Using Multiple Lines of Evidence to Assess Aquatic Resource Condition for Western Public Lands: A Case Study from the Northern Great Basin, USA

Macroinvertebrate bioassessment tools are used to determine the biological condition of streams, but they do not identify the stressors responsible for degraded conditions nor the source of a given stressor. To address these issues, we selected sites on BLM land in Northeast California and Northwest Nevada using a spatially explicit random sample. At each site we measured biological, chemical, and physical attributes to make condition determinations. Using a multimetric index, we found 51.5% of stream km within the study area have degraded biological condition. Of the chemical and physical indicators, total nitrogen, total phosphorus, and canopy cover were the most pervasive stressors. We found that 68.0% of stream km have excessive total nitrogen, 41.7% have canopy cover below expected conditions, and 36.9% have excessive total phosphorus. Using random forest models, 27.8% of the variability in biological condition was explained by total nitrogen and phosphorus concentrations, riparian habitat complexity, intermittent stream density, and watershed area. We hypothesize that the occurrence of biologically relevant stressors will be related to the extent of cattle grazing within a given watershed, the predominate land-use in the study area. For public land management agencies, identifying priority stressors and their likely sources is critical to effective and efficient adaptive management