Using bacterial growth on insects to assess nutrient impacts in streams

A combination field and laboratory study was conducted to evaluate the abil ity of a recently developed bioindicator to detect detrimental nutrient con ditions in streams. The method utilizes bacterial growth on aquatic insects to determine nutrient impacts. Field investigations indicated that elevate d concentrations of nitrate and phosphate were associated with growth of fi lamentous bacteria on insect body surfaces, and that there was a significan t reduction in the density of major insect taxa in the nutrient-enriched st ream 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% cov erage of the body surface. A threshold for catastrophic mortality occurred at about 25% body coverage; there were few survivors above that amount. Bas ed on these findings, the diagnostic endpoint for the bioindicator is 25% b ody coverage by bacterial growth, a level that signifies major impacts and is also easy to detect visually. This study provides additional evidence th at the insect-bacteria bioindicator is a reliable tool for assessing nutrie nt 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.