Relative importance of two correlated variables on aquatic macroinvertebrate communities in a Colorado Front Range river: selenium and urbanization.

Urbanization and elevated selenium can negatively affect aquatic macroinvertebrate communities. These factors are often highly correlated in streams along the Front Range of the Rocky Mountains, USA. Understanding which factor is the primary driver of macroinvertebrate community health would aid in the development of appropriate management actions to mitigate their influence. Data collected from three sites in the Big Thompson River, Colorado, in seven different years from 2006 to 2016 were used to develop a random forest model to determine the relative importance of an index of urbanization and dissolved selenium levels in predicting the relative status of macroinvertebrate communities as measured by Colorado multi-metric index (MMI) scores. Based on the percent increase in mean square error (%incMSE) variable importance measure, dissolved selenium has a greater influence on MMI scores than a development index (19.5% vs 15.2%). There is a significantly negative relationship between MMI scores and dissolved selenium levels. This relationship predicts a MMI score of 29.2 under the current dissolved selenium standard of 4.6 µg/L and 38.6 under the suggested lower standard of 3.1 µg/L. Although improvements in MMI score may result with any reduction in dissolved selenium, other factors may limit the maximum expected MMI scores in the absence of selenium as the intercept of the relationship is 57.9 on a scale of 0-100.