IDENTIFYING LIMITS ON AQUATIC INSECT DENSITY EXPOSED TO METALS IN THE PRESENCE OF CO-LIMITING FACTORS

Quantile regression was used to estimate the effect of metal concentrations on the density of a caddisfly (Arctopsyche grandis) and two mayflies (Rhithrogena spp. and Drunella spp.) at 125 discrete locations in Colorado. Akiaki Information Criteria were used to evaluate which co-limiting factors (metals, basin area, elevation, stream temperature, and discharge) likely limited each quantile (0.05^th to 0.95^th by 0.05) of taxa density. Densities differed heterogeneously with increasing metal concentration and were described by multiple quantile slopes and limiting factors. High quantiles (> 50^th) of density were more reliably related to metal concentrations and generally had higher rates of change (steeper slope) than lower quantiles (<50^th). Caddisfly density was not limited by metals concentrations. Maximum densities (90^th quantile) of the two mayflies declined by 56 to 60 percent at metal concentrations previously thought safe for aquatic life.