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Freshwater macroinvertebrates are organisms that have no backbone (invertebrate), are large enough to be seen without a microscope (macro), and live underwater in streams, rivers, ponds, and lakes (freshwater). Many stream invertebrates are insect larvae. While such macroinvertebrates as dragonflies, mayflies, black flies, stoneflies, and caddis flies spend the adult stage of their lives on land and in the air, all of these insects spend the larval—and some the pupal—stage of their lives totally underwater. They have special adaptations, including gills, as larvae to help them live underwater. Other stream macroinvertebrates live their entire life underwater. These include crustaceans such as crayfish, scuds, and sowbugs; arachnids such as water mites; mollusks such as snails, clams, and mussels; and worms such as leeches, flatworms, and aquatic earthworms. Scientists have discovered that certain macroinvertebrates are sensitive to pollution and can survive only in clean or slightly polluted water. Others are somewhat sensitive and can live in clean water or water with a moderate level of pollution. Still others are tolerant of pollution and can live in any water condition, even heavily polluted water. A clean, healthy stream ecosystem will have many sensitive organisms as well as somewhat sensitive and tolerant ones. The unpolluted stream ecosystem will likely exhibit biodiversity, meaning a great variety of organisms will be found. A stream in fair condition with some pollution will have more tolerant organisms but also some somewhat sensitive and a few sensitive species. A polluted stream ecosystem will have mostly tolerant organisms and a few somewhat sensitive ones. The polluted stream will not exhibit much biodiversity, because fewer organisms are able to survive in its poor conditions. Professional scientists and certified Save Our Streams volunteers monitor stream health by collecting and identifying samples of the macroinvertebrates living there. The data is analyzed using several different counts. For this lesson, the analysis has been simplified so that students will compare the health of three simulated streams by looking only at the percent of tolerant macroinvertebrates.
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