Certain macroinvertebrates are sensitive to pollution, while others are highly tolerant of it. The number and diversity (richness) of pollution-sensitive (e.g., mayflies, stoneflies, caddisflies) and pollution-tolerant macroinvertebrates (e.g., midges, black flies, worms) can be used to describe water quality conditions. Because the macroinvertebrate samples were collected from a known area of stream bottom the data could be summarized as a density (individuals/m2) for individual families or groups of families.
Not all macroinvertebrates were identified to the family level because of specimen size, damage, or taxonomic limitations. Thus, our estimates of richness may slightly underestimate actual richness. From 1991 to 1993 macroinvertebrates were, in part, only identified to order because volunteers were still learning identification skills.
Besides density of certain macroinvertebrate groups, water quality conditions can also be described by various metrics that are commonly used in water quality monitoring programs. Metrics take in to account diversity (richness) and/or composition (percentages) of certain macroinvertebrate groups. Many metrics can be combined to create an index (e.g., Macroinvertebrate Aggregated Index for Streams (MAIS)), a single value that rates water quality for that site).
Epeorus mayfly larvae
The quantitative sampling method allowed macroinvertebrate counts to be expressed as a density (e.g., individuals/m2) that was used to compare across sites and years. We examined densities of pollution-sensitive taxa [e.g., many Ephemeroptera (mayflies), Plecoptera (stoneflies), Trichoptera (caddisflies)] and pollution-tolerant taxa [e.g., many Diptera (true flies), Odonata (dragonflies, damselflies), Coleoptera (beetles)]. In response to moderate exposure to pollution, a decrease in density of pollution-sensitive taxa accompanied by an increase in density of pollution-tolerant taxa would be predicted.
Densities of Ephemeroptera, Plecoptera, and Trichoptera (EPT) are commonly pooled together and analyzed as a group to assess changes in water/habitat quality in streams and rivers. Species in this group are generally more pollution-sensitive than other taxa; thus, a decrease in EPT density would be predicted in response to moderate exposure to pollution. All density data were natural logarithm transformed, a standard procedure to correct for the clumped spatial dispersion of invertebrate populations in rivers.
To classify stream condition using the macroinvertebrate data, we calculated a Macroinvertebrate Aggregated Index for Streams (MAIS) that uses family level identification that integrates various types of information into a single number that can be used to compare streams. It summarizes the values of 10 metrics: Ephemeroptera Richness, EPT Richness, Intolerant Taxa Richness, % Ephemeroptera, % EPT, % 5 Dominant Taxa, Simpson Diversity, Hilsenhoff’s Biotic Index (HBI), % Scrapers, and % Haptobenthos. Values for the individual metrics are transformed into a score of 0, 1 and 2, and then combined into a MAIS score. MAIS scores are predicted to decrease in response to a decrease in water/habitat quality. Streams are classified based on MAIS scores as follows:
- 13.1-20 classify a site as “Good”
- 6.1-13 classify a site as “Fair”
- 0-6 classify a site as “Poor”