A method for evaluating sediment-induced macroinvertebrate community composition changes in Idaho streams.

Area of Science:

• Environmental Science
• Aquatic Ecology
• Water Quality Management

Background:

• Human activities increase stream sediment, altering aquatic life composition and abundance.
• Limited numeric water quality standards for streambed sediment necessitate local benchmark development.
• Assessing sediment impacts requires reliable benchmarks for surface fine sediment (SF) and macroinvertebrate communities.

Purpose of the Study:

• Develop and validate benchmarks for stream surface fine sediment (SF) and macroinvertebrate fine sediment biotic index (FSBI).
• Create a framework to evaluate sediment-induced changes in macroinvertebrate communities in Idaho streams.
• Provide tools for managers to assess sediment impairment under the Clean Water Act.

Main Methods:

• Calculated FSBI reference benchmarks using the 25th percentile of reference sites.
• Developed SF benchmarks using quantile regression for reach-specific upper bounds.
• Employed logistic regression to predict FSBI condition based on SF values.
• Evaluated benchmark-macroinvertebrate associations using relative risk and multiple datasets.

Main Results:

• SF reference benchmarks showed stronger associations with poor macroinvertebrate condition than stressor-response benchmarks.
• Poor macroinvertebrate condition was 1.8-3 times more likely when SF reference benchmarks were exceeded.
• A proposed framework rates evidence for sediment impact as 'unlikely,' 'mixed,' or 'likely' based on benchmark achievement.

Conclusions:

• Developed and validated SF and FSBI benchmarks provide effective tools for assessing sediment impacts on aquatic life.
• Exceeding SF reference benchmarks is a strong indicator of impaired macroinvertebrate communities.
• A weight-of-evidence approach combining benchmark ratings with other data is recommended for robust sediment impairment assessments.