Sedimentation is widely acknowledged as a major cause of degradation of instream habitats (Wood et al. 2005). During rain storms and snowmelt, dirt and gravel roads bleed sediment into ditches that often drain into streams. These roads are a major source of stream sediment loads, especially harmful fine sediments, and roads contribute more sediment to streams than any other land management activity (USDA 2000). Sedimentation is directly related to a decrease in benthic macroinvertebrate density and a change in diversity according to a number of studies. In this paper I review some of the impacts of sedimentation on benthic invertebrates and explain how examining macroinvertebrate diversity can help determine overall aquatic ecosystem health.

Overview of Impacts
Wood and Armitage (1997) define four primary ways in which fine sediments impair macroinvertebrate diversity and health: 1) altering substrate composition and changing its suitability for some taxa; 2) increasing drift due to sediment deposition or substrate instability; 3) affecting respiration due to silt deposition on respiration structures or low oxygen concentrations associated with silt deposits; 4) impeding filter feeding by increasing suspended sediment concentration, reducing the food value of periphyton, killing aquatic flora, and reducing the density of prey items. In addition, through drift caused by scouring the streambed, macroinvertebrates can become more susceptible to predation or experience damaged respiratory systems (Newcombe and MacDonald 1991).

Roads are also responsible for chemical contamination of streams. For example, Forrow and Maltby (2000) investigated the mechanistic basis for reduced leaf processing in a stream contaminated with motorway (superhighway) runoff. They found the feeding rate of Gammarus pulex (Amphipoda), the dominant detrivore at the site, was significantly reduced downstream of the motorway discharge. Approximately 70 percent of the reduction in feeding rate could be accounted for by the direct effects of exposure to contaminated sediment.

Increased stream temperature and reduced dissolved oxygen content of streams can also be attributed to road activities such as the clearing of stream-side vegetation and the input of sediments. Fine sediment reduces dissolved oxygen content of the affected stream as suspended solids absorb heat from sunlight and increase stream temperature. Temperatures greater than 21oC (70oF) can severely stress most coldwater macroinvertebrates (Frondork 2001).

Using Macroinvertebrates for Stream Assessments
Bioassessment of rivers and streams can reveal water quality and stream ecosystem impairment. Aquatic benthic macroinvertebrates are especially useful indicators as each species has a specific tolerance for water conditions (Frondork 2001). These aquatic biota are affected by the physical, chemical and biological conditions of the stream and may show impacts from habitat loss not detected by traditional water quality assessments. As monitors of environmental quality, macroinvertebrates can reveal episodic as well as cumulative pollution and habitat alteration. The use of macroinvertebrates as bioindicators has been shown to be one of the most reliable and cost-effective assessment tools of water and habitat quality in streams throughout the world (King et al. 2000).

Macroinvertebrate assessment is crucial for determining aquatic ecosystem health in roaded landscapes. The presence of roads has been shown to be highly correlated with changes in species composition, population sizes, and hydrologic and geomorphic processes that shape aquatic and riparian systems (Trombulak and Frissell 2000). Macroinvertebrate diversity and abundance are affected by roads; their physical and behavioral changes can pinpoint sources of road-caused habitat impact.

Various models have been used to assess macroinvertebrate response to road induced aquatic changes. The heterogeneity of stream ecosystems, the variable responses of macroinvertebrates, and the differences between agency models suggest that analysis of the reference conditions and the model used is essential in interpreting bioassessment results. Though impact is evident, collaboration between agencies and citizen scientist working groups to define model standards is needed for remediation of problems indicated by bioassessment results.

EPT Index
The most general macroinvertebrate assessment model uses the EPT index. This index claims that although different insect taxa vary widely in their sensitivity to sedimentation, the taxa from the orders Ephemeroptera (E), Plecoptera (P), and Trichoptera (T) behave similarly. However, a taxonomic group can exhibit a great deal of heterogeneity (Lenat et al. 1981), so an assessment method like the EPT may be insensitive to changes in species composition unless composition is altered along with overall taxa richness (Hawkins et al. 2000).

Multimetric and multivariate approaches can increase a model’s accuracy. These models evaluate the sampled community by comparing observed conditions to what conditions or taxa are expected to occur in the absence of disturbance. The sampling method is important to consider as well. Gradient sampling designs have been shown to be more sensitive and powerful statistically than designs based on random allocation of samples (King et al. 2000).

The type of model used in macroinvertebrate assessment significantly affects determination of water quality impairment. Identifying the specific impact on a macroinvertebrate population may also be difficult due to the geomorphological and geochemical controls on the physical and chemical characteristics of streams. Many of the environmental variables are interrelated (Griffith et al. 2001) and as a result, community assemblages will be correlated with these variables, though species distributions may be directly affected by only one or a subset of the variables (Griffith et al. 2001). In addition to the physical variations that may influence data acquisition and interpretation, state agencies use different methods and models to biologically assess water quality. (Barbour et al. 1999).

Multimetric Assessments
US EPA Region IV has suggested adopting national multimetric assessment methods, sharing information on successful approaches to decision criteria, developing regional reference conditions across political boundaries, and developing shared ecological databases. They have also initiated cooperative efforts to increase exchange of biological data in shared ecoregions or basins. Conducting side-by-side assessments with multi-agency projects and using a single method would also assist in stream classification and developing regional reference conditions by ensuring that differences in assessment results are a consequence of natural differences in biotic communities and not investigator bias (Housten et al. 2002).

Conclusion
Roads cause a variety of impacts on stream ecosystem health and water quality. The use of macroinvertebrate assessment can reveal these impacts if properly conducted. Careful environmental analysis of the site, data comparison to reference sites and species-specific response models can provide accurate assessment of stream impairment and can generate predictions of macroinvertebrate response to road-caused impacts. Comparison of macroinvertebrate assessment results based on methods and models collected by various citizen groups and state agencies will facilitate an accurate understanding of road-caused impacts on stream health.

-Christine Morris is a graduate student in Environmental Studies at the University of Montana.



References
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