Benthic macroinvertebrates are aquatic animals without backbones that are large enough to see without a microscope. They include worms, crustaceans, and immature forms of aquatic insects such as stonefly and mayfly nymphs.
Benthic macroinvertebrates can be important indicators of water quality. Unlike fish, these organisms are not very mobile and are therefore less able to escape the effects of pollution and sedimentation. Many species of mayfly nymphs, caddisfly larvae, and stonefly larvae are not very tolerant of pollution and can only survive in swift, cool, well oxygenated water. Their presence is generally a sign of good water quality. Other pollution intolerant species include hellgrammites, freshwater clams, and water pennies.
Blackfly larvae, midges, leeches, and aquatic worms are somewhat more tolerant of pollution and can be found in both good and poor quality water. If these species are the only ones present, they may be indicators of silty water with low dissolved oxygen.