Macroinvertebrate Data
The biological assessment of water quality involves examining both the quantity and quality of macroinvertebrates present. The Shannon Index of Species Diversity is calculated to assess the quantity of macroinvertebrates present. This index relates the density of individuals of each species (or order) present in a habitat to the total number of species (or orders) present in that habitat. From this, one can determine whether most of the individual macroinvertebrates present belong to just a few of the orders present, or if most of the individual macroinvertebrates present are evenly distributed across most of the orders present. The EPT Index is calculated to assess the quality of macroinvertebrates present. It is a ratio of pollution-sensitive insects to pollution-resistant insects.

In order to better demonstrate these two calculations, a step-by-step example of each is provided here using biological data collected at Site A in Bear Creek Watershed presented in the below table.

The Shannon Index of Species Diversity measures diversity by the formula ;

where H = diversity index, s= number of species (or orders), i = species (or order) number, and p= proportion of individuals of the total sample belonging to the ith species (or order).

H = -{[(6/6150)*(log 6/6150)] + [(2/6150)*(log 2/6150)] + [(5/6150)*(log 5/6150)] + [(165/6150)*(log 165/6150)] + [(5695/6150)*(log 5695/6150)] + [(5/6150)*(log 5/6150)] + [(272/6150)*(log 272/6150)]

H = -[(-0.002937292) + (-0.001134259) + (-0.002512118) + (-0.042159275) + (-0.030911707) + (-0.002512118) + (-0.059897771)]

H = 0.14206454

Remember, the higher the value of H, the greater the probability that the next individual chosen at random from a sample of species containing N individuals will NOT belong to the same species as the previous one (i.e. the greater H, the greater the diversity of species within an area).

The EPT Index is calculated as the sum of the number of insects from the orders Ephemeroptera, Plecoptera, and Trichoptera divided by the total number of insects of the species Diptera: Chironomidae:

EPT = (Ephemeroptera + Plecoptera + Trichoptera) / Diptera: Chironomidae.

EPT = (6 + 2 + 5) / 5695

EPT = 13 / 5695

EPT = 0.002282704

The greater the EPT value, the greater the proportion of pollution-sensitive insects (i.e. the greater the quality of water).

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