Quality Assessment: Biological: Macroinvertebrates
from top to bottom:
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.
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