The abundance and composition of benthic macroinvertebrates are influenced by stream flow that in turn may be affected by Project operations; thus, benthic macroinvertebrates may provide an indirect indication of aquatic habitat integrity in relation to instream flows (as well as flow fluctuations, as described in Section 4.2.4). Assessments are usually conducted to evaluate impacts that have occurred, rather than to prescribe future desired conditions, or mitigation measures. There are two main approaches to assessing the effects of instream flows on macroinvertebrate communities. Of these approaches, the index of biotic integrity (IBI) approach (Karr and Chu 1997, Karr 1999) is more typically employed for instream flows studies, while the functional feeding group (FFG) approach (Cummins and Klug 1979) has been utilized for benthic macroinvertebrate community alteration assessments in relation to land management and more recently for instream flows.
In the FFG approach, collected samples are typically sorted in the field into functional feeding groups according to the classification system developed by Cummins and Klug (1979), later modified by Merritt and Cummins (1996). The separation of macroinvertebrates into FFGs is based on the mechanisms by which they acquire food; the morphological and behavioral adaptations for acquiring food are readily observed in the field with live, freshly collected specimens. For example, shredders often have generalized mouthparts for tearing pieces of matter from larger materials, whereas scrapers have modified mouthparts, such as slightly cupped mandibles with brush-like structures at the base that allow them to scrape food from rocks. Filtering collectors have specific body modifications such as filtering hairs on forelegs, or behavioural mechanisms such as net-spinning to collect fine particles suspended in the water. The composition of these FFGs can then be used to characterize potential effects of a hydroelectric project. Though this method is relatively easy and inexpensive, information gained is qualitative, and may not provide the data necessary to make strong inferences about the effect of a given hydroelectric project on the macroinvertebrate community. Although sorting is conducted in the field, laboratory analyses of selected samples is usually conducted to validate the field counts and to calculate fish drift-feeding indices.
For the IBI approach, just as with the FFG approach, substrate is sampled using a mesh collection net or artificial substrate. However, instead of sorting specimens in the field as with the FFG approach, samples are preserved and processed in the laboratory. Representative sub-samples are generally identified to family, with identification to genus or species as required and/or feasible. Using these identifications, assemblage composition is assessed by calculation of metrics that are based on the presence/absence and abundance of individual taxa with different levels of tolerance to environmental stressors. The sampling objective is to examine relative differences in assemblage composition between areas within the study area and outside of its potential influence using local and regional control sites. Metrics calculated may include:
- Richness-total number of unique taxa (many times with particular focus on the orders Ephemeroptera, Plecoptera, and Trichoptera).
- Sensitive taxa-total taxa from the orders Diptera, Ephemeroptera, Plecoptera, and Trichoptera.
- Sediment-sensitive taxa-total taxa from members of the orders Ephemeroptera, Plecoptera, and Trichoptera that qualify as "intolerant" or "sensitive."
- Hilsenhoff Biotic Index (HBI)-Index of organic pollution developed by Hilsenhoff (1988). Also known as community tolerance.
- Percent tolerant taxa-Proportion of individuals from taxa tolerant to disturbance.
- Percent sediment-tolerant taxa-Proportion of individuals from taxa tolerant to fine sediment.
- Percent dominant (single taxa)-Proportion of individuals from the single-most abundant taxon in the sample.
Based on a scoring system derived from these metrics, assessment of alterations in macroinvertebrates due to project effects on instream flows can be performed. As with the FFG approach, results are qualitative, and therefore may not have a strong nexus to specific project effects. Repeated sampling during alternate seasons and expert involvement is recommended to more accurately characterize the effect of instream flows on macroinvertebrate communities. This approach has been applied at the Carmen-Smith Hydroelectric Project (FERC Project No. 2242), on the McKenzie River, Oregon.
4.1.6.1 Selected references
Cummins, K. W., and M. J. Klug. 1979. Feeding ecology of stream invertebrates. Annual Review of Ecology and Systematics 10: 147-172.
Fore, L. S., J. R. Karr, and R. W. Wisseman. 1996. Assessing invertebrate responses to human activities: evaluating alternative approaches. Journal of the North American Benthological Society 15: 212-231.
Gislason, J. C. 1980. Effects of flow fluctuations due to hydroelectric peaking on benthic insects and periphyton of the Skagit River, Washington. Doctoral dissertation, University of Washington, Seattle.
Gislason, J. C. 1985. Aquatic insect abundance in a regulated stream under fluctuating and stable diel flow patterns. North American Journal of Fisheries Management 5: 39-46.
Gore, J. A., J. B. Layzer, and J. Mead. 2001. Macroinvertebrate instream flow studies after 20 years: a role in stream management and restoration. Regulated Rivers: Research and Management 17: 527-542.
Hilsenhoff, W. L. 1988. Rapid field assessment of organic pollution with a family-level biotic index. Journal of the North American Benthological Society 7: 65-68.
Karr, J. R. 1999. Defining and measuring river health. Freshwater Biology 41: 221-234
Karr, J. R., and E. W. Chu. 1997. Biological monitoring and assessment: using multi-metric indexes effectively. EPA 235-R97-001. University of Washington, Seattle.
Karr, J. R., and E. W. Chu. 1999. Restoring life in running waters: better biological monitoring. Island Press, Washington, D. C.
Merritt, R. W., and K. W. Cummins. 1996. An introduction to the aquatic insects of North America. Kendall/Hunt Publishing Company, Dubuque, Iowa.
Monk, W. A., P. J. Wood, D. M. Hannah, D. A. Wilson, C. A. Extence, and R. P. Chadd. 2006. Flow variability and macroinvertebrate community response within riverine systems. River Research and Applications 22: 595-615. doi:10.1002/rra.933.
Stillwater Sciences. 2006e. Aquatic habitats and instream flows at the Carmen-Smith Hydroelectric Project, upper McKenzie River basin, Oregon. Final report. Prepared by Stillwater Sciences, Arcata, California for Eugene Water & Electric Board, Eugene, Oregon.
