the opening is shaped like a D) against the bottom of the stream in a riffle area while the other person stands in front of the net and kicks the rocks in a approximately a 2 square foot area in front of the net for 2 minutes. D-frames have a fine mesh that allows water to travel through while trapping macro invertebrates. The nets can then be rinsed into basins and counted or rinsed into bags and taken to the lab. Be sure to have filters available to sift out sediment, but retain the macro invertebrates.
Sediment drags are similar to kick nets. It is possible to use a d-frame net for this as well. It this procedure, someone scoops into the sediment on the bottom of the stream and rinses it into a basin or bag as before.
Leaf packs are very easy to use, but require much more time. Prepare leaf packs by filling screen mesh bags with leaves, try to use the same kinds as the trees surrounding your stream. Make sure the mesh is large enough for the macros to climb in. Weight each bag with a rock and attach 3 or 4 to a brick or other weight. Place 3 to 4 bricks in various locations at each sample site. Allow the bricks to remain for about a month before collecting them. When collecting them be sure to look carefully, bricks are often moved by strong currents. Bag the leaf packs and take them back to the lab to count and identify the macro invertebrates.
Do not be surprised if the organisms you gather appear different from the adult, this is normal in insect development. Some of the macros go through incomplete metamorphosis so the nymph grows in size, but changes little in appearance. Others undergo complete metamorphosis, in which the larva looks nothing like the developed adult. At times even separate stages in the development of either type of insect can appear quite different. If there are any questions about the identity of a captured microorganism, be sure to make use of the available help from the conservation district or accessible qualified macro inverterbrate identifier.
Be sure to return most of your samples to the stream to prevent population
depletion, questionable, interesting or required samples can be removed and kept
for further reference.