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Image of a decopod crustacean, or 'crawdad.'  This image links to a more detailed image. Image of three aquatic beetle larvae.  This image links to a more detailed image. Image of two Mayflies grazing on algae.  This image links to a more detailed image. Image of a predacious stonefly.  This image links to a more detailed image. Image of four case-building caddisflies.  This image links to a more detailed image. Image of a muscle, mollusk, or Bivalve (clamed-up).  This image links to a more detailed image. Water Quality Assessment: Biological: Macroinvertebrates
To assess the biological attributes of water quality, scientists generally examine benthic macroinvertebrates. These organisms are abundant, easier to capture than fish, and easier to identify than algae or protozoa. Benthic macroinvertebrates include crustaceans, mollusks, worms, and many species of insect larva such as mayflies, stoneflies, caddisflies, and beetles. Samples of macroinvertebrates can be collected over areas of uniform size using a Hess sampler (below) in large (fifth- and sixth-order) streams. A Surber sampler (below) is usually used in smaller streams. Generally, three samples are collected from one riffle per study site.

Photos from top to bottom:
1. A decopod crustacean or "crawdad."
Photo courtesy of Dr. Ken Rastall.
2. Aquatic beetle larvae.
Photo courtesy of James Hodges, Jr.
3. Mayflies grazing on algae.
Photo courtesy of Larry Willis.
4. A predacious stonefly.
Photo courtesy of James Hodges, Jr.
5. Case-building caddisflies.
Photo courtesy of James Hodges, Jr.
6. A muscle, mollusk, or bi-valve--all clammed up.
Photo courtesy of Dr. Ken Rastall.

Macroinvertebrates from each sample are identified and counted. The density of organisms per square meter of stream bottom at each site is estimated from the average of the samples collected there. A calculation of species diversity such as the Shannon Index of Species Diversity can be performed on this data. The Shannon Index of Species Diversity is often performed on macroinvertebrate order data rather than species data.

The abundance of macroinvertebrates belonging to the orders Ephemeroptera, Plecoptera, and Trichoptera should be noted. These three orders constitute the EPT Index of a stream. Since these orders of macroinvertebrates are highly sensitive to pollution, they are often used as water quality indicators. Their presence indicates a high quality of water, while their absence suggests water may be polluted. The EPT Index is calculated as the sum of the number of Ephemeroptera, Plecoptera, and Trichoptera divided by the total number of midges. Midges (Diptera: Chironomidae) are a species of fly that are present in large numbers in nearly all streams.
Image of some students using a Hess sampler to collect benthic sample in a large stream in the Wheeling Creek watershed. Image of some students using a Surber sampler to collect benthic sample in a small stream in the Wheeling Creek watershed.
Students using a Hess sampler to collect benthic sample in a large stream in the Wheeling Creek watershed. Photo courtesy of Dr. Ben Stout. Students using a Surber sampler to collect benthic sample in a small stream in the Wheeling Creek watershed. Photo courtesy of Dr. Ben Stout.

 

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Macroinvertebrates

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