Using Bacterial Growth on Insects to Assess Nutrient Impacts in Streams

A combination field and laboratory study was conducted to evaluate the ability of a recently developed bioindicator to detect detrimental nutrient conditions in streams. The method utilizes bacterial growth on aquatic insects to determine nutrient impacts. Field investigations indicated that elevated concentrations of nitrate and phosphate were associated with growth of filamentous bacteria on insect body surfaces, and that there was a significant reduction in the density of major insect taxa in the nutrient-enriched stream reaches. Laboratory investigations confirmed a strong linkage between bacterial growth and reduced survival of insects. Survival was examined for insects with bacterial infestation ranging from 0% to greater than 50% coverage of the body surface. A threshold for catastrophic mortality occurred at about 25% body coverage; there were few survivors above that amount. Based on these findings, the diagnostic endpoint for the bioindicator is 25% body coverage by bacterial growth, a level that signifies major impacts and is also easy to detect visually. This study provides additional evidence that the insect-bacteria bioindicator is a reliable tool for assessing nutrient impacts on stream macroinvertebrate communities. The bioindicator should prove useful for identifying nutrient-impacted sites as well as monitoring the success of management actions to improve water quality.