Background Information on Macroinvertebrates
Macroinvertebrates are animals without a backbone, and which are large enough to be seen by the unaided eye. 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 habitats and integrate the availability of food, type of stream bottom, predators, water temperature and speed, and the water quality itself into their survival. To understand how to interpret population changes in aquatic communities, one must first understand the distribution of these organisms under natural conditions.
The following information is generally true for macroinvertebrate populations in unpolluted water. One of the most important factors for the aquatic invertebrate is the rate of flow of the water. This rate will influence the type of stream bottom, the temperature, and the amount of dissolved oxygen in the water. If the gradient is steep and the water flow is fast, as it usually is in the headwaters of a stream, then the amount of dissolved oxygen will be high, the amount of carbon dioxide in the water will be low. The temperature of the water will be low and the stream bottom will be bedrock or large cobbles since all smaller material is carried downstream. The invertebrates in this part of the stream have many different adaptations which prevent them from being swept away. Their bodies may be flattened, (such as the water penny), so that the water flows over them as they cling to a rock. The organism may be like the blackfly larva which attaches to the rocks by means of suckers or like the caddisfly larva which builds itself a case of pebbles and sand and attaches this case to the underside of rocks with silken threads. The oxygen content of the riffle (area of broken surface water) is high, the number of predators is low and food is brought by the current. The different insects have different methods of capturing food. These methods vary from the silken net made by one of the caddisflies to strain food out of the current to mayflies which scrape algae off the rocks and 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 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 in quieter waters. 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, however, some of these organisms may reappear. Meanwhile, back 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 very polluted you may find only the most tolerant species such as air breathing snails, rat-tailed maggots, and leeches. Although you may find large numbers of these creatures, the overall diversity will be quite low.
Tips on collecting biological data
Be sure to follow the same collecting technique each time your group gathers data, as this will allow a more accurate comparison of the data collected over time. When collecting biological data, remember to take samplings from three different riffle areas at your study site.
Use the same size hand held screen each time you sample. Make certain no macroinvertebrates can pass under or over the screen. Remind students to pass all fist-sized or larger rocks to other students on the shore for close scrutiny. Be sure to return these rocks to the streambed. Students working in the water should then thoroughly dislodge the stream bottom beginning furthest from the screen and working toward it in order to capture all macroinvertebrates.
Tips on identification
Identifying the macroinvertebrates found during sampling can be overwhelming at first to students, so care should be taken as to how this task is handled. Groups will vary in their degree of readiness for this task.
Introduce your students to dichotomous keys and practice using such a key to identify major groups of aquatic macroinvertebrates and then to identify the orders of aquatic insects. You may wish to use preserved specimens or pictures in the classroom for this purpose. Once the students are familiar with keying out macroinvertebrates to major groups and insects 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 families. For example, once members of the order Ephemeroptera (mayflies) are separated from other macroinvertebrates, students could then decide how many subgroups of mayflies could be made. As long as you counted the numbers of different kinds of mayflies and know that you had, for example, seventeen type A mayflies, eight type B mayflies and two type C mayflies you could still calculate the diversity index necessary to keep an accounting of the relative water quality of your stream.
If you and your students are willing and able, you may tackle identifying the aquatic insects down to the family classification by using a complete key to aquatic insects. You should work toward this goal.
Back to Interpreting Biological Results
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