Streamwatch Project
Data Analysis
Macroinvertebrate data were summarized as estimates of density for individual taxa or groups of taxa and as community structure metrics that are commonly used in water quality monitoring programs. 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. Due to initial skill level of volunteers macroinvertebrates from 1991 to 1993 were in part identified only to order.
Density
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 species 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 ln transformed, a standard procedure to correct for the clumped spatial dispersion of invertebrate populations in rivers (Elliott 1977).
MAIS Index
To classify stream using the macroinvertebrate data, we calculated a Macroinvertebrate Aggregated Index for Streams (MAIS) that uses family level identification integrate various types of information into a single number that can be used to compare streams. The MAIS score was developed by Smith and Voshell (1997) based on benthic macroinvertebrate data from streams in Maryland (51 sites), Pennsylvania (53 sites), Virginia (126 sites) and West Virginia (200 sites). It summarizes the values of 10 metrics:
- Ephemeroptera Richness
- EPT Richness
- Intolerant Taxa Richness
- % Ephemeroptera
- % EPT
- % 5 Dominant Taxa
- Simpson Diversity
- HBI (Hilsenhof Biotic Index)
- % Scrapers
- % 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"
The difference between Good and Poor sites is dramatic. For example, EPT Richness (the number of mayfly, stonefly, and caddisfly families) might be 11-12 at the highest scoring Good sites, but only 1-3 at the Poor sites.
MDIBI Index
Maryland Index of Biotic Integrity (MDIBI) uses family level identifications to calculate seven metrics (Stribling et al. 1998).
- Total Richness
- EPT Richness
- Ephemeroptera Richness
- Diptera Richness
- % Ephemeroptera
- Intolerant Richness
- Beck’s Biotic Index
Values for the individual metrics are transformed into a score of 1, 3 or 5, and then combined into a MDIBI Score. MDIBI Scores are predicted to decrease in response to a decrease in water/habitat quality. Streams were classified based on MDIBI Scores using the modified scale:
- 21-28 Very Good (Non-impacted)
- 14-20.9 Good (Somewhat impacted)
- 7-13.9 Fair (Moderately impacted)
- 0-6.9 Poor (Severely impacted)
VASOS Index
The Virginia Save Our Streams (VASOS) Multimetric Index protocol uses a combination of family- and order-level identifications to calculate six metrics:
- % EP and most T
- % Hydropsychid caddisflies
- % Lunged snails
- % Coleoptera
- % Tolerant taxa
- % Non-Insects
Values for the individual metrics are transformed into a score of 0, 1 and 2, and then combined into a VASOS Score. VASOS Scores are predicted to decrease in response to a decrease in water/habitat quality. Streams are classified based on VASOS Scores as follows:
- 9-12 Acceptable ecological condition
- 7.1-8.9 Ecological conditions cannot be determined at this time (Gray Zone)
- 0-7 Unacceptable ecological condition