Using macroinvertebrate response to inform sediment criteria development in mountain streams

The phrase biologically-based sediment criterion indicates that biological data is used to develop regional sediment criteria that will protect and maintain self-sustaining populations of native sediment-sensitive biota. To develop biologically-based sediment criteria we must quantitatively link a gradient of sediment effects with biotic response. In this study we related an index of biotic integrity (IBI) for aquatic macroinvertebrates to reachwide measures of areal percent streambed surficial fines (≤ 0.06 mm) and sand and fines (≤ 2 mm) for 525 mountain streams in the western U. S. Quantile regression predicted declines in potential macroinvertebrate IBI of 4.6% for each 10% increase in areal percent fines (≤0.06 mm) and 3.5% for each 10% increase in areal percent sand and fines (≤ 2 mm). To collect evidence for minimum-effect sediment thresholds, we recorded the range of fine sediment values at 169 least-disturbed reference sites in our sample and calculated sediment tolerance values for eight sediment-intolerant macroinvertebrate species for both particle size classes. We also gathered evidence from a literature review of studies that quantitatively linked macroinvertebrate response to the pertinent size classes of streambed accumulated sediment. The body of evidence suggested that sediment thresholds that would sustain sediment-intolerant macroinvertebrates were 3% for silt-sized fines and 10% for sand and fines. We found the sediment-intolerant macroinvertebrate species selected for our study to be more sensitive to fine sediment accumulation than four sediment-sensitive salmonid species that had minimum-effect sediment thresholds of 5% for fines and 13% for sand and fines. Documenting the varying sensitivities and responses of different taxa to the effects of fine sediment accumulation in streams helps to inform the criteria-setting process.