Linking Stream Sediment Metrics and Aquatic Macroinvertebrate Status in East Tennessee Streams

Excess sediment is one of the leading causes of impairment of the rivers and streams of Tennessee and in the United States. Sediment acts as a pollutant by degrading the quality of habitat for fish and other aquatic life. The Tennessee Department of Environment and Conservation (TDEC) currently assesses the quality of streams in Tennessee by conducting benthic macroinvertebrate surveys. These surveys use seven biometrics to calculate Tennessee Macroinvertebrate Index (TMI) scores. This method is well tested and reliable, but it has not been shown to directly link changes in stream sediment characteristics to macroinvertebrate status. Finding a stream sediment metric that could indicate water quality with the same reliability would be valuable for watershed management. The purpose of this study was to investigate the relationship between channel-bed sediment in riffles and benthic macroinvertebrate status in East Tennessee streams in order to determine whether fine sediment metrics might be useful indicators of stream health. This was accomplished by comparing bed sediment characteristics to the status of aquatic macroinvertebrates as reported by the TMI and by the seven biometrics used to derive the index. Research focused on channel bed sediment in riffle areas in 20 streams within the Ridge and Valley Ecoregion. Channel bed sediment size was characterized using a modified Wolman pebble count. Additional pebble count data from previous studies by Terrell (2011) and Williams (2005) were also added to the data set. Fine (<2 mm) sediment was collected and its composition was determined through laser particle size analysis. No signficant correlation was found between fine sediment size classes and TMI biometrics. Metrics for channel bed sediments significantly correlated with nearly all TMI biometrics. The highest correlations were between TMI scores and sediment metrics for larger particle sizes (D₅₀ [diameter at which 50% of the sample was smaller] and D₈₄ [diameter at which 84% of the sample was smaller]) rather than fine sediment. Statistical tests for differences in sediment metrics between levels of TMI impairment suggest that the D₅₀ and D₈₄ of bed sediments could potentially be used to determine whether a stream is considered to be impaired.