DCVA
Assessment Methods
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What are benthic macroinvertebrates? Benthic macroinvertebrates are organisms that dwell on the stream bottom and include many immature insect larvae and nymphs (mayflies, caddisflies and hellgrammites to name a few), crayfish, clams, worms, etc. These organisms spend months to years living on the stream bottom and their presence and the overall composition of the benthic community indicate changes in water quality and habitat degradation in the stream over time. How do you make any sense of all those bugs?Biologists use metrics to analyze samples of benthic invertebrates taken from a stream. Metrics are measures that summarize the numbers of organisms, the types of organisms, and the pollution tolerance of organisms. Taxa Richness, EPT and HBI are commonly used metrics. Taxa richness is a measure of the number of distinct families of critters in the stream. For example, a healthy stream has many different organisms. A healthy stream often has 20-25 families of different organisms. An impacted stream tends to have fewer types of organisms, and most of the critters present are tolerant to pollution and can live in poor habitat. As stream health improves, the number of different organisms increases and taxa richness generally increases. Taxa richness is also seasonal . The number of different organisms usually peaks in the spring, before the first hatch of mature insects occurs. Mayflies, stoneflies and caddisflies are sensitive to changes in water quality and habitat. The Latin names for the orders of these insects are Ephemeroptera, Plecoptera and Tricoptera. The number of different families, genera or species in these three orders can be counted and summarized into another common index, the number of Ephemeroptera, Plecoptera and Tricoptera (hence EPT) organisms. Healthy streams often have 10-15 different families of mayflies, stoneflies and caddisflies present. EPT is a subset of total taxa richness, and it also peaks in the spring. The Hilsenhoff Biotic Index, or HBI, is another common index that is designed to summarize the organic pollution tolerance of stream critters. This index was designed when raw sewage in streams and rivers was still a widespread concern. However, it is still useful today since many activities add organic pollution to the water (treated waste water, septic tanks, agriculture, etc.) Each family is assigned a tolerance value of 0 to 10. The most sensitive, or clean-water, organisms are lower on the scale. The more tolerant organisms are on the higher end of the scale. The index value ranges from 0 to 10, increasing as water quality decreases. Common values for very clean, healthy streams are often less than 4.5. This means that the average tolerance value of all the critters in the stream is 4.5. Slow-moving, lower gradient streams, like those found in the coastal plain, will have higher values for this index than fast-moving higher gradient streams. The slower-moving streams should not be considered impaired or polluted, just naturally different.
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The Darby Creek Valley Association
monitors the health of the watershed by sampling the benthic
macroinvertebrates. We use the U.S. Environmental Protection
Agency's 